Universal basic income

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Universal_basic_income

In 2013, eight million 5-centime coins (one per inhabitant) were dumped on the Bundesplatz in Bern to support the 2016 Swiss referendum for a basic income (which was rejected 77% – 23%).

Universal basic income (UBI) is a social welfare proposal in which all members of a given population regularly receive a minimum income in the form of an unconditional transfer payment, i.e., without a means test or need to perform work. In contrast, a guaranteed minimum income (GMI) is paid only to those who do not already receive an income that is enough to live on. A UBI would be received independently of any other income. If the level is sufficient to meet a person's basic needs (i.e., at or above the poverty line), it is considered a full basic income; if it is less than that amount, it is called a partial basic income. As of 2025, no country has implemented a full UBI system, but two countries—Mongolia and Iran—have had a partial UBI in the past. There have been numerous pilot projects, and the idea is discussed in many countries. Some have labelled UBI as utopian due to its historical origin.

There are several welfare arrangements that can be considered similar to basic income, although they are not unconditional. Many countries have a system of child benefit, which is essentially a basic income for guardians of children. A pension may be a basic income for retired persons. There are also quasi-basic income programs that are limited to certain population groups or time periods, like Bolsa Familia in Brazil, which is concentrated on the poor, or the Thamarat Program in Sudan, which was introduced by the transitional government to ease the effects of the economic crisis inherited from the Bashir regime. Likewise, the economic impact of the COVID-19 pandemic prompted some countries to send direct payments to its citizens. The Alaska Permanent Fund is a fund for all residents of the U.S. state of Alaska which averages $1,600 annually (in 2019 currency), and is sometimes described as the only example of a real basic income in practice. A negative income tax (NIT) can be viewed as a basic income for certain income groups in which citizens receive less and less money until this effect is reversed the more a person earns.

Critics claim that a basic income at an appropriate level for all citizens is not financially feasible, fear that the introduction of a basic income would lead to fewer people working, and consider it socially unjust that everyone should receive the same amount of money regardless of their individual needs. Proponents say it is indeed financeable, arguing that such a system, instead of many individual means-tested social benefits, would eliminate more expensive social administration and bureaucratic efforts, and expect that unattractive jobs would have to be better paid and their working conditions improved because there would have to be an incentive to do them when already receiving an income, which would increase the willingness to work. Advocates also argue that a basic income is fair because it ensures that everyone has a sufficient financial basis to build on and less financial pressure, thus allowing people to find work that suits their interests and strengths.

Early examples of unconditional payments to citizens date back to antiquity, and the first proposals to introduce a regular unconditionally paid income for all citizens were developed and disseminated between the 16th and 18th centuries. After the Industrial Revolution, public awareness and support for the concept increased. At least since the mid-20th century, basic income has repeatedly been the subject of political debates. In the 21st century, several discussions are related to the debate about basic income, including those concerning the automation of large parts of the human workforce through artificial intelligence (AI), and associated questions regarding the future of the necessity of work. A key issue in these debates is whether automation and AI will significantly reduce the number of available jobs and whether a basic income could help prevent or alleviate such problems by allowing everyone to benefit from a society's wealth, as well as whether a UBI could be a stepping stone to a resource-based or post-scarcity economy.

History

Antiquity

See also: Cura Annonae

Ancient Egypt had a strong, unified theocratic state that owned key parts of the Egyptian economy, including granaries that dispensed grain to the public during hard times.

In a 46 BC triumph, Roman general and dictator Julius Caesar gave each common Roman citizen 100 denarii. Following his assassination in 44 BC, Caesar's will left 300 sestertii (or 75 denarii) to each citizen. Trajan, emperor of Rome from 98 to 117 AD, personally gave 650 denarii (equivalent to perhaps US$430 in 2023) to all common Roman citizens who applied.

16th century

In his Utopia (1516), English statesman and philosopher Thomas More depicts a society in which every person receives a guaranteed income. In this book, basic income is proposed as an answer to the statement "No penalty on earth will stop people from stealing, if it's their only way of getting food", stating:

instead of inflicting these horrible punishments, it would be far more to the point to provide everyone with some means of livelihood, so that nobody's under the frightful necessity of becoming first a thief, and then a corpse.

Spanish scholar Johannes Ludovicus Vives (1492–1540) proposed that the municipal government should be responsible for securing a subsistence minimum to all its residents "not on the grounds of justice but for the sake of a more effective exercise of morally required charity." Vives also argued that to qualify for poor relief, the recipient must "deserve the help he or she gets by proving his or her willingness to work."

18th century

English-born American philosopher Thomas Paine authored Common Sense (1776) and The American Crisis (1776–1783), the two most influential pamphlets at the start of the American Revolution. His essay, Agrarian Justice, was published in 1797. In it, he proposed concrete reforms to abolish poverty. In particular, he proposed a universal social insurance system comprising old-age pensions and disability support, and universal stakeholder grants for young adults, funded by a 10% inheritance tax focused on land, it is also considered one of the earliest proposals for a social security system. Thomas Paine summarized his view by stating that "Men did not make the earth. It is the value of the improvements only, and not the earth itself, that is individual property. Every proprietor owes to the community a ground rent for the land which he holds." Paine saw inheritance as being partly a common fund and wanted to supplement the citizen's dividend in a tax on inheritance transfers.

In 1797, English Radical Thomas Spence published The Rights of Infants as a response to Thomas Paine's Agrarian Justice. In this essay Spence proposes the introduction of an unconditional basic income to all members of the community. Such allowance would be financed through the socialization of land and the benefits of the rents received by each municipality. A part of everyone's earnings would be seized by the State, and given to others.

19th century

Henry George proposed to create a pension and disability system, and a broad social support system from a single tax on land and natural resource value. Social support would be distributed to residents "as a right" instead of as charity. George mentioned, but did not stress, the possibility of direct cash distribution of land rent. His ideas gave rise to the economic philosophy now known as Georgism or the "single tax movement", which is an economic ideology holding that, although people should own the value they produce themselves, the economic rent derived from land—including from all natural resources, the commons, and urban locations—should belong equally to all members of society. Some Georgists refer to unconditional basic income funded by the single tax as a citizen's dividend in reference to Thomas Paine's proposal from the 19th century.

Early 20th century

Around 1920, support for basic income started growing, primarily in England.

Bertrand Russell (1872–1970) argued for a new social model that combined the advantages of socialism and anarchism, and that basic income should be a vital component in that new society. In his 1918 book Roads to Freedom, Russell wrote "... the plan we are advocating amounts essentially to this: that a certain small income, sufficient for necessaries, should be secured to all, whether they work or not, and that a larger income – as much larger as might be warranted by the total amount of commodities produced – should be given to those who are willing to engage in some work which the community recognizes as useful..."

In the United Kingdom at the end of World War I, Dennis and Mabel Milner, a Quaker married couple of the Labour Party, published a short pamphlet entitled "Scheme for a State Bonus" (1918) that argued for the "introduction of an income paid unconditionally on a weekly basis to all citizens of the United Kingdom." They considered it a moral right for everyone to have the means to subsistence, and thus it should not be conditional on work or willingness to work.

C. H. Douglas was an engineer who became concerned that most British citizens could not afford to buy the goods that were produced, despite the rising productivity in British industry. His solution to this paradox was a new social system he called social credit, a combination of monetary reform and basic income.

In 1944 and 1945, the Beveridge Committee, led by the British economist William Beveridge, developed a proposal for a comprehensive new welfare system of social insurance, means-tested benefits, and unconditional allowances for children. Committee member Lady Rhys-Williams argued that the incomes for adults should be more like a basic income. She was also the first to develop the negative income tax model. Her son Sir Brandon Rhys-Williams proposed a basic income to a parliamentary committee in 1982, and soon after that in 1984, the Basic Income Research Group, now the Citizen's Basic Income Trust, began to conduct and disseminate research on basic income.

Late 20th century

Milton Friedman proposed a negative income tax (NIT), which effectively sanctioned a basic income for all, in his 1962 book Capitalism and Freedom. In his 1964 State of the Union address, U.S. President Lyndon B. Johnson declared an "unconditional war on poverty," implemented in coming years with sweeping legislation. Johnson broadened the agenda to the Great Society, including education, civil rights, health care, and support for the arts. In this political climate, the idea of a guaranteed income for every American also took root. Notably, a 1968 document, signed by 1200 economists, called for a guaranteed income for every American. Four ambitious basic income experiments started on the related concept of negative income tax. President Richard Nixon explained the Family Assistance Plan's purpose as to provide both a safety net for the poor and a financial incentive for welfare recipients to work. Congress eventually approved a guaranteed minimum income for the elderly and the disabled.

In the mid-1970s, the main competitor to basic income and negative income tax, the Earned income tax credit (EITC) and its advocates won over enough legislators for the US Congress to pass laws on that policy. In 1986, the Basic Income European Network (later renamed the Basic Income Earth Network (BIEN)) was founded, with academic conferences every second year. Other advocates included the green political movement, other activists, and some groups of unemployed people.

In the late 20th century, discussions were held around automatization and jobless growth, the possibility of combining economic growth with ecologically sustainable development, and how to reform the welfare state bureaucracy. Basic income was interwoven in these and many other debates. During the BIEN academic conferences, scholars published papers about basic income from a wide variety of perspectives.

21st century

Further information: List of advocates of basic income

In recent years, the idea has come to the forefront more than before. The Swiss referendum about basic income in 2016 was covered in media worldwide, despite its rejection. Famous business people like Elon Musk, Pierre Omidyar, and Andrew Yang have lent their support, as have high-profile politicians like Jeremy Corbyn and Tulsi Gabbard. The Institute for Public Policy Research predicted that 59% of tasks currently done by humans could be affected by AI in the next three to five years. Universal basic Income could help fill the gap left by this "jobs apocalypse."

In 2019-2021, in Stockton, California, then-Mayor Michael Tubbs initiated a 24-month pilot program of guaranteed income for 125 residents as part of the privately funded S.E.E.D. project there.

In the 2020 Democratic Party primaries, political newcomer Andrew Yang touted basic income as his core policy. His policy, referred to as a "Freedom Dividend", would have provided adult American citizens US$1,000 a month independent of employment status.

On 21 January 2021, in California, the two-year donor-funded Compton Pledge began distributing monthly guaranteed income payments to a "pre-verified" pool of low-income residents, in a program gauged for a maximum of 800 recipients, at which point it would be one of the larger among 25 U.S. cities exploring this approach to community economics.

Beginning in December 2021, Tacoma, Washington, piloted "Growing Resilience in Tacoma" (GRIT), a guaranteed income initiative that provided $500 a month to 110 families. GRIT is part of the University of Pennsylvania's Center for Guaranteed Income Research larger study. A report on the results of the GRIT experiment was published in 2024.

Response to COVID-19

As a response to the COVID-19 pandemic and its economic impact, universal basic income and similar proposals such as helicopter money and cash transfers were increasingly discussed across the world. Most countries implemented forms of partial unemployment schemes, which effectively subsidized workers' incomes without a work requirement. Around ninety countries and regions including the United States, Spain, Hong Kong, and Japan introduced temporary direct cash transfer programs to their citizens.

In Europe, a petition calling for an "emergency basic income" gathered more than 200,000 signatures, and polls suggested widespread support in public opinion for it. Unlike the various stimulus packages of the US administration, the EU's stimulus plans did not include any form of income-support policies.

Basic income vs negative income tax

See also: Negative income tax

Two ways of looking at basic income when combined with a flat income tax, both of which result in the same net income (orange line): 1. (red) stipend with conventional tax for income above the stipend. 2. (blue) negative tax for low-income people and conventional tax for high-income people.

The associated diagram shows a basic income/negative tax system combined with flat income tax (the same percentage in tax for every income level). Axis y is here the pre-tax salary given by the employer and y' is the net income.

Negative income tax

For low earnings, there is no income tax in the negative income tax system. They receive money, in the form of a negative income tax, but they do not pay any tax. Then, as their labour income increases, this benefit, this money from the state, gradually decreases. That decrease is to be seen as a mechanism for the poor, instead of the poor paying tax.

Basic income

That is, however, not the case in the corresponding basic income system in the diagram at right. There, everyone typically pays income taxes. But on the other hand, everyone also gets the same amount of basic income.

But the net income is the same

But, as the orange line in the diagram shows, the net income is anyway the same. No matter how much or how little one earns, the amount of money received is the same, regardless of which of these two systems are used.

Basic income and negative income tax are generally seen to be similar in economic net effects, but there are some differences:

Philippe Van Parijs in his library

• Psychological. Philip Harvey accepts that "both systems would have the same redistributive effect and tax earned income at the same marginal rate" but does not agree that "the two systems would be perceived by taxpayers as costing the same".
• Tax profile. Tony Atkinson made a distinction based on whether the tax profile was flat (for basic income) or variable (for NIT).
• Timing. Philippe Van Parijs states that "the economic equivalence between the two programs should not hide the fact that they have different effects on recipients because of the different timing of payments: ex-ante in Basic Income, ex-post in Negative Income Tax".

Perspectives and arguments

Automation

There is a prevailing opinion that we are in an era of technological unemployment – that technology is increasingly making skilled workers obsolete.

Prof. Mark MacCarthy (2014)

One central rationale for basic income is the belief that automation and robotisation could result in technological unemployment, leading to a world with fewer paid jobs. A key question in this context is whether a basic income could help prevent or alleviate such problems by allowing everyone to benefit from a society's wealth, as well as whether a UBI could be a stepping stone to a resource-based or post-scarcity economy.

U.S. presidential candidate and nonprofit founder Andrew Yang has stated that automation caused the loss of 4 million manufacturing jobs and advocated for a UBI (which he calls a Freedom Dividend) of $1,000/month rather than worker retraining programs. Yang has stated that he is influenced by Martin Ford. Ford believes that the emerging technologies will fail to deliver much employment; on the contrary, because the new industries will "rarely, if ever, be highly labor-intensive". Similar ideas have been debated before in history—that "the machines will take the jobs". What is new is the existence of several academic studies that forecast a future with substantially less employment, in the decades to come. Additionally, US President Barack Obama stated that he believes that the growth of artificial intelligence will lead to an increased discussion around the idea of "unconditional free money for everyone".

Economics and costs

See also: Cost of living

Some proponents of UBI have argued that basic income could increase economic growth because it would sustain people while they invest in education to get higher-skilled and well-paid jobs. However, there is also a discussion of basic income within the degrowth movement, which argues against economic growth.

Advocates contend that the guaranteed financial security of a UBI will increase the population's willingness to take risks, which would create a culture of inventiveness and strengthen entrepreneurial spirit.

The cost of a basic income is one of the biggest questions in the public debate as well as in the research and depends on many things. It first and foremost depends on the level of the basic income as such, and it also depends on many technical points regarding exactly how it is constructed.

While opponents claim that a basic income at an adequate level for all citizens cannot be financed, their supporters propose that it could indeed be financed, with some advocating a strong redistribution and restructuring of bureaucracy and administration for this purpose.

According to statements of American Enterprise Institute-affiliated Libertarian/conservative scholar Charles Murray, recalled and sanctioned in 2016 by the George Gibbs Chair in Political Economy and Senior Research Fellow at the Mercatus Center at George Mason University and nationally syndicated columnist Veronique de Rugy, as of 2014, the annual cost of a UBI in the US would have been about $200 billion less expensive than the US social safety-net system put in place at that date. By 2020, it would have been nearly a trillion dollars less expensive.

American economist Karl Widerquist argues that simply multiplying the amount of the grant by the population would be a naive calculation, as this is the gross costs of UBI and does not take into account that UBI is a system where people pay taxes on a regular basis and receive the grant at the same time.

According to Swiss economist Thomas Straubhaar, the concept of UBI is basically financeable without any problems. He describes it as "at its core, nothing more than a fundamental tax reform" that "bundles all social policy measures into a single instrument, the basic income paid out unconditionally." He also considers a universal basic income to be socially just, arguing that although all citizens would receive the same amount in the form of the basic income at the beginning of the month, the rich would have lost significantly more money through taxes at the end of the month than they would have received through the basic income, while the opposite is the case for poorer people, similar to the concept of a negative income tax.

Recent theoretical work has introduced alternative mathematical approaches to the allocation of basic income. One such example is the Boltzmann fair division model, which applies the Boltzmann distribution from statistical mechanics to resource or income allocation. In this framework, each individual's share is assigned probabilistically according to an exponential function of a specified attribute (such as need or contribution), providing a flexible mechanism to balance fairness and efficiency in basic income distribution.

Inflation of labor and rental costs

One of the most common arguments against UBI stems from the upward pressure on prices, in particular for labor and housing rents, which would likely cause inflation. Public policy choices such as rent controls or land value taxation would likely affect the inflationary potential of universal basic income.

Work

Many critics of basic income argue that people, in general, will work less, which in turn means less tax revenue and less money for the state and local governments. Studies include:

• In negative income tax experiments in the United States in 1970, there was a five percent decline in the hours worked. The work reduction was largest for second earners in two-earner households and weakest for primary earners. The reduction in hours was higher when the benefit was higher.
• In the Mincome experiment in rural Dauphin, Manitoba, also in the 1970s, there were slight reductions in hours worked during the experiment. However, the only two groups who worked significantly less were new mothers, and teenagers working to support their families. New mothers spent this time with their infant children, and working teenagers put significant additional time into their schooling.
• A 2024 study investigated the impact of $1,000/month UBI over a period of 3 years for 1,000 randomized low-income participants in two U.S. states, which represented an around 40% increase in household income. The study found an income effect with a decrease of non-UBI income by $1,500/year, a decrease in non-UBI household income by 21% of the UBI transfer, a 2% decrease in labor market participation, no significant change in time spent on childcare, no self-reported decrease in barriers to employment, a null result for changes in job quality, an increase in entrepreneurial orientation but no significant change in entrepreneurial activity, while enrollment in tertiary education showed a slight increase for participants below 30 years of age.

Although it is difficult to know for sure what will happen if a whole country introduces basic income, there are nevertheless some studies that have attempted to look at this question:

• A study from 2017 showed no evidence that people worked less because of the Iranian subsidy reform (a basic income reform).

Regarding the question of basic income vs jobs, there is also the aspect of so-called welfare traps. Proponents of basic income often argue that with a basic income, unattractive jobs would necessarily have to be better paid and their working conditions improved, so that people still do them without need, reducing these traps.

Philosophy and morality

See also: Community cohesion and Solidarity

By definition, universal basic income does not make a distinction between "deserving" and "undeserving" individuals when making payments. Opponents argue that this lack of discrimination is unfair: "Those who genuinely choose idleness or unproductive activities cannot expect those who have committed to doing productive work to subsidize their livelihood. Responsibility is central to fairness."

Proponents usually view UBI as a fundamental human right that enables an adequate standard of living which every citizen should have access to in modern society. It would be a kind of foundation guaranteed for everyone, on which one could build and never fall below that subsistence level.

It is also argued that this lack of discrimination between those who supposedly deserve it and those who do not is a way to reduce social stigma.

In addition, proponents of UBI may argue that the "deserving" and "undeserving" categories are a superficial classification, as people who are not in regular gainful employment also contribute to society, e.g. by raising children, caring for people, or doing other value-creating activities which are not institutionalized. UBI would provide a balance here and thus overcomes a concept of work that is reduced to pure gainful employment and disregards sideline activities too much.

Health and poverty

See also: Social determinants of mental health, Social determinants of health in poverty, and Social determinants of health

The first comprehensive systematic review of the health impact of basic income (or rather unconditional cash transfers in general) in low- and middle-income countries, a study that included 21 studies of which 16 were randomized controlled trials, found a clinically meaningful reduction in the likelihood of being sick by an estimated 27%. Unconditional cash transfers, according to the study, may also improve food security and dietary diversity. Children in recipient families are also more likely to attend school and the cash transfers may increase money spent on health care. A 2022 update of this review confirmed these findings based on a body of evidence (35 studies, the majority being large randomized controlled trials) and additionally found that unconditional cash transfers also reduce the likelihood of living in extreme poverty.

The Canadian Medical Association passed a motion in 2015 in support of basic income and for basic income trials in Canada.

Advocates

Main article: List of advocates of universal basic income

Pilot programs and experiments

Main article: Universal basic income pilots

See also: Universal basic income by country

Omitara, one of the two poor villages in Namibia where a local basic income was tested in 2008–2009

Since the 1960s, but in particular since the late 2000s, several pilot programs and experiments on basic income have been conducted around the world. Some examples include:

1960s−1970s

• Experiments with negative income tax in the United States and Canada in the 1960s and 1970s.
• The province of Manitoba, Canada experimented with Mincome, a basic guaranteed income, in the 1970s. In the town of Dauphin, Manitoba, labor decreased by 13%, less than expected. This program was ended after issues with the cost becoming unsustainable started to arise.

2000−2009

• The basic income grant in Namibia launched in 2008 and ended in 2009.
• An independent pilot implemented in São Paulo, Brazil launched in 2009.

2010−2019

• Basic income trials ran in 2011–2012 in several villages in India, whose government has proposed a guaranteed basic income for all citizens. It was found that basic income in the region raised the education rate of young people by 25%.
• Iran became the first country to introduce a system of UBI in December 2010. It was paid to all citizens and replaced the gasoline subsidies, electricity, and some food, that the country applied for years to reduce inequalities and poverty. The sum corresponded in 2012 to approximately US$40 per person per month, US$480 per year for a single person, and US$2,300 for a family of five people.
• In Spain, the ingreso mínimo vital, the income guarantee system, is an economic benefit guaranteed by the social security in Spain, but in 2016 was considered in need of reform.
• In South Korea the Youth Allowance Program was started in 2016 in the City of Seongnam, which would give every 24-year-old citizen 250,000 won (~US$215) every quarter in the form of a "local currency" that could only be used in local businesses. This program was later expanded to the entire province of Gyeonggi in 2018.
• The GiveDirectly experiment in a disadvantaged village of Nairobi, Kenya, benefitting over 20,000 people living in rural Kenya, was the longest-running basic income pilot as of November 2017, which is set to run for 12 years.
• A project called Eight in a village in Fort Portal, Uganda, was launched by a nonprofit organization in January 2017, providing income for 56 adults and 88 children through mobile money.
• A two-year pilot the Finnish government began in January 2017 involved 2,000 subjects. In April 2018, the Finnish government rejected a request for funds to extend and expand the program from Kela (Finland's social security agency).
• An experiment in the city of Utrecht, Netherlands launched in early 2017, that is testing different rates of aid.
• A three-year basic income pilot that the Ontario provincial government, Canada, launched in the cities of Hamilton, Thunder Bay and Lindsay in July 2017. Although called basic income, it was only made available to those with a low income and funding would be removed if they obtained employment, making it more related to the current welfare system than true basic income. The pilot project was canceled on 31 July 2018 by the newly elected Progressive Conservative government under Ontario Premier Doug Ford.
• In Israel, in 2018 a non-profit initiative GoodDollar started with an objective to build a global economic framework for providing universal, sustainable, and scalable basic income through the new digital asset technology of blockchain. The non-profit aims to launch a peer-to-peer money transfer network in which money can be distributed to those most in need, regardless of their location, based on the principles of UBI. The project raised US$1 million from a financial company.
• The Rythu Bandhu scheme is a welfare scheme started in the state of Telangana, India, in May 2018, aimed at helping farmers. Each farm owner receives 4,000 INR per acre twice a year for rabi and kharif harvests. To finance the program a budget allocation of 120 billion INR (US$1.55 Billion as of May 2022) was made in the 2018–2019 state budget.

2020−present

• Swiss non-profit Social Income started paying out basic incomes in the form of mobile money in 2020 to people in need in Sierra Leone. Contributions finance the international initiative from people worldwide, who donate 1% of their monthly paychecks.
• In May 2020, Spain introduced a minimum basic income, reaching about 2% of the population, in response to COVID-19 in order to "fight a spike in poverty due to the coronavirus pandemic". It was expected to cost state coffers three billion euros ($3.5 billion) a year."
• In August 2020, a project in Germany started that gives a €1,200 monthly basic income in a lottery system to citizens who applied online. The crowdsourced project lasted three years and be compared against 1,380 people who do not receive basic income. When the project was finished in August 2023, Mein Grundeinkommen calculated that a tax-financed universal basic income of €1,200 per month could be financed for every adult in Germany that would make 80% of adults better off.
• In October 2020, HudsonUP was launched in Hudson, New York, by The Spark of Hudson and Humanity Forward Foundation to give $500 monthly basic income to 25 residents. It will last five years and be compared against 50 people who are not receiving basic income.
• In May 2021, the government of Wales, which has devolved powers in matters of Social Welfare within the UK, announced the trialling of a universal basic income scheme to "see whether the promises that basic income holds out are genuinely delivered". From July 2022 over 500 people leaving care in Wales were offered £1600 per month in a 3-year £20 million pilot scheme, to evaluate the effect on the lives of those involved in the hope of providing independence and security to people.
• In July 2022, Chicago began a year-long guaranteed income program by sending $500 a month to 5,000 households for one year in a lottery system to citizens who applied online. A similar program was launched in late 2022 by Cook County, Illinois (which encompasses the entirety of Chicago as well as several suburbs) which sent monthly $500 payments to 3,250 residents with a household income at or below 250% of the federal poverty level for two years.
• In June 2023, The Guardian reported that a universal basic income of £1,600 a month is to be trialled in two places in England – Jarrow and East Finchley.
• In February 2025, South Korea announced a "farmers' opportunity income" plan to be integrated into their basic income for farmers, and deployed in 24 cities and counties of the Gyeonggi Province. An estimated 210,000 selected farmers and fishermen will receive either 1.8 million won annually or 50,000 won monthly.
• In March 2025, the Government of Delhi in India approved the "Mahila Samridhi Yojana" plan, under which eligible women would receive a monthly allowance of ₹2,500, based on financial status for women below the poverty line.
• In November 2025, the government of Marshall Islands introduced a national universal basic income scheme under which every resident citizen receives quarterly payments of about US$200. It is financed by a trust fund created under an agreement with the United States, which in part aims to compensate the Marshall Islands for nuclear testing there. The fund holds more than $1.3bn in assets, with the USA committing a further $500m through to 2027.

Payments with similarities

Alaska Permanent Fund

Main article: Alaska Permanent Fund

The Permanent Fund of Alaska in the United States provides a kind of yearly basic income based on the oil and gas revenues of the state to nearly all state residents. More precisely the fund resembles a sovereign wealth fund, investing resource revenues into bonds, stocks, and other conservative investment options with the intent to generate renewable revenue for future generations. The fund has had a noticeable yet diminishing effect on reducing poverty among rural Alaska Indigenous people, notably in the elderly population. However, the payment is not high enough to cover basic expenses, averaging $1,600 annually per resident in 2019 currency (As of 2019 it has never exceeded $2,100), and is not a fixed, guaranteed amount. For these reasons, it is not always considered a basic income. However, some consider it to be the only example of a real basic income.

Wealth Partaking Scheme

Main article: Wealth Partaking Scheme

Macau's Wealth Partaking Scheme provides some annual basic income to permanent residents, funded by revenues from the city's casinos. However, the amount disbursed is not sufficient to cover basic living expenses, so it is not considered a basic income.

Bolsa Família

Main article: Bolsa Família

Bolsa Família is a large social welfare program in Brazil that provides money to many low-income families in the country. The system is related to basic income, but has more conditions, like asking the recipients to keep their children in school until graduation. As of March 2020, the program covers 13.8 million families, and pays an average of $34 per month, in a country where the minimum wage is $190 per month.

Eastern Band of Cherokee Indians

The Eastern Band of Cherokee Indians (ECBI) opened Harrah's Cherokee Casino in 1997 and it has generated jobs and revenue for the tribe, providing money that the EBCI applies to its people's education, welfare and culture. Each member of the tribe is paid an annual income that started at $500 and has increased to $10,000 as of 2015.

Other welfare programs

• Pension: A payment that in some countries is guaranteed to all citizens above a certain age. The difference from true basic income is that it is restricted to people over a certain age.
• Child benefit: A program similar to pensions but restricted to parents of children, usually allocated based on the number of children.
• Conditional cash transfer: A regular payment given to families, but only to the poor. It is usually dependent on basic conditions such as sending their children to school or having them vaccinated. Programs include Bolsa Família in Brazil and Programa Prospera in Mexico.
• Guaranteed minimum income differs from a basic income in that it is restricted to those in search of work and possibly other restrictions, such as savings being below a certain level. Example programs are unemployment benefits in the UK, the revenu de solidarité active in France, and citizens' income in Italy.

Petitions, polls and referendums

• 2008: An official petition for basic income was launched in Germany by Susanne Wiest. The petition was accepted, and Susanne Wiest was invited for a hearing at the German parliament's Commission of Petitions. After the hearing, the petition was closed as "unrealizable".
• 2013–2014: A European Citizens' Initiative collected 280,000 signatures demanding that the European Commission study the concept of an unconditional basic income.
• 2015: A citizen's initiative in Spain received 185,000 signatures, short of the required number to mandate that the Spanish parliament discuss the proposal.
• 2016: The world's first universal basic income referendum in Switzerland on 5 June 2016 was rejected with a 76.9% majority. Also in 2016, a poll showed that 58% of the EU's population were aware of basic income, and 64% would have voted in favour of the idea.
• 2017: Politico/Morning Consult asked 1,994 Americans about their opinions on several political issues including national basic income; 43% either "strongly supported" or "somewhat supported" the idea.
• 2018: The results of a poll by Gallup conducted last year between September and October were published. 48% of respondents supported universal basic income.
• 2019: In November, an Austrian initiative received approximately 70,000 signatures but failed to reach the 100,000 signatures needed for a parliamentary discussion. The initiative was started by Peter Hofer. His proposal suggested a basic income sourced from a financial transaction tax, of €1,200, for every Austrian citizen.
• 2020: A study by Oxford University found that 71% of Europeans were in favour of basic income. The study was conducted in March, with 12,000 respondents and in 27 EU-member states and the UK. A YouGov poll likewise found a majority for universal basic income in United Kingdom and a poll by University of Chicago found that 51% of Americans aged 18–36 supported a monthly basic income of $1,000. In the UK there was also a letter, signed by over 170 MPs and Lords from multiple political parties, calling on the government to introduce a universal basic income during the COVID-19 pandemic.
• 2020: A Pew Research Center survey, conducted online in August 2020, of 11,000 U.S. adults found that a majority (54%) opposed the federal government providing a guaranteed income of $1,000 per month to all adults, while 45% supported it.
• 2020: In a poll by Hill-HarrisX, 55% of Americans voted in favour of UBI in August, up from 49% in September 2019 and 43% in February 2019.
• 2020: The results of an online survey of 2,031 participants conducted in 2018 in Germany were published: 51% were either "very much in favor" or "in favor" of UBI being introduced.
• 2020: An October survey of 1,026 Australians by YouGov found a 58% support for universal basic income.
• 2021: A Change.org petition calling for monthly stimulus checks in the amount of $2,000 per adult and $1,000 per child for the remainder of the COVID-19 pandemic had received almost 3 million signatures.

Proto-metabolism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Proto-metabolism 

A proto-metabolism is a network of linked chemical reactions in a prebiotic environment that resembled modern metabolic pathways but was primarily controlled by external factors such as minerals, environmental conditions, and geochemistry rather than by internally produced molecular components. Combining ongoing research in astrobiology and prebiotic chemistry, work in this area focuses on reconstructing the connections between potential metabolic processes that may have occurred in early Earth conditions. Proto-metabolism is believed to be simpler than modern metabolism and the Last Universal Common Ancestor (LUCA), as simple organic molecules likely gave rise to more complex metabolic networks. Prebiotic chemists have demonstrated abiotic generation of many simple organic molecules including amino acids,^[3] fatty acids,^[4] simple sugars, and nucleobases. There are multiple scenarios bridging prebiotic chemistry to early metabolic networks that occurred before the origins of life, also known as abiogenesis. In addition, there are hypotheses made on the evolution of biochemical pathways including the metabolism-first hypothesis, which theorizes how reaction networks dissipate free energy from which genetic molecules and proto-cell membranes later emerge. To determine the composition of key early metabolic networks, scientists have also used top-down approaches to study LUCA and modern metabolism.

Proto metabolism and minimal metabolism

Two related but distinct concepts are important for understanding the origins of metabolism:

Proto metabolism refers to the actual metabolic like chemistry that existed on prebiotic Earth. Kee and Monnard describe the "internal catalytic network, often referred to as 'metabolism'" as "the set of catalysts/catalytic assemblies that a protocell would have required to process resources into its own building blocks." Critically, early proto metabolic systems relied on external support: "From its simplest form, based on encapsulated metal-ions and complexes or even mineral particles, this reaction system would have gradually evolved first into RNA-based and, over time, into protein catalytic networks, i.e. towards metabolic bio-machinery."

Minimal metabolism is a theoretical concept describing the minimum requirements for chemistry to become truly metabolic. Lauber et al. (2021) define it as "a heuristic construct, halfway between chemistry and biology" that stands "at the interface between non-equilibrium complex chemistries and biological systems." Unlike proto metabolism that describes non genetic and non enzymatic reaction networks driven by the environment, minerals, and simple organics, minimal metabolism represents the next stage: the earliest cellular metabolic system with some enzymes and genetic control, capable of supporting growth and division while still depending on environmental gradients.

Heterotrophic versus autotrophic origin

The heterotrophic hypothesis, also known as the Oparin–Haldane hypothesis, proposes that the first organisms were heterotrophs that obtained energy and carbon from organic molecules accumulated through abiotic synthesis in the primitive environment. Alexander Oparin (1924) and J.B.S. Haldane (1929) independently argued that since heterotrophic anaerobes are metabolically simpler than autotrophs, heterotrophy must have evolved first. This concept traces back to Charles Darwin's 1871 speculation about life originating in "some warm little pond" containing ammonia, phosphoric salts, and energy sources where protein compounds could form. The Miller–Urey experiment (1953) provided experimental support by demonstrating abiotic synthesis of amino acids and other biochemically significant molecules under simulated early Earth conditions. The autotrophic hypothesis proposes that the earliest life forms were autotrophs capable of synthesizing organic molecules from inorganic carbon (CO₂) using geochemical energy. Günter Wächtershäuser's iron–sulfur world theory suggests that life originated at hydrothermal vents where iron sulfide and nickel sulfide minerals catalyzed carbon fixation from volcanic gases. Experimental work demonstrated synthesis of activated acetic acid and peptide bond formation on (Fe,Ni)S surfaces under prebiotic conditions. The two hypotheses are not mutually exclusive; the FeS/H₂S reducing chemistry central to autotrophic models is also consistent with heterotrophic scenarios.

Autocatalytic prebiotic chemistries

Further information: Autocatalysis and Autocatalytic set

Autocatalytic reactions are reactions where the reaction product acts as a catalyst for its own formation. Many researchers that study proto-metabolism agree that early metabolic networks likely originated as a set of chemical reactions that form self-sustaining networks. This set of reactions is commonly referred to as an autocatalytic set. Some prebiotic chemistries focus on these autocatalytic reactions including the formose reaction, HCN oligomerization, and formamide chemistry.

Formose reaction

Further information: Formose reaction

Discovered in 1861 by Aleksandr Butlerov, the formose reaction is a set of two reactions converting formaldehyde (CH₂O) to a mixture of simple sugars. Formaldehyde is an intermediate in the oxidation of simple carbon molecules (e.g. methane) and was likely present in early Earth's atmosphere. The first reaction is the slow conversion of formaldehyde (C1 carbon) to glycolaldehyde (C2 carbon) and occurs through an unknown mechanism. The second reaction is the faster and autocatalytic formation of higher weight aldoses and ketoses. The kinetics of the formose reaction are often described as autocatalytic, as the alkaline reaction uses lowest molecular weight sugars as feedstocks or input molecules into the reaction. Self-organized autocatalytic networks, like the formose reaction, would allow for adaptation to changing prebiotic environmental conditions. As a proof-of-concept, Robinson and colleagues demonstrated how changing environmental conditions and catalyst availability can impact the resultant sugar products.

In the past, many researchers have suggested the importance of this reaction for abiogenesis and the origins of metabolism because it can lead to ribose. Ribose is a building block of RNA and an important precursor in proto-metabolism. However, there are limitations for the formose reaction to be the chemical origin of sugars including the low chemoselectivity for ribose and high complexity of the final reaction mixture. In addition, a direct joining of ribose, a nucleobase, and phosphate to make a ribonucleotide (the building block of RNA) is not currently chemically feasible. Alternative prebiotic mechanisms have been proposed including cyanosulfidic prebiotic chemistries.

HCN oligomerization

On Earth, hydrogen cyanide (HCN) is made in volcanos, lightning, and reducing atmospheres like the Miller-Urey experiment. On the Hadean Earth, large impactor events and active hydrothermal processes likely contributed to widespread metal production and metal-based proto-metabolism. Hydrogen cyanide has also been detected in meteorites and atmospheres in the outer solar system.

HCN-derived polymers are the oligomer or hydrolysis products of HCN. These polymers can be synthesized from HCN or cyanide salts often in alkaline conditions, but they have been observed in a wide range of experimental conditions. HCN readily reacts with itself to produce many HCN polymers and biologically relevant compounds like nucleobases, amino acids, and carboxylic acids. The diversity of products could point to a plausible proto-metabolic network of HCN oligomerization reactions. Although, some groups point to low HCN concentrations in early Earth and low chemioselectivity of key biologically relevant products, similar to the formose reaction. Others have shown that abundant HCN is produced after large impacts and that high specificity and yield can be achieved.

Formamide chemistry

Further information: Formamide-based prebiotic chemistry

Formamide (NH₂CHO) is the simplest naturally occurring amide. Similar to HCN, formamide can form naturally. Formamide has specific physical and stability properties possibly suitable for a universal prebiotic precursor for early proto-metabolic networks. For example, it has four universal atomic elements ubiquitous to life: C, H, O, N. The presence of unique functional groups involving oxygen and nitrogen support reaction chemistries to build key biomolecules like amino acids, sugars, nucleosides and other key intermediates of other prebiotic reactions (e.g. the citric acid cycle). In addition, early Earth geological features like hydrothermal pores might support formamide chemistry and synthesis of key prebiotic biomolecules with concentration requirements.

Overall, formamide chemistry can support connections and substrates needed to support prebiotic biomolecule synthesis including the formose reaction, Strecker synthesis, HCN oligomerization, or the Fischer-Tropsch process. In addition, formamide can be easily concentrated through evaporation reactions as it has a boiling point of 210C. Although this reaction has high versatility across one-carbon atom precursors, the connections between different biosynthetic pathways are yet to be directly explored experimentally.

Experimental reconstruction

Many research groups are actively attempting experimental reconstruction of the interactions between prebiotic reactions. One major consideration is the ability for these reactions to operate in the same environmental conditions. These one-pot syntheses would likely push the reaction towards specific subgroups of molecules. The key to building proto-metabolic scenarios involves coupling constructive and interconversion reactions. Constructive reactions use autocatalytic prebiotic chemistries to increase the structural complexity of the original molecule, while interconversion reactions connect different prebiotic chemistries by changing the functional groups appended to the original molecule. A functional group is a group of atoms that has similar properties whenever it appears in different molecules. These interconversion reactions and functional group transformations can lead to new prebiotic chemistries and precursor molecules.

Cyanosulfidic scenario

Further information: Cyanosulfidic Prebiotic Synthesis

Cyanosulfidic scenarios are mechanisms for proto-metabolism proposed by the Eschenmoser and Sutherland groups. Research from the Eschenmoser group suggested that interactions between HCN and aldehydes can catalyze the formation of diaminomaleodinitrile (DAMN). Iterations of this cycle would generate multiple intermediate metabolites and key biomolecular precursors through functional group transformations by hydrolytic and redox processes. To expand upon this finding, the Sutherland group experimentally assessed the assembly of biomolecular building blocks from prebiotic intermediates and one-carbon feedstocks. They synthesized precursors of ribonucleotides, amino acids and lipids from the reactants of hydrogen cyanide, acetylene, acrylonitrile (product of cyanide and acetylene), and dihydroxyacetone (stable triose isomer of glyceraldehyde and phosphate). These reactions are driven by UV light and use hydrogen sulfide (H₂S) as the primary reductant in these reactions. As each of these synthesis reactions was tested independently and some reactions require periodic input of additional reactants, these biomolecular precursors were not strictly generated through a one-pot synthesis expected of early Earth environments. In the same work, these authors argue that flow chemistry or the movement of reactants through water could generate the conditions favorable for the synthesis of these molecules.

Glyoxylate scenario

Further information: TCA cycle

Eschenmoser also proposed a parallel scenario where the connections between prebiotic reactions would be connected by glyoxylate, a simple α-ketoacid, produced by HCN oligomerization and hydrolysis. In this work, Eschenmoser proposes potential schemes to generate both informational oligomers and other key autocatalytic reactions from plausible one-carbon sources (HCN, CO, CO₂).

The Krishnamurthy group at Scripps experimentally expanded on this theory. In mild aqueous conditions, they demonstrated that the reaction of glyoxylate and pyruvate can produce a series of α-ketoacid intermediates constituting the reductive tricarboxylic acid (TCA) cycle. This reaction proceeded without metal or enzyme catalysts as glyoxylate acted as both the carbon source and reducing agent in the reaction. Similarly, the Moran group have also reported pyruvate and glyoxylate can react in warm iron-rich water to produce TCA intermediates and some amino acids. Their work has successfully reconstructed 9 out of 11 TCA intermediates and 5 universal metabolic precursors. Additional experimental analysis is needed to connect this scenario to modern metabolism.

Energy sources

Unlike proto-metabolism, the bioenergetic pathways powering modern metabolism are well understood. In early Earth conditions, there were mainly three kinds of energy to support early metabolic pathways: high energy sources to catalyze monomers, lower energy sources to support condensation or polymerization, and energy carriers that support transfer of energy from the environment to metabolic networks. Examples of high energy sources include photochemical energy from ultraviolet light, atmospheric electric discharge, and geological electrochemical energy. These energy sources would support synthesis of biological monomers or feedstocks for proto-metabolism. In contrast, examples of lower energy sources for assembly of more complex molecules include anhydrous heat, mineral-catalyzed synthesis, and sugar-driven reactions. Energy carrier molecules could allow for propagation of the energy through the metabolic networks likely resembled modern energy carriers including ATP and NADH. Both energy carriers are nucleotide-based molecules and likely originated early in metabolism.

Gene-first versus metabolism-first hypotheses

Gene-first

The gene-first hypothesis proposes that the first living systems were self-replicating informational molecules, most likely RNA, capable of both storing genetic information and catalyzing chemical reactions. Under this model, simple self-replicating molecules would evolve under selective pressures into increasingly complex organisms.

A key advantage of the gene-first hypothesis is that a single molecule capable of template-directed replication can evolve readily if modifications breed true, whereas metabolic networks are fundamentally resistant to evolutionary change.

An autocatalytic network is a set of chemical reactions where the products of some reactions act as catalysts to accelerate other reactions in the same network. In simpler terms, chemicals in the network help each other form more chemicals, creating a self-sustaining and self-replicating system. A common challenge in metabolism-first origin-of-life theories is that chemical networks without genes tend to resist change. Imagine a system where one chemical (A) helps make a second chemical (B), and this second chemical (B) helps make more of the first (A). This creates a self-replicating loop but is hard to evolve because even if a better version of the first chemical (A') appears, the system only makes the original kind (B), not the improved one (B'). For this system to evolve, the better first (A`) chemical would also need to help make a better second chemical (B'), which then helps make more of the better first chemical (A'), a very unlikely chain. In more complex networks involving multiple chemicals helping make each other, this difficulty of evolving increases dramatically as the network grows. .^[52]

The discovery of ribozymes—RNA molecules with catalytic activity—by Thomas Cech and Sidney Altman, recognized with the Nobel Prize in Chemistry in 1989, provided crucial support for this hypothesis.

Metabolism-first

Further information: Abiogenesis

Metabolism-first hypothesis suggests that autocatalytic networks of metabolic reactions were the first forms of life. This is an alternative hypothesis to RNA-world, which is a genes-first hypothesis. It was first proposed by Martynas Ycas in 1955. A lot of recent work in this area is focused in computational modeling of theoretical prebiotic networks.

Metabolism-first proponents postulate that replication and genetic machinery could not arise without the accumulation of the molecules needed for replication. Alone, simple connections between prebiotic synthesis reactions could form key organic molecules and once encapsulated by a membrane would constitute the first cells. These reactions could be catalyzed by various inorganic molecules or ions and stabilized by solid surfaces. Molecular self-replicators and enzymes would emerge later, with these future metabolisms better resembling modern metabolism.

One critique for the metabolism-first hypothesis for abiogenesis is they would also need self-replicating abilities with a high degree of fidelity. If not, the chemical networks with greater fitness in early Earth would not be preserved. There is limited experimental evidence for these theories, so additional exploration in this area is needed to determine the feasibility of a metabolism-first origins of life.

Natural selection

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Natural_selection

A diagram demonstrating mutation and selection

Natural selection is the differential survival and reproduction of individuals due to differences in the relative fitness endowed on them by their own particular complement of observable characteristics. It is a key law or mechanism of evolution which changes the heritable traits characteristic of a population or species over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.

For Darwin natural selection was a law or principle which resulted from three different kinds of process: inheritance, including the transmission of heritable material from parent to offspring and its development (ontogeny) in the offspring; variation, which partly resulted from an organism's own agency (see phenotype; Baldwin effect); and the struggle for existence, which included both competition between organisms and cooperation or 'mutual aid' (particularly in 'social' plants and social animals).

Variation of traits, both genotypic and phenotypic, exists within all populations of organisms. However, some traits are more likely to facilitate survival and reproductive success. Thus, these traits are more likely to be passed on to the next generation. These traits can also become more common within a population if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specific niche, microevolution occurs. If new traits become more favoured due to changes in the broader environment, macroevolution occurs. Sometimes, new species can arise especially if these new traits are radically different from the traits possessed by their predecessors. (Reference?)

The likelihood of these traits being 'selected' and passed down are determined by many factors. Some are likely to be passed down because they adapt well to their environments. Others are passed down because these traits are actively preferred by mating partners, which is known as sexual selection. Female bodies also prefer traits that confer the lowest cost to their reproductive health, which is known as fecundity selection.

Natural selection is a cornerstone of modern biology. The concept, published by Darwin and Alfred Russel Wallace in a joint presentation of papers in 1858, was elaborated in Darwin's influential 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. He described natural selection as analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favoured for reproduction. The concept of natural selection originally developed in the absence of a valid theory of heredity; at the time of Darwin's writing, science had yet to develop modern theories of genetics. The union of traditional Darwinian evolution with subsequent discoveries in classical genetics formed the modern synthesis of the mid-20th century.

New evidence has prompted 21st century evolutionary biologists to challenge the 20th century's gene-centred view of evolution, producing several extended evolutionary syntheses which bring organisms back to the heart of the theory of natural selection. Convergently, the growth of molecular genetics has led to evolutionary developmental biology, which compares the developmental processes of different organisms to infer how developmental processes evolved. While it is now recognised that genotypes can slowly change by random genetic drift, natural selection remains the primary explanation for adaptive evolution.

Historical development

Main article: History of evolutionary thought

Pre-Darwinian theories

Aristotle considered whether different forms could have appeared, only the useful ones surviving.

Several philosophers of the classical era, including Empedocles and his intellectual successor, the Roman poet Lucretius, expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by Aristotle in Book II of Physics. He posited natural teleology in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof. Nevertheless, he accepted in his biology that new types of animals, monstrosities (τερας), can occur in very rare instances (Generation of Animals, Book IV). As quoted in Darwin's 1872 edition of The Origin of Species, Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived:

So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, and whatsoever things were not thus constituted, perished, and still perish.

— Aristotle, Physics, Book II, Chapter 8

But Aristotle rejected this possibility in the next paragraph, making clear that he is talking about the development of animals as embryos with the phrase "either invariably or normally come about", not the origin of species:

... Yet it is impossible that this should be the true view. For teeth and all other natural things either invariably or normally come about in a given way; but of not one of the results of chance or spontaneity is this true. We do not ascribe to chance or mere coincidence the frequency of rain in winter, but frequent rain in summer we do; nor heat in the dog-days, but only if we have it in winter. If then, it is agreed that things are either the result of coincidence or for an end, and these cannot be the result of coincidence or spontaneity, it follows that they must be for an end; and that such things are all due to nature even the champions of the theory which is before us would agree. Therefore action for an end is present in things which come to be and are by nature.

— Aristotle, Physics, Book II, Chapter 8

The struggle for existence was later described by the Islamic writer Al-Jahiz in the 9th century, particularly in the context of top-down population regulation, but not in reference to individual variation or natural selection.

At the turn of the 16th century Leonardo da Vinci collected a set of fossils of ammonites as well as other biological material. He extensively reasoned in his writings that the shapes of animals are not given once and forever by the "upper power" but instead are generated in different forms naturally and then selected for reproduction by their compatibility with the environment.

The more recent classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis and others, including Darwin's grandfather, Erasmus Darwin.

Until the early 19th century, the prevailing view in Western societies was that differences between individuals of a species were uninteresting departures from their Platonic ideals (or typus) of created kinds. However, the theory of uniformitarianism in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. The success of this theory raised awareness of the vast scale of geological time and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species.

The early 19th-century zoologist Jean-Baptiste Lamarck suggested the inheritance of acquired characteristics as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causing transmutation of species. This theory, Lamarckism, was an influence on the Soviet biologist Trofim Lysenko's ill-fated antagonism to mainstream genetic theory as late as the mid-20th century.

Between 1835 and 1837, the zoologist Edward Blyth worked on the area of variation, artificial selection, and how a similar process occurs in nature. Darwin acknowledged Blyth's ideas in the first chapter on variation of On the Origin of Species.

Darwin's theory

Main articles: Inception of Darwin's theory and Development of Darwin's theory

Further information: Coloration evidence for natural selection

Modern biology began in the nineteenth century with Charles Darwin's work on evolution by natural selection.

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved". The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them and that variation is heritable, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to the evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.

Part of Thomas Malthus's table of population growth in England 1780–1810, from his Essay on the Principle of Population, 6th edition, 1826

Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS Beagle (1831–1836), and by the work of a political economist, Thomas Robert Malthus, who, in An Essay on the Principle of Population (1798), noted that population (if unchecked) increases exponentially, whereas the food supply grows only arithmetically; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence". When Darwin read Malthus in 1838 he was already primed by his work as a naturalist to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species." Darwin wrote:

If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.

— Darwin summarising natural selection in the fourth chapter of On the Origin of Species

Once he had this hypothesis, Darwin was meticulous about gathering and refining evidence of consilience to meet standards of methodology before making his scientific theory public. He was in the process of writing his "big book" to present his research when the naturalist Alfred Russel Wallace independently conceived of the principle and described it in an essay he sent to Darwin to forward to Charles Lyell. Lyell and Joseph Dalton Hooker decided to present his essay together with unpublished writings that Darwin had sent to fellow naturalists, and On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection was read to the Linnean Society of London announcing co-discovery of the principle in July 1858. Darwin published a detailed account of his evidence and conclusions in On the Origin of Species in 1859. In later editions Darwin acknowledged that earlier writers—like William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar basic ideas. However, they had not developed their ideas, or presented evidence to persuade others that the concept was useful.

Charles Darwin noted that pigeon fanciers had created many kinds of pigeon, such as Tumblers (1, 12), Fantails (13), and Pouters (14) by selective breeding.

Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "artificial selection"; in his early manuscripts he referred to a "Nature" which would do the selection. At the time, mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, but, even in 1859, Darwin clearly stated that selection was only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification". The final edition of The Origin of Species documented several other contributors to evolutionary modification: sexual selection; the inherited effects of the use and disuse of parts (see Baldwin effect); "the direct action of external conditions" (a process which has been revived in some 21st century evolutionary biologies); and "variations which seem to us in our ignorance to arise spontaneously" (see mutation). In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural Preservation".

For Darwin and his contemporaries, evolution was in essence synonymous with evolution by natural selection. After the publication of On the Origin of Species, educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a law or mechanism, partly because it was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its "unguided" and non-progressive nature, a response that has been characterised as the single most significant impediment to the idea's acceptance. However, some thinkers enthusiastically embraced natural selection; after reading Darwin, Herbert Spencer introduced the phrase survival of the fittest, which became a popular summary of the theory. The fifth edition of On the Origin of Species published in 1869 included Spencer's phrase as an alternative to natural selection, with credit given: "But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient." Although the phrase is still often used by non-biologists, modern biologists avoid it because it is tautological if "fittest" is read to mean "functionally superior" and is applied to individuals rather than considered as an averaged quantity over populations.

The modern synthesis

Main article: Modern synthesis (20th century)

Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of genetics were invented. Although the Moravian monk Gregor Mendel, the father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900. With the early 20th-century integration of evolution with Mendel's laws of inheritance, the so-called modern synthesis, scientists generally came to accept natural selection. The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wrote The Genetical Theory of Natural Selection (1930). J. B. S. Haldane introduced the concept of the "cost" of natural selection. Sewall Wright elucidated the nature of selection and adaptation. In his book Genetics and the Origin of Species (1937), Theodosius Dobzhansky established the idea that mutation, once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity.

A second synthesis

Evolutionary developmental biology relates the evolution of form to the precise pattern of gene activity, here gap genes in the fruit fly, during embryonic development.

Main article: Evolutionary developmental biology § History

Ernst Mayr recognised the key importance of reproductive isolation for speciation in his Systematics and the Origin of Species (1942). W. D. Hamilton conceived of kin selection in 1964. This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. A second synthesis was brought about at the end of the 20th century by advances in molecular genetics, creating the field of evolutionary developmental biology ("evo-devo"), which seeks to explain the evolution of form in terms of the genetic regulatory programs which control the development of the embryo at molecular level. Natural selection is here understood to act on embryonic development to change the morphology of the adult body.

21st century developments

Main article: Extended evolutionary synthesis

Darwin's argument in On the Origin of Species portrayed natural selection as a law which resulted from other processes: inheritance (including both the transmission and development of heritable material); what we now call 'phenotypic' variation; and the metaphorical struggle for existence among living organisms. The 20th century's dominant theories of evolutionary biology treated natural selection differently, as if it were itself a causal mechanism, the agency of which was attributed either to the machinations of selfish genes or to 'the environment'. Which meant that living organisms themselves dropped out of scientists' theoretical picture. Under the pressure of evidence, 21st century evolutionary biology has seen growing criticism of the 20th century's gene-centred view of evolution. In consequence we now have an array of extended evolutionary syntheses which have returned the agency of living organisms to the heart of the theory of natural selection.

Terminology

The term natural selection is most often defined as the differential survival and reproduction of different phenotypic variations, where these are supported by heritable traits. It is sometimes helpful to distinguish between the processes or mechanisms which result in selection and selection's effects. Traits that endow greater reproductive success on an organism are said to be selected for, while those that reduce success are selected against.

Mechanism

Heritable variation, differential reproduction

During the Industrial Revolution, pollution killed many lichens, leaving tree trunks dark. A dark (melanic) morph of the peppered moth largely replaced the formerly usual light morph (both shown here). Since the moths are subject to predation by birds hunting by sight, the colour change offers better camouflage against the changed background, suggesting natural selection at work.

Main article: Phenotype

Natural or phenotypic variation occurs among the individuals of any population of organisms. Some variations may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring. If the variations that give these individuals a reproductive advantage are also supported by heritable traits which are passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of flying squirrels, fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way the natural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described. This gives the appearance of purpose, but in natural selection there is no intentional choice. Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it.

The peppered moth exists in both light and dark colours in Great Britain, but during the Industrial Revolution, many of the trees on which the moths rested became blackened by soot, giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial Manchester were dark. The balance was reversed by the effect of the Clean Air Act 1956, and the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution. A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies the camouflage of moths to predation risk. Modern genetic studies show that the switch from light to dark coloration is due to a transposable element insertion into the first intron of the gene cortex.

An example of natural selection in the wild involving a much larger number of genes is given by ash trees in Britain, under selection by an invasive fungus causing ash dieback. This fungus has killed large numbers of ash trees in Europe, and damaged many others, though some trees remain healthy. The genetic basis of health under ash dieback pressure has been shown to be quantitative and highly polygenic. Using genomic prediction models trained on planted trials, geneticists have shown that natural selection is acting on a woodland in Surrey England, causing the new generation of ash trees to be, on average, more genetically resistant to ash dieback than their parents generation. This is due to selection for beneficial gene combinations from among the variation present in the parents.

Fitness

Main article: Fitness (biology)

The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection". Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman. An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment. In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. Richard Lenski's classic E. coli long-term evolution experiment is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest"). The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.

Competition

Main article: Competition (biology)

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on a limited supply of a resource such as food, water, or territory. Competition may be within or between species, and may be direct or indirect. Species less suited to compete should in theory either adapt or die out, since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among larger clades.

Competition is modelled by r/K selection theory, which is based on Robert MacArthur and E. O. Wilson's work on island biogeography. In this theory, selective pressures drive evolution in one of two stereotyped directions: r- or K-selection. These terms, r and K, can be illustrated in a logistic model of population dynamics:

$${\displaystyle \frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)}$$ where r is the growth rate of the population (N), and K is the carrying capacity of its local environmental setting. Typically, r-selected species exploit empty niches, and produce many offspring, each with a relatively low probability of surviving to adulthood. In contrast, K-selected species are strong competitors in crowded niches, and invest more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood.

Social species

Main article: Cooperation (evolution)

Foreshadowing a central theme in 21st century evolutionary biology, Darwin argued that natural selection operated differently in social than in non-social species. The members of social species aided their conspecifics to survive, either passively (as in social plants) or both passively and actively, as in social animals. Darwin called plants like grasses and thistles social, because, in a "somewhat strained sense", they help each other by increasing their mutual chances of cross-fertilization (and hence vigour), and by reducing the depredations of their "devourers" (e.g. birds eating their seeds). This meant that often, if social plants "did not live in numbers, they could not live at all."

When it came to animals, Darwin said a truly social animal sought society beyond its own family. Unlike marmosets and tamarins, gorillas, lions, and tigers were not social in Darwin's sense, because, while they "no doubt" felt sympathy for the suffering of their young, they did not sympathize with "any other animal" beyond their own family.

In addition to the passive kinds of mutual aid that advantaged social plants, social animals could gain additional benefits through efficiencies due to divisions of labour like those found in social insects. Beyond this, some social species of bird and mammal actively signaled danger to other members of their community, some even posting sentinels to warn the group of approaching enemies. Thus rabbits stamp their hind-feet, and female seals act as look-outs. Social creatures may also actively groom each other, removing parasites, or licking each other’s wounds. Animals like wolves, killer whales, and pelicans hunt in concert, sometimes with a combined strategy. Social animals mutually defend each other too, and thereby show their "heroism." All these advantages mean that, in social animals, unlike non-social species, natural selection "will adapt the structure of each individual for the benefit of the whole community; if the community profits by the selected change." In The Descent of Man, Darwin attributes the evolution of all the most human of human characteristics—rationality, intellect, language, conscience, moral qualities, and culture—to the fact that our pre-human ancestors were group-living social animals par excellence.

Although the gene-centred view of evolution promulgated by the 20th century's modern synthesis in evolutionary biology denied the possibility of community or group selection of the kind proposed by Darwin, 21st century evolutionists are less dismissive.

Classification

1: directional selection: a single extreme phenotype favoured.
2, stabilizing selection: intermediate favoured over extremes.
3: disruptive selection: extremes favoured over intermediate.
X-axis: phenotypic trait
Y-axis: number of organisms
Group A: original population
Group B: after selection

Natural selection can act on any heritable phenotypic trait, and selective pressure can be altered by any aspect of the environment, including sexual selection and competition or cooperation with members of the same or other species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.

Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.

By effect on a trait

Selection has different effects on phenotypic traits. Stabilizing selection acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous. Directional selection favours extreme values of a trait. The uncommon disruptive selection also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and univariate then both higher and lower trait levels are favoured. Disruptive selection can be a precursor to speciation.

By effect on genetic diversity

Alternatively, selection can be divided according to its effect on genetic diversity. Purifying or negative selection acts to remove genetic variation from the population (and is opposed by de novo mutation, which introduces new variation. In contrast, balancing selection acts to maintain genetic variation in a population, even in the absence of de novo mutation, by negative frequency-dependent selection. One mechanism for this is heterozygote advantage, where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human ABO blood group locus has been explained in this way.

Different types of selection act at each life cycle stage of a sexually reproducing organism.

By life cycle stage

Another option is to classify selection by the life cycle stage at which it acts. Some biologists recognise just two types: viability (or survival) selection, which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase the rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on gamete survival, and compatibility selection, acting on zygote formation.

By unit of selection

Selection can also be classified by the level or unit of selection. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. Gene selection acts directly at the level of the gene. In kin selection and intragenomic conflict, gene-level selection provides a more apt explanation of the underlying process. Group selection, if it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.

By resource being competed for

The peacock's elaborate plumage is mentioned by Darwin as an example of sexual selection, and is a classic example of Fisherian runaway, driven to its conspicuous size and coloration through mate choice by females over many generations.

Further information: Sexual selection

Finally, selection can be classified according to the resource being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. Ecological selection is natural selection via any means other than sexual selection, such as kin selection, competition, and infanticide. Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism.

Sexual selection as first articulated by Darwin (using the example of the peacock's tail) refers specifically to competition for mates, which can be intrasexual, between individuals of the same sex, that is male–male competition, or intersexual, where one gender chooses mates, most often with males displaying and females choosing. However, in some species, mate choice is primarily by males, as in some fishes of the family Syngnathidae.

Phenotypic traits can be displayed in one sex and desired in the other sex, causing a positive feedback loop called a Fisherian runaway, for example, the extravagant plumage of some male birds such as the peacock. An alternate theory proposed by the same Ronald Fisher in 1930 is the sexy son hypothesis, that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of stags, which are used in combat with other stags. More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.

Arms races

Selection in action: resistance to antibiotics grows through the survival of individuals less affected by the antibiotic. Their offspring inherit the resistance.

Further information: Antimicrobial resistance

Natural selection is seen in action in the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928, antibiotics have been used to fight bacterial diseases. The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to the point that the methicillin-resistant Staphylococcus aureus (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs. Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. This is an evolutionary arms race, in which bacteria develop strains less susceptible to antibiotics, while medical researchers attempt to develop new antibiotics that can kill them. A similar situation occurs with pesticide resistance in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the spread of a gene in the butterfly Hypolimnas bolina suppressing male-killing activity by Wolbachia bacteria parasites on the island of Samoa, where the spread of the gene is known to have occurred over a period of just five years.

Evolution by means of natural selection

Main articles: Evolution and Darwinism

Without phenotypic variation, there would be no evolution by natural selection. A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that affects phenotypic fitness differences. Genetic variation is the result of mutations, genetic recombinations and alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes). Any of these changes might have an effect that is highly advantageous or highly disadvantageous for phenotypic variations, but large effects on phenotypes are rare.

In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in noncoding DNA or resulted in a synonymous substitution. However, many mutations in non-coding DNA have deleterious effects. Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious.

Some mutations occur in "toolkit" or regulatory genes. Changes in these often have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable embryos. Some nonlethal regulatory mutations occur in HOX genes in humans, which can result in a cervical rib or polydactyly, an increase in the number of fingers or toes. When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in the population. Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in a vestigial manifestation of the trait, also called evolutionary baggage. In many circumstances, the apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known as preadaptation. A famous example of a vestigial structure, the eye of the blind mole-rat, is believed to retain function in photoperiod perception.

Speciation

Main article: Speciation

Speciation requires a degree of reproductive isolation—that is, a reduction in gene flow. However, it is intrinsic to the concept of a species that hybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur in allopatric speciation with geographically separated populations, which can diverge with different sets of mutations. E. B. Poulton realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene. Selection against the heterozygote would then directly create reproductive isolation, leading to the Bateson–Dobzhansky–Muller model, further elaborated by H. Allen Orr and Sergey Gavrilets. With reinforcement, however, natural selection can favour an increase in pre-zygotic isolation, influencing the process of speciation directly.

Genetic basis

Genotype and phenotype

Main article: Genotype–phenotype distinction

Natural selection results from the ways an organism's phenotypes, or observable characteristics, bear on its capacity to reproduce. Phenotypes are plastic which means they are less directly determined by a given organism's genetic make-up (genotype) than by the way that particular organism develops and behaves in the theatre of agency which constitutes its habitat or environment. When different organisms in a population possess different versions of a gene affecting a certain phenotypic trait, each of these versions is known as an allele. (An example is the ABO blood type antigens in humans, where three alleles govern the phenotype.) It is these genetic variations which affect fitness-relevant differences in phenotypic traits and so underpin the evolution of new adaptations and, ultimately, new species.

Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can support a continuum of possible phenotypic values.

Directionality of selection

Main article: Directional selection

When some component of a phenotypic trait is heritable, selection alters the frequencies of the different alleles, or variants of the gene that affect the variants of the observed trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional, stabilizing, and disruptive selection. Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until the allele is fixed and the entire population shares the fitter phenotype. Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selection conserves functional genetic features, such as protein-coding genes or regulatory sequences, over time by selective pressure against deleterious variants. Disruptive (or diversifying) selection is selection favouring extreme trait values over intermediate trait values. Disruptive selection may cause sympatric speciation through niche partitioning.

Some forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in diploid species (with pairs of chromosomes) when heterozygous individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous for sickle-cell anaemia. Maintenance of allelic variation can also occur through disruptive or diversifying selection, which favours genotypes that depart from the average in either direction (that is, the opposite of over-dominance), and can result in a bimodal distribution of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of game theory have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution of reciprocal altruism.

Selection, genetic variation, and drift

Main articles: Genetic variation and Genetic drift

A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness. Motoo Kimura's neutral theory of molecular evolution by genetic drift proposes that this variation accounts for a large fraction of observed genetic diversity. Neutral events can radically reduce genetic variation through population bottlenecks, which among other things can cause the founder effect in initially small new populations. When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness. However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a mutation–selection balance. The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavourable the mutation proves to be.

Genetic linkage occurs when the loci of two alleles are close on a chromosome. During the formation of gametes, recombination reshuffles the alleles. The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them. Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, closely linked alleles can also become more common by "genetic hitchhiking", whether they are neutral or even slightly deleterious. A strong selective sweep results in a region of the genome where the positively selected haplotype (the allele and its neighbours) are in essence the only ones that exist in the population. Selective sweeps can be detected by measuring linkage disequilibrium, or whether a given haplotype is overrepresented in the population. Since a selective sweep also results in selection of neighbouring alleles, the presence of a block of strong linkage disequilibrium might indicate a 'recent' selective sweep near the centre of the block.

Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation tends to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it does not produce clear blocks of linkage disequilibrium, although with low recombination it can still lead to slightly negative linkage disequilibrium overall.

Impact

Main article: Universal Darwinism

Darwin's ideas, along with those of Adam Smith and Karl Marx, had a profound influence on 19th century thought, including his radical claim that "elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner" evolved from the simplest forms of life by a few simple principles. This inspired some of Darwin's most ardent supporters—and provoked the strongest opposition. Natural selection had the power, according to Stephen Jay Gould, to "dethrone some of the deepest and most traditional comforts of Western thought", such as the belief that humans have a special place in the world.

In the words of the philosopher Daniel Dennett, "Darwin's dangerous idea" of evolution by natural selection is a "universal acid," which cannot be kept restricted to any vessel or container, as it soon leaks out, working its way into ever-wider surroundings. Thus, in the last decades, the concept of natural selection has spread from evolutionary biology to other disciplines, including evolutionary computation, quantum Darwinism, evolutionary economics, evolutionary epistemology, evolutionary psychology, and cosmological natural selection. This unlimited applicability has been called universal Darwinism.

Origin of life

Main article: Abiogenesis

How life originated from inorganic matter remains an unresolved problem in biology. One prominent hypothesis is that life first appeared in the form of short self-replicating RNA polymers. On this view, life may have come into existence when RNA chains first experienced the basic conditions, as conceived by Charles Darwin, for natural selection to operate. These conditions are: heritability, variation of type, and competition for limited resources. The fitness of an early RNA replicator would likely have been a function of adaptive capacities that were intrinsic (i.e., determined by the nucleotide sequence) and the availability of resources. The three primary adaptive capacities could logically have been: (1) the capacity to replicate with moderate fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and process resources. These capacities would have been determined initially by the folded configurations (including those configurations with ribozyme activity) of the RNA replicators that, in turn, would have been encoded in their individual nucleotide sequences.

Cell and molecular biology

In 1881, the embryologist Wilhelm Roux published Der Kampf der Theile im Organismus (The Struggle of Parts in the Organism) in which he suggested that the development of an organism results from a Darwinian competition between the parts of the embryo, occurring at all levels, from molecules to organs. In recent years, a modern version of this theory has been proposed by Jean-Jacques Kupiec. According to this cellular Darwinism, random variation at the molecular level generates diversity in cell types whereas cell interactions impose a characteristic order on the developing embryo.

Social and psychological theory

Main article: Evolutionary psychology

The social implications of the theory of evolution by natural selection also became the source of continuing controversy. Friedrich Engels, a German political philosopher and co-originator of the ideology of communism, wrote in 1872 that "Darwin did not know what a bitter satire he wrote on mankind, and especially on his countrymen, when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the animal kingdom." Herbert Spencer and the eugenics advocate Francis Galton's interpretation of natural selection as necessarily progressive, leading to supposed advances in intelligence and civilisation, became a justification for colonialism, eugenics, and social Darwinism. For example, in 1940, Konrad Lorenz, in writings that he subsequently disowned, used the theory as a justification for policies of the Nazi state. He wrote "... selection for toughness, heroism, and social utility ... must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect." Others have developed ideas that human societies and culture evolve by mechanisms analogous to those that apply to evolution of species.

More recently, work among anthropologists and psychologists has led to the development of sociobiology and later of evolutionary psychology, a field that attempts to explain features of human psychology in terms of adaptation to the ancestral environment. The most prominent example of evolutionary psychology, notably advanced in the early work of Noam Chomsky and later by Steven Pinker, is the hypothesis that the human brain has adapted to acquire the grammatical rules of natural language. Other aspects of human behaviour and social structures, from specific cultural norms such as incest avoidance to broader patterns such as gender roles, have been hypothesised to have similar origins as adaptations to the early environment in which modern humans evolved. By analogy to the action of natural selection on genes, the concept of memes—"units of cultural transmission," or culture's equivalents of genes undergoing selection and recombination—has arisen, first described in this form by Richard Dawkins in 1976 and subsequently expanded upon by philosophers such as Daniel Dennett as explanations for complex cultural activities, including human consciousness.

Information and systems theory

In 1922, Alfred J. Lotka proposed that natural selection might be understood as a physical principle that could be described in terms of the use of energy by a system, a concept later developed by Howard T. Odum as the maximum power principle in thermodynamics, whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation.

The principles of natural selection have inspired a variety of computational techniques, such as "soft" artificial life, that simulate selective processes and can be highly efficient in 'adapting' entities to an environment defined by a specified fitness function. For example, a class of heuristic optimisation algorithms known as genetic algorithms, pioneered by John Henry Holland in the 1970s and expanded upon by David E. Goldberg, identify optimal solutions by simulated reproduction and mutation of a population of solutions defined by an initial probability distribution. Such algorithms are particularly useful when applied to problems whose energy landscape is very rough or has many local minima.

In fiction

Main article: Evolution in fiction

Darwinian evolution by natural selection is pervasive in literature, whether taken optimistically in terms of how humanity may evolve towards perfection, or pessimistically in terms of the dire consequences of the interaction of human nature and the struggle for survival. Among major responses is Samuel Butler's 1872 pessimistic Erewhon ("nowhere", written mostly backwards). In 1893 H. G. Wells imagined "The Man of the Year Million", transformed by natural selection into a being with a huge head and eyes, and shrunken body.