Social effects of evolutionary theory

From Wikipedia, the free encyclopedia

The social effects of evolutionary thought have been considerable. As the scientific explanation of life's diversity has developed, it has often displaced alternative, sometimes very widely held, explanations. Because the theory of evolution includes an explanation of humanity's origins, it has had a profound impact on human societies. Some have vigorously denied acceptance of the scientific explanation due to its perceived religious implications (e.g. its implied rejection of the special creation of humans presumably described in the Bible). This has led to a vigorous conflict between creation and evolution in public education, primarily in the United States.

Evolution and ethics

The theory of evolution by natural selection has also been adopted as a foundation for various ethical and social systems, such as social Darwinism, an idea that preceded the publication of The Origin of Species, popular in the 19th century, which holds that "the survival of the fittest" (a phrase coined in 1851 by Herbert Spencer, 6 years before Darwin published his theory of evolution) explains and justifies differences in wealth and success among societies and people. A similar interpretation was one created by Darwin's cousin, Francis Galton, known as eugenics, which claimed that human civilization was subverting natural selection by allowing the less bright and less healthy to survive and out-breed the more smart and more healthy.

Later advocates of this theory suggested radical and often coercive social measures in an attempt to "correct" this imbalance. Thomas Huxley spent much time demonstrating through a series of thought experiments that it would not only be immoral, but impossible, Stephen Jay Gould and others have argued that social Darwinism is based on misconceptions of evolutionary theory, and many ethicists regard it as a case of the is-ought problem. After the atrocities of the Holocaust became linked with eugenics, it greatly fell out of favor with public and scientific opinion, though it was never universally accepted by either, and at no point in Nazi literature is Charles Darwin or the scientific theory of evolution mentioned.

In his book The End of Faith, Sam Harris argues that Nazism was largely a continuation of Christian anti-Semitism. Jim Walker compiled a list of 129 quotes from Mein Kampf in which Hitler described himself as a Christian, or mentioned God, Jesus or a biblical passage. Some argue that six million of the people killed during the Holocaust were killed because of their religion (Judaism) not their race, "strength," or any reason with an obvious link to the mechanism of Darwinian evolution. Hitler often used Christian beliefs like, "Jews killed Jesus," to justify his anti-Semitism.

The notion that humans share ancestors with other animals has also affected how some people view the relationship between humans and other species. Many proponents of animal rights hold that if animals and humans are of the same nature, then rights cannot be distinct to humans.

Charles Darwin, in fact, considered "sympathy" to be one of the most important moral virtues — and that it was, indeed, a product of natural selection and a trait beneficial to social animals (including humans). Darwin further argued that the most "sympathetic" societies would consequently be the most "successful." He also stated that our sympathy should be extended to "all sentient beings":

As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all the members of the same nation, though personally unknown to him. This point being once reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races. If, indeed, such men are separated from him by great differences in appearance or habits, experience unfortunately shows us how long it is, before we look at them as our fellow-creatures. ... This virtue, one of the noblest with which man is endowed, seems to arise incidentally from our sympathies becoming more tender and more widely diffused, until they are extended to all sentient beings. As soon as this virtue is honored and practiced by some few men, it spreads through instruction and example to the young, and eventually becomes incorporated in public opinion.

— Charles Darwin; The Descent of Man, 1871

Thomas Huxley: Evolution and Ethics

Thomas Huxley, Darwin's Bulldog, spent much of his book Evolution and Ethics debunking Social Darwinism, piece by piece. The following is a summary of his arguments in the Prolegomena, the most detailed and comprehensive of the two sections devoted to it. It should be noted that Huxley is here attempting to disprove the science behind Social Darwinism; as such, the moral arguments only come in later in the essay.

Consider a garden. Without constant upkeep, it would return to the state of nature, even the very walls surrounding it crumbling in sufficient time, but by constant diligence of the gardener, may be maintained in a state of art. This "state of art" is not permanent: It is instead the replacement of natural selection by artificial selection through the human energy expended in maintaining it.

This artificial selection is, however, part of natural selection: It is the action upon a set of species by the human species by way of the human species expending energy through evolved intelligence on its choice of selection. It is thus no less natural than, for example, a predator expending energy through evolved instinct on preferentially hunting a certain prey species. The presence of humans may change the dynamic, but in a perfectly natural way. Hence, it is part of the cosmic process, that is natural laws, even though the "histological process" may remove many aspects of the "struggle for existence" that is a key part of the natural laws that apply to biology, from its preferred plant species by substituting human work for work done by the species itself.

Not only is the state of nature hostile to the state of art of the garden; but the principle of the horticultural process, by which the latter is created and maintained, is antithetic to that of the cosmic process. The characteristic feature of the latter is the intense and unceasing competition of the struggle for existence. The characteristic of the former is the elimination of that struggle, by the removal of the conditions which give rise to it. The tendency of the cosmic process is to bring about the adjustment of the forms of plant life to the current conditions; the tendency of the horticultural process is the adjustment of the conditions to the needs of the forms of plant life which the gardener desires to raise.

Nature uses unrestricted breeding to let hundreds compete for the natural resources that would only support one, and uses frost and drought to kill off the weak and unlucky, requiring not just strength, but "flexibility and good fortune." However, a gardener restricts multiplication, gives each plant sufficient space and nourishment, protects from frost and drought—and, in every other way, attempts to modify the conditions to benefit the forms that most nearly approach the result he desires. However, though the gardener's actions may have circumvented natural selection, he can still improve the species, should he find them wanting, through selective breeding. The struggle for existence is not actually required for improvement: only heritability, variation, and some form of selective pressure.

Can we then apply this to humans? Let's see how far we can take the analogy with respect to colonization:

Suppose a shipload of English colonists sent to form a settlement, in such a country as Tasmania was in the middle of the last century. On landing, they find themselves in the midst of a state of nature, widely different from that left behind them in everything but the most general physical conditions. The common plants, the common birds and quadrupeds, are as totally distinct as the men from anything to be seen on the side of the globe from which they come. The colonists proceed to put an end to this state of things over as large an area as they desire to occupy. They clear away the native vegetation, extirpate or drive out the animal population, so far as may be necessary, and take measures to defend themselves from the re-immigration of either. In their place, they introduce English grain and fruit trees; English dogs, sheep, cattle, horses; and English men; in fact, they set up a new Flora and Fauna and a new variety of mankind, within the old state of nature. Their farms and pastures represent a garden on a great scale, and themselves the gardeners who have to keep it up, in watchful antagonism to the old regime. Considered as a whole, the colony is a composite unit introduced into the old state of nature; and, thenceforward, a competitor in the struggle for existence, to conquer or be vanquished.

Under the conditions supposed, there is no doubt of the result, if the work of the colonists be carried out energetically and with intelligent combination of all their forces. On the other hand, if they are slothful, stupid, and careless; or if they waste their energies in contests with one another, the chances are that the old state of nature will have the best of it. The native savage will destroy the immigrant civilized man; of the English animals and plants some will be extirpated by their indigenous rivals, others will pass into the feral state and themselves become components of the state of nature. In a few decades, all other traces of the settlement will have vanished.

However, as yet we lack an organized gardener. Let us imagine an idealized one: an administrative authority of intelligence and foresight as much greater than men as men are to their livestock. The unwanted native species - men, animals, or plants - are all weeded out and destroyed. Those to replace them are chosen with a view to his ideal of the colony, just as a gardener tries to create through his selection his ideal garden. And, finally, to ensure that no struggle for existence between the colonists interferes with the struggle against nature, he provides them with sufficient food, housing, and so on. "With every step of this progress in civilization, the colonists would become more and more independent of the state of nature; more and more, their lives would be conditioned by a state of art. In order to attain his ends, the administrator would have to avail himself of the courage, industry, and co-operative intelligence of the settlers; and it is plain that the interest of the community would be best served by increasing the proportion of persons who possess such qualities, and diminishing that of persons devoid of them. In other words, by selection directed towards an ideal."

However, though this might create a paradise where every aspect of nature works to support its colonists, problems arise: "as soon as the colonists began to multiply, the administrator would have to face the tendency to the reintroduction of the cosmic struggle into his artificial fabric, in consequence of the competition, not merely for the commodities, but for the means of existence. When the colony reached the limit of possible expansion, the surplus population must be disposed of somehow; or the fierce struggle for existence must recommence and destroy that peace, which is the fundamental condition of the maintenance of the state of art against the state of nature.

If the administrator is guided purely by scientific considerations, he would work to restrict the population by removing "the hopelessly diseased, the infirm aged, the weak or deformed in body or in mind, and the excess of infants born," just as a "gardener pulls up defective and superfluous plants, or the breeder destroys undesirable cattle. Only the strong and the healthy, carefully matched, with a view to the progeny best adapted to the purposes of the administrator, would be permitted to perpetuate their kind."

And so we have reached Social Darwinism. However, we do not have an idealized administrator:

Of the more thoroughgoing of the multitudinous attempts to apply the principles of cosmic evolution, or what are supposed to be such, to social and political problems, which have appeared of late years, a considerable proportion appear to me to be based upon the notion that human society is competent to furnish, from its own resources, an administrator of the kind I have imagined. The pigeons, in short, are to be their own Sir John Sebright. A despotic government, whether individual or collective, is to be endowed with the preternatural intelligence, and with what, I am afraid, many will consider the preternatural ruthlessness, required for the purpose of carrying out the principle of improvement by selection, with the somewhat drastic thoroughness upon which the success of the method depends. Experience certainly does not justify us in limiting the ruthlessness of individual "saviors of society"; and, on the well-known grounds of the aphorism which denies both body and soul to corporations, it seems probable (indeed the belief is not without support in history) that a collective despotism, a mob got to believe in its own divine right by demagogic missionaries, would be capable of more thorough work in this direction than any single tyrant, puffed up with the same illusion, has ever achieved. But intelligence is another affair. The fact that "saviors of society" take to that trade is evidence enough that they have none to spare. And such as they possess is generally sold to the capitalists of physical force on whose resources they depend. However, I doubt whether even the keenest judge of character, if he had before him a hundred boys and girls under fourteen, could pick out, with the least chance of success, those who should be kept, as certain to be serviceable members of the polity, and those who should be chloroformed, as equally sure to be stupid, idle, or vicious. The "points" of a good or of a bad citizen are really far harder to discern than those of a puppy or a short-horn calf; many do not show themselves before the practical difficulties of life stimulate manhood to full exertion. And by that time the mischief is done. The evil stock, if it be one, has had time to multiply, and selection is nullified.

However, humans are not cattle, nor flowers: the organization of human society is kept together by

...bonds of such a singular character, that the attempt to perfect society after his fashion would run serious risk of loosening them. They do not even correspond to social insects such as bees: With bees, "The members of the society are each organically predestined to the performance of one particular class of functions only. If they were endowed with desires, each could desire to perform none but those offices for which its organization specially fits it; and which, in view of the good of the whole, it is proper it should do. Among mankind, on the contrary, there is no such predestination to a sharply defined place in the social organism. However much men may differ in the quality of their intellects, the intensity of their passions, and the delicacy of their sensations, it cannot be said that one is fitted by his organization to be an agricultural laborer and nothing else, and another to be a landowner and nothing else. Moreover, with all their enormous differences in natural endowment, men agree in one thing, and that is their innate desire to enjoy the pleasures and to escape the pains of life; and, in short, to do nothing but that which it pleases them to do, without the least reference to the welfare of the society into which they are born, checked only by sympathy, familial and social bonds, and fear of the judgment of ones fellow man. "Every forward step of social progress brings men into closer relations with their fellows, and increases the importance of the pleasures and pains derived from sympathy.

In short, he describes a creation of morality.

Since morality is what keeps the desire for selfishness in check, it is necessary to the propagation of society, with one requirement: the punishment of wrongdoers being necessary for the continuation of society, self-restraint must not be taken so far that wrongdoers may act unrestrained: Without the protection of society against them, "The followers of the "golden rule" may indulge in hopes of heaven, but they must reckon with the certainty that other people will be masters of the earth."

Huxley sums up this section of his argument against Social Darwinism:

I have further shown cause for the belief that direct selection, after the fashion of the horticulturist and the breeder, neither has played, nor can play, any important part in the evolution of society; apart from other reasons, because I do not see how such selection could be practiced without a serious weakening, it may be the destruction, of the bonds which hold society together. It strikes me that men who are accustomed to contemplate the active or passive extirpation of the weak, the unfortunate, and the superfluous; who justify that conduct on the ground that it has the sanction of the cosmic process, and is the only way of ensuring the progress of the race; who, if they are consistent, must rank medicine among the black arts and count the physician a mischievous preserver of the unfit; on whose matrimonial undertakings the principles of the stud have the chief influence; whose whole lives, therefore, are an education in the noble art of suppressing natural affection and sympathy, are not likely to have any large stock of these commodities left. But, without them, there is no conscience, nor any restraint on the conduct of men, except the calculation of self-interest, the balancing of certain present gratifications against doubtful future pains; and experience tells us how much that is worth. Every day, we see firm believers in the hell of the theologians commit acts by which, as they believe when cool, they risk eternal punishment; while they hold back from those which are opposed to the sympathies of their associates.

Huxley finishes with a series of short, further evidences against Social Darwinism, including:

• Historical evidences against: Consider the vast changes of society between the Tudor and the Victorian eras; however, human nature, as evidenced by their writing, remains the same. "In my belief, the innate qualities, physical, intellectual, and moral, of our nation have remained substantially the same for the last four or five centuries. If the struggle for existence has affected us to any serious extent (and I doubt it) it has been, indirectly, through our military and industrial wars with other nations."
• Whether some qualities are virtues or vices depends on circumstance: "The benevolence and open-handed generosity which adorn a rich man, may make a pauper of a poor one; the energy and courage to which the successful soldier owes his rise, the cool and daring subtlety to which the great financier owes his fortune, may very easily, under unfavorable conditions, lead their possessors to the gallows, or to the hulks. Moreover, it is fairly probable that the children of a "failure" will receive from their other parent just that little modification of character which makes all the difference. I sometimes wonder whether people, who talk so freely about extirpating the unfit, ever dispassionately consider their own history. Surely, one must be very "fit," indeed, not to know of an occasion, or perhaps two, in one's life, when it would have been only too easy to qualify for a place among the "unfit.""
• Evolution and Ethics at Project Gutenberg

Evolution and religion

Before Darwin's argument and presentation of the evidence for evolution, Western religions generally discounted or condemned any claims that diversity of life is the result of an evolutionary process, as did most scientists in the English scientific establishment. However, evolution was accepted by some religious groups such as the Unitarian church and the liberal Anglican theologians who went on to publish Essays and Reviews. as well as by many scientists in France and Scotland and some in England, notably Robert Edmund Grant. Literal or authoritative interpretations of Scripture hold that a supreme being directly created humans and other animals as separate Created kinds, which to some means species. This view is commonly referred to as creationism. From the 1920s to the present in the US, there has been a strong religious backlash to the teaching of evolution theory, particularly by conservative evangelicals. They have expressed concerns about the effects of the teaching of evolution on society and their faith (see Creation-evolution controversy).

In response to the wide scientific acceptance of the theory of evolution, many religions have formally or informally synthesized the scientific and religious viewpoints. Several important 20th century scientists (Fisher, Dobzhansky) whose work confirmed Darwin's theory, were also Christians who saw no incompatibility between their experimental and theoretical confirmations of evolution and their faith. Some religions have adopted a theistic evolution viewpoint, where God provides a divine spark that ignited the process of evolution and (or), where God has guided evolution in one way or another.

Evolution and the Roman Catholic Church

The Roman Catholic Church, beginning in 1950 with Pope Pius XII's encyclical Humani Generis, took up a neutral position with regard to evolution. "The Church does not forbid that...research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter."

In an October 22, 1996, address to the Pontifical Academy of Science, Pope John Paul II updated the Church's position, recognizing that Evolution is "more than a hypothesis" - "In his encyclical Humani Generis, my predecessor Pius XII has already affirmed that there is no conflict between evolution and the doctrine of the faith regarding man and his vocation... Today, more than a half-century after the appearance of that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis. In fact it is remarkable that this theory has had progressively greater influence on the spirit of researchers, following a series of discoveries in different scholarly disciplines."

Islamic views on Evolution

Classical figures have not discussed the subject as it has only come up in the 19th century. Contemporaries have come up with several distinct stances. One stance is that adaptation, or evolution on a micro scale, is accepted within a species, but cross-species evolution, that is evolution from one species into another species, is not as the human beginning is considered to be miraculous. However, this traditional thought would not conflict with the view that human-like beings could have been created around the same time as human beings, which, in this view, would explain the fossil records that look human but are not. Another stance is that since evolution is the simplest explanation it is the most reasonable to accept under the condition that it is not random but occurs only with the permission of God every step of the way. One particular argument that supports the idea that evolution is possible is the one stating that in that the stages of human development in evolution are akin to the distinct stages of development acknowledged in the Koran. The final stance completely rejects cross-species evolution across all organisms, but approves of adaptation (micro evolution).

Evolutionary theory and the political left

Many important political figures on the left have never publicized their views on biology, and so their opinions of evolutionary theory are unknown. To some extent, Marxists are the exception. Karl Marx, Friedrich Engels and Vladimir Lenin supported Darwin's evolutionary theory. Marx even sent Darwin a copy of his book Das Kapital, though Darwin never wrote back to him. Karl Marx's work was based on a material view of the world that showed natural causes and effects for all aspects of human society and economy. He recognized that Darwin's work provided a similar material explanation for all of nature, thus supporting Marx's worldview.

In 1861 Karl Marx wrote to his friend Ferdinand Lassalle, "Darwin’s work is most important and suits my purpose in that it provides a basis in natural science for the historical class struggle. ... Despite all shortcomings, it is here that, for the first time, 'teleology' in natural science is not only dealt a mortal blow but its rational meaning is empirically explained."

Most later Marxists agreed with this view, but some – particularly those in the early Soviet Union – believed that evolutionary theory conflicted with their economic and social ideals. As a result, they came to support Lamarckism instead – the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring. This led to the practice of Lysenkoism, which caused agricultural problems.

In his book, Mutual Aid: A Factor of Evolution, anarcho-communist Peter Kropotkin argued that co-operation and mutual aid are as important in the evolution of the species as competition and mutual strife, if not more so.

On the contemporary moderate left, some authors such as Peter Singer (in his book, A Darwinian Left) support Darwinism but reach different political and economic lessons than more conservative observers. Richard Dawkins' book, The Selfish Gene, has a chapter, "Nice guys finish first," that attempts to explain the role of altruism and cooperation in evolution and how social animals not only cannot survive without such traits, but how evolution will create them. Dawkins explains that when an animal sacrifices itself or uses its resources for the survival of other members of the same species, its genes, present on the other animals, survive. For example, if a mother dies to save three of its pups, one and a half copies (on average) of its genes will survive, because there is a 50% chance of a particular gene being present in its offspring. Dawkins also made a documentary of the same name. According to the documentary, Dawkins added that chapter as a way of overcoming modern day misinterpretations of the concept of survival of the fittest.

Evolution in relation to Social Darwinism and Imperialism

"Social Darwinism" is a derogatory term associated with the 19th century Malthusian theory developed by Whig philosopher Herbert Spencer. It is associated with evolutionary theory but now widely regarded as unwarranted. Social Darwinism was later expanded by others into ideas about "survival of the fittest" in commerce and human societies as a whole, and led to claims that social inequality, sexism, racism and imperialism were justified. However, these ideas contradict Darwin's own views, and contemporary scientists and philosophers consider these ideas to be neither mandated by evolutionary theory nor supported by data.

Social Darwinism is further linked with nationalism and imperialism. During the age of New Imperialism, the concepts of evolution justified the exploitation of "lesser breeds without the law" by "superior races." To elitists, strong nations were composed of white people who were successful at expanding their empires, and as such, these strong nations would survive in the struggle for dominance. With this attitude, Europeans, except for Christian missionaries, seldom adopted the customs and languages of local people under their empires. Christian missionaries, on the other hand, were the very first individuals to meet new peoples and develop writing systems for local inhabitants' languages that lacked one. Being critics of Darwinism, they ardently opposed slavery and provided an education and religious instruction to the new peoples they interacted with since they felt that this was their duty as Christians.

Adaptive radiation

From Wikipedia, the free encyclopedia

Four of the 14 finch species found in the Galápagos Archipelago, which are thought to have evolved via an adaptive radiation that diversified their beak shapes, enabling them to exploit different food sources.

In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits. The prototypical example of adaptive radiation is finch speciation on the Galapagos ("Darwin's finches"), but examples are known from around the world.

Characteristics

Four features can be used to identify an adaptive radiation:

1. A common ancestry of component species: specifically a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.
2. A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.
3. Trait utility: the performance or fitness advantages of trait values in their corresponding environments.
4. Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.

Conditions

Adaptive radiation tends to take place under the following conditions:

1. A new habitat has opened up: a volcano, for example, can create new ground in the middle of the ocean. This is the case in places like Hawaii and the Galapagos. For aquatic species, the formation of a large new lake habitat could serve the same purpose; the tectonic movement that formed the East African Rift, ultimately leading to the creation of the Rift Valley Lakes, is an example of this. An extinction event could effectively achieve this same result, opening up niches that were previously occupied by species that no longer exist.
2. This new habitat is relatively isolated. When a volcano erupts on the mainland and destroys an adjacent forest, it is likely that the terrestrial plant and animal species that used to live in the destroyed region will recolonize without evolving greatly. However, if a newly formed habitat is isolated, the species that colonize it will likely be somewhat random and uncommon arrivals.
3. The new habitat needs a wide availability of niche space. The rare colonist can only adaptively radiate into as many forms as there are niches.

Examples

Darwin's finches

Darwin's finches are an often-used textbook example of adaptive radiation. Today represented by approximately 15 species, Darwin's finches are Galapagos endemics famously adapted for a specialized feeding behavior (although one species, the Cocos finch (Pinaroloxias inornata), is not found in the Galapagos but on the island of Cocos south of Costa Rica). Darwin's finches are not actually finches in the true sense, but are members of the tanager family Thraupidae, and are derived from a single ancestor that arrived in the Galapagos from mainland South America perhaps just 3 million years ago. Excluding the Cocos finch, each species of Darwin's finch is generally widely distributed in the Galapagos and fills the same niche on each island. For the ground finches, this niche is a diet of seeds, and they have thick bills to facilitate the consumption of these hard materials. The ground finches are further specialized to eat seeds of a particular size: the large ground finch (Geospiza magnirostris) is the largest species of Darwin's finch and has the thickest beak for breaking open the toughest seeds, the small ground finch (Geospiza fuliginosa) has a smaller beak for eating smaller seeds, and the medium ground finch (Geospiza fortis) has a beak of intermediate size for optimal consumption of intermediately sized seeds (relative to G. magnirostris and G. fuliginosa). There is some overlap: for example, the most robust medium ground finches could have beaks larger than those of the smallest large ground finches. Because of this overlap, it can be difficult to tell the species apart by eye, though their songs differ. These three species often occur sympatrically, and during the rainy season in the Galapagos when food is plentiful, they specialize little and eat the same, easily accessible foods. It was not well-understood why their beaks were so adapted until Peter and Rosemary Grant studied their feeding behavior in the long dry season, and discovered that when food is scarce, the ground finches use their specialized beaks to eat the seeds that they are best suited to eat and thus avoid starvation.

The other finches in the Galapagos are similarly uniquely adapted for their particular niche. The cactus finches (Geospiza sp.) have somewhat longer beaks than the ground finches that serve the dual purpose of allowing them to feed on Opuntia cactus nectar and pollen while these plants are flowering, but on seeds during the rest of the year. The warbler-finches (Certhidea sp.) have short, pointed beaks for eating insects. The woodpecker finch (Camarhynchus pallidus) has a slender beak which it uses to pick at wood in search of insects; it also uses small sticks to reach insect prey inside the wood, making it one of the few animals that use tools.

The mechanism by which the finches initially diversified is still an area of active research. One proposition is that the finches were able to have a non-adaptive, allopatric speciation event on separate islands in the archipelago, such that when they reconverged on some islands they were able to maintain reproductive isolation. Once they occurred in sympatry, niche specialization was favored so that the different species competed less directly for resources. This second, sympatric event was adaptive radiation.

Cichlids of the African Great Lakes

The haplochromine cichlid fishes in the Great Lakes of the East African Rift (particularly in Lake Tanganyika, Lake Malawi, and Lake Victoria) form the most speciose modern example of adaptive radiation. These lakes are believed to be home to about 2,000 different species of cichlid, spanning a wide range of ecological roles and morphological characteristics. Cichlids in these lakes fill nearly all of the roles typically filled by a large number of fish families, including those of predators, scavengers, and herbivores, with varying dentitions and head shapes to match their dietary habits. In each case, the radiation events are only a few million years old, making the very high level of speciation particularly remarkable. Several factors could be responsible for this diversity: the availability of a multitude of niches probably favored specialization, as few other fish taxa are present in the lakes (meaning that sympatric speciation was the most probable mechanism for initial specialization). Also, continual changes in the water level of the lakes during the Pleistocene (which often turned the largest lakes into several smaller ones) could have created the conditions for secondary allopatric speciation.

Tanganyika cichlids

Lake Tanganyika is the site from which nearly all the cichlid lineages of East Africa (including both riverine and lake species) originated. Thus, the species in the lake constitute a single adaptive radiation event but do not form a single monophyletic clade. Lake Tanganyika is also the least speciose of the three largest African Great Lakes, with only around 200 species of cichlid; however, these cichlids are more morphologically divergent and ecologically distinct than their counterparts in lakes Malawi and Victoria, an artifact of Lake Tanganyika's older cichlid fauna. Lake Tanganyika itself is believed to have formed 9-12 million years ago, putting a recent cap on the age of the lake's cichlid fauna. Many of Tanganyika's cichlids live very specialized lifestyles. The giant or emperor cichlid (Boulengerochromis microlepis) is a piscivore often ranked the largest of all cichlids (though it competes for this title with South America's Cichla temensis, the speckled peacock bass). It is thought that giant cichlids spawn only a single time, breeding in their third year and defending their young until they reach a large size, before dying of starvation some time thereafter. The three species of Altolamprologus are also piscivores, but with laterally compressed bodies and thick scales enabling them to chase prey into thin cracks in rocks without damaging their skin. Plecodus straeleni has evolved large, strangely curved teeth that are designed to scrape scales off of the sides of other fish, scales being its main source of food. Gnathochromis permaxillaris possesses a large mouth with a protruding upper lip, and feeds by opening this mouth downward onto the sandy lake bottom, sucking in small invertebrates. A number of Tanganyika's cichlids are shell-brooders, meaning that mating pairs lay and fertilize their eggs inside of empty shells on the lake bottom. Lamprologus callipterus is the most unique egg-brooding species, with 15 cm-long males amassing collections of shells and guarding them in the hopes of attracting females (about 6 cm in length) to lay eggs in these shells. These dominant males must defend their territories from three types of rival: (1) other dominant males looking to steal shells; (2) younger, "sneaker" males looking to fertilize eggs in a dominant male's territory; and (3) tiny, 2–4 cm "parasitic dwarf" males that also attempt to rush in and fertilize eggs in the dominant male's territory. These parasitic dwarf males never grow to the size of dominant males, and the male offspring of dominant and parasitic dwarf males grow with 100% fidelity into the form of their fathers. A number of other highly specialized Tanganyika cichlids exist aside from these examples, including those adapted for life in open lake water up to 200m deep.

Malawi cichlids

The cichlids of Lake Malawi constitute a "species flock" of up to 1000 endemic species. Only seven cichlid species in Lake Malawi are not a part of the species flock: the Eastern happy (Astatotilapia calliptera), the sungwa (Serranochromis robustus), and five tilapia species (genera Oreochromis and Coptodon). All of the other cichlid species in the lake are descendants of a single original colonist species, which itself was descended from Tanganyikan ancestors. The common ancestor of Malawi's species flock is believed to have reached the lake 3.4 million years ago at the earliest, making Malawi cichlids' diversification into their present numbers particularly rapid. Malawi's cichlids span a similarly range of feeding behaviors to those of Tanganyika, but also show signs of a much more recent origin. For example, all members of the Malawi species flock are mouth-brooders, meaning the female keeps her eggs in her mouth until they hatch; in almost all species, the eggs are also fertilized in the female's mouth, and in a few species, the females continue to guard their fry in their mouth after they hatch. Males of most species display predominantly blue coloration when mating. However, a number of particularly divergent species are known from Malawi, including the piscivorous Nimbochromis livingtonii, which lies on its side in the substrate until small cichlids, perhaps drawn to its broken white patterning, come to inspect the predator - at which point they are swiftly eaten.

Victoria cichlids

Lake Victoria's cichlids are also a species flock, once composed of some 500 or more species. The deliberate introduction of the Nile Perch (Lates niloticus) in the 1950s proved disastrous for Victoria cichlids, and the collective biomass of the Victoria cichlid species flock has decreased substantially and an unknown number of species have become extinct. However, the original range of morphological and behavioral diversity seen in the lake's cichlid fauna is still mostly present today, if endangered. These again include cichlids specialized for niches across the trophic spectrum, as in Tanganyika and Malawi, but again, there are standouts. Victoria is famously home to a large number of piscivorous cichlid species, some of which feed by sucking the contents out of mouthbrooding females' mouths. Victoria's cichlids constitute a far younger radiation than even that of Lake Malawi, with estimates of the age of the flock ranging from 200,000 years to as little as 14,000.

Adaptive radiation in Hawaii

An ʻiʻiwi (Drepanis coccinea). Note the long, curved beak for sipping nectar from tubular flowers.

Hawaii has served as the site of a number of adaptive radiation events, owing to its isolation, recent origin, and large land area. The three most famous examples of these radiations are presented below, though insects like the Hawaiian drosophilid flies and Hyposmocoma moths have also undergone adaptive radiation.

Hawaiian honeycreepers

The Hawaiian honeycreepers form a large, highly morphologically diverse species group that began radiating in the early days of the Hawaiian archipelago. While today only 17 species are known to persist in Hawaii (3 more may or may not be extinct), there were more than 50 species prior to Polynesian colonization of the archipelago (between 18 and 21 species have gone extinct since the discovery of the islands by westerners). The Hawaiian honeycreepers are known for their beaks, which are specialized to satisfy a wide range of dietary needs: for example, the beak of the ʻakiapōlāʻau (Hemignathus wilsoni) is characterized by a short, sharp lower mandible for scraping bark off of trees, and the much longer, curved upper mandible is used to probe the wood underneath for insects. Meanwhile, the ʻiʻiwi (Drepanis coccinea) has a very long curved beak for reaching nectar deep in Lobelia flowers. An entire clade of Hawaiian honeycreepers, the tribe Psittirostrini, is composed of thick-billed, mostly seed-eating birds, like the Laysan finch (Telespiza cantans). In at least some cases, similar morphologies and behaviors appear to have evolved convergently among the Hawaiian honeycreepers; for example, the short, pointed beaks of Loxops and Oreomystis evolved separately despite once forming the justification for lumping the two genera together. The Hawaiian honeycreepers are believed to have descended from a single common ancestor some 15 to 20 million years ago, though estimates range as low as 3.5 million years.

Hawaiian silverswords

A mixture of blooming and non-blooming Haleakalā silverswords (Argyroxiphium sandwicense macrocephalum).

Adaptive radiation is not a strictly vertebrate phenomenon, and examples are also known from among plants. The most famous example of adaptive radiation in plants is quite possibly the Hawaiian silverswords, named for alpine desert-dwelling Argyroxiphium species with long, silvery leaves that live for up to 20 years before growing a single flowering stalk and then dying. The Hawaiian silversword alliance consists of twenty-eight species of Hawaiian plants which, aside from the namesake silverswords, includes trees, shrubs, vines, cushion plants, and more. The silversword alliance is believed to have originated in Hawaii no more than 6 million years ago, making this one of Hawaii's youngest adaptive radiation events. This means that the silverswords evolved on Hawaii's modern high islands, and descended from a single common ancestor that arrived on Kauai from western North America. The closest modern relatives of the silverswords today are California tarweeds of the family Asteraceae.

Hawaiian lobelioids

Hawaii is also the site of a separate major floral adaptive radiation event: the Hawaiian lobelioids. The Hawaiian lobelioids are significantly more speciose than the silverswords, perhaps because they have been present in Hawaii for so much longer: they descended from a single common ancestor who arrived in the archipelago up to 15 million years ago. Today the Hawaiian lobelioids form a clade of over 125 species, including succulents, trees, shrubs, epiphytes, etc. A large number of species have been lost to extinction and many of the surviving species endangered.

Caribbean anoles

Anole lizards are distributed broadly in the New World, from the Southeastern US to South America. With over 400 species currently recognized, often placed in a single genus (Anolis), they constitute one of the largest radiation events among all lizards. Anole radiation on the mainland has largely been a process of speciation, and is not adaptive to any great degree, but anoles on each of the Greater Antilles (Cuba, Hispaniola, Puerto Rico, and Jamaica) have adaptively radiated in separate, convergent ways. On each of these islands, anoles have evolved with such a consistent set of morphological adaptations that each species can be assigned to one of six "ecomorphs": trunk–ground, trunk–crown, grass–bush, crown–giant, twig, and trunk. Take, for example, crown–giants from each of these islands: the Cuban Anolis luteogularis, Hispaniola's Anolis ricordii, Puerto Rico's Anolis cuvieri, and Jamaica's Anolis garmani (Cuba and Hispaniola are both home to more than one species of crown–giant). These anoles are all large, canopy-dwelling species with large heads and large lamellae (scales on the undersides of the fingers and toes that are important for traction in climbing), and yet none of these species are particularly closely related and appear to have evolved these similar traits independently. The same can be said of the other five ecomorphs across the Caribbean's four largest islands. Much like in the case of the cichlids of the three largest African Great Lakes, each of these islands is home to its own convergent Anolis adaptive radiation event.

Other examples

Presented above are the most well-documented examples of modern adaptive radiation, but other examples are known. On Madagascar, birds of the family Vangidae are marked by very distinct beak shapes to suit their ecological roles. Madagascan mantellid frogs have radiated into forms that mirror other tropical frog faunas, with the brightly colored mantellas (Mantella) having evolved convergently with the Neotropical poison dart frogs of Dendrobatidae, while the arboreal Boophis species are the Madagascan equivalent of tree frogs and glass frogs. The pseudoxyrhophiine snakes of Madagascar have evolved into fossorial, arboreal, terrestrial, and semi-aquatic forms that converge with the colubroid faunas in the rest of the world. These Madagascan examples are significantly older than most of the other examples presented here: Madagascar's fauna has been evolving in isolation since the island split from India some 88 million years ago, and the Mantellidae originated around 50 mya. Older examples are known: the K-Pg extinction event, which caused the disappearance of the dinosaurs and most other reptilian megafauna 65 million years ago, is seen as having triggered a global adaptive radiation event that created the mammal diversity that exists today.

Adaptation

From Wikipedia, the free encyclopedia

In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic or adaptive trait, with a functional role in each individual organism, that is maintained and has been evolved by natural selection.

Organisms face a succession of environmental challenges as they grow, and show adaptive plasticity as traits develop in response to the imposed conditions. This gives them resilience to varying environments.

History

Adaptation is an observable fact of life accepted by philosophers and natural historians from ancient times, independently of their views on evolution, but their explanations differed. Empedocles did not believe that adaptation required a final cause (~ purpose), but thought that it "came about naturally, since such things survived." Aristotle did believe in final causes, but assumed that species were fixed.

The second of Jean-Baptiste Lamarck's two factors (the first being a complexifying force) was an adaptive force that causes animals with a given body plan to adapt to circumstances by inheritance of acquired characteristics, creating a diversity of species and genera.

In natural theology, adaptation was interpreted as the work of a deity and as evidence for the existence of God. William Paley believed that organisms were perfectly adapted to the lives they led, an argument that shadowed Gottfried Wilhelm Leibniz, who had argued that God had brought about "the best of all possible worlds." Voltaire's Dr. Pangloss is a parody of this optimistic idea, and David Hume also argued against design. The Bridgewater Treatises are a product of natural theology, though some of the authors managed to present their work in a fairly neutral manner. The series was lampooned by Robert Knox, who held quasi-evolutionary views, as the Bilgewater Treatises. Charles Darwin broke with the tradition by emphasising the flaws and limitations which occurred in the animal and plant worlds.

Jean-Baptiste Lamarck proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus "the influence of circumstances," usually expressed as use and disuse. This second, subsidiary element of his theory is what is now called Lamarckism, a proto-evolutionary hypothesis of the inheritance of acquired characteristics, intended to explain adaptations by natural means.

Other natural historians, such as Buffon, accepted adaptation, and some also accepted evolution, without voicing their opinions as to the mechanism. This illustrates the real merit of Darwin and Alfred Russel Wallace, and secondary figures such as Henry Walter Bates, for putting forward a mechanism whose significance had only been glimpsed previously. A century later, experimental field studies and breeding experiments by people such as E. B. Ford and Theodosius Dobzhansky produced evidence that natural selection was not only the 'engine' behind adaptation, but was a much stronger force than had previously been thought.

General principles

The significance of an adaptation can only be understood in relation to the total biology of the species.

— Julian Huxley, Evolution: The Modern Synthesis

What adaptation is

Adaptation is primarily a process rather than a physical form or part of a body. An internal parasite (such as a liver fluke) can illustrate the distinction: such a parasite may have a very simple bodily structure, but nevertheless the organism is highly adapted to its specific environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, which is often quite complex. However, as a practical term, "adaptation" often refers to a product: those features of a species which result from the process. Many aspects of an animal or plant can be correctly called adaptations, though there are always some features whose function remains in doubt. By using the term adaptation for the evolutionary process, and adaptive trait for the bodily part or function (the product), one may distinguish the two different senses of the word.

Adaptation is one of the two main processes that explain the observed diversity of species, such as the different species of Darwin's finches. The other process is speciation, in which new species arise, typically through reproductive isolation. A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African lakes, where the question of reproductive isolation is complex.

Adaptation is not always a simple matter where the ideal phenotype evolves for a given external environment. An organism must be viable at all stages of its development and at all stages of its evolution. This places constraints on the evolution of development, behaviour, and structure of organisms. The main constraint, over which there has been much debate, is the requirement that each genetic and phenotypic change during evolution should be relatively small, because developmental systems are so complex and interlinked. However, it is not clear what "relatively small" should mean, for example polyploidy in plants is a reasonably common large genetic change. The origin of eukaryotic endosymbiosis is a more dramatic example.

All adaptations help organisms survive in their ecological niches. The adaptive traits may be structural, behavioural or physiological. Structural adaptations are physical features of an organism, such as shape, body covering, armament, and internal organization. Behavioural adaptations are inherited systems of behaviour, whether inherited in detail as instincts, or as a neuropsychological capacity for learning. Examples include searching for food, mating, and vocalizations. Physiological adaptations permit the organism to perform special functions such as making venom, secreting slime, and phototropism), but also involve more general functions such as growth and development, temperature regulation, ionic balance and other aspects of homeostasis. Adaptation affects all aspects of the life of an organism.

The following definitions are given by the evolutionary biologist Theodosius Dobzhansky:

1. Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.

2. Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.

3. An adaptive trait is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.

What adaptation is not

Some generalists, such as birds, have the flexibility to adapt to urban areas.

Adaptation differs from flexibility, acclimatization, and learning. Flexibility deals with the relative capacity of an organism to maintain itself in different habitats: its degree of specialization. Acclimatization describes automatic physiological adjustments during life; learning means improvement in behavioral performance during life. These terms are preferred to adaptation for changes during life which are not inherited by the next generation.

Flexibility stems from phenotypic plasticity, the ability of an organism with a given genotype to change its phenotype in response to changes in its habitat, or to move to a different habitat. The degree of flexibility is inherited, and varies between individuals. A highly specialized animal or plant lives only in a well-defined habitat, eats a specific type of food, and cannot survive if its needs are not met. Many herbivores are like this; extreme examples are koalas which depend on Eucalyptus, and giant pandas which require bamboo. A generalist, on the other hand, eats a range of food, and can survive in many different conditions. Examples are humans, rats, crabs and many carnivores. The tendency to behave in a specialized or exploratory manner is inherited—it is an adaptation. Rather different is developmental flexibility: "An animal or plant is developmentally flexible if when it is raised in or transferred to new conditions, it changes in structure so that it is better fitted to survive in the new environment," writes evolutionary biologist John Maynard Smith.

If humans move to a higher altitude, respiration and physical exertion become a problem, but after spending time in high altitude conditions they acclimatize to the reduced partial pressure of oxygen, such as by producing more red blood cells. The ability to acclimatize is an adaptation, but the acclimatization itself is not. Fecundity goes down, but deaths from some tropical diseases also go down. Over a longer period of time, some people are better able to reproduce at high altitudes than others. They contribute more heavily to later generations, and gradually by natural selection the whole population becomes adapted to the new conditions. This has demonstrably occurred, as the observed performance of long-term communities at higher altitude is significantly better than the performance of new arrivals, even when the new arrivals have had time to acclimatize.

Adaptedness and fitness

In this sketch of a fitness landscape, a population can evolve by following the arrows to the adaptive peak at point B, and the points A and C are local optima where a population could become trapped.

There is a relationship between adaptedness and the concept of fitness used in population genetics. Differences in fitness between genotypes predict the rate of evolution by natural selection. Natural selection changes the relative frequencies of alternative phenotypes, insofar as they are heritable. However, a phenotype with high adaptedness may not have high fitness. Dobzhansky mentioned the example of the Californian redwood, which is highly adapted, but a relict species in danger of extinction. Elliott Sober commented that adaptation was a retrospective concept since it implied something about the history of a trait, whereas fitness predicts a trait's future.

1. Relative fitness. The average contribution to the next generation by a genotype or a class of genotypes, relative to the contributions of other genotypes in the population. This is also known as Darwinian fitness, selection coefficient, and other terms.

2. Absolute fitness. The absolute contribution to the next generation by a genotype or a class of genotypes. Also known as the Malthusian parameter when applied to the population as a whole.

3. Adaptedness. The extent to which a phenotype fits its local ecological niche. Researchers can sometimes test this through a reciprocal transplant.

Sewall Wright proposed that populations occupy adaptive peaks on a fitness landscape. To evolve to another, higher peak, a population would first have to pass through a valley of maladaptive intermediate stages, and might be "trapped" on a peak that is not optimally adapted.

Genetic basis

A large diversity of genome DNAs in a species is the basis for adaptation and differentiation. A large population is needed to carry sufficient diversity. According to the misrepair-accumulation aging theory, The misrepair mechanism is important in maintaining a sufficient number of individuals in a species. misrepair is a way of repair for increasing the surviving chance of an organism when it has severe injuries. Without misrepairs, no individual could survive to reproduction age. Thus misrepair mechanism is an essential mechanism for the survival of a species and for maintaining the number of individuals. Although individuals die from aging, genome DNAs are being recopied and transmitted by individuals generation by generation. In addition, the DNA misrepairs in germ cells contribute also to the diversity of genome DNAs.

Types

Adaptation is the heart and soul of evolution.

— Niles Eldredge, Reinventing Darwin: The Great Debate at the High Table of Evolutionary Theory

Changes in habitat

Before Darwin, adaptation was seen as a fixed relationship between an organism and its habitat. It was not appreciated that as the climate changed, so did the habitat; and as the habitat changed, so did the biota. Also, habitats are subject to changes in their biota: for example, invasions of species from other areas. The relative numbers of species in a given habitat are always changing. Change is the rule, though much depends on the speed and degree of the change. When the habitat changes, three main things may happen to a resident population: habitat tracking, genetic change or extinction. In fact, all three things may occur in sequence. Of these three effects only genetic change brings about adaptation. When a habitat changes, the resident population typically moves to more suitable places; this is the typical response of flying insects or oceanic organisms, which have wide (though not unlimited) opportunity for movement. This common response is called habitat tracking. It is one explanation put forward for the periods of apparent stasis in the fossil record (the punctuated equilibrium theory).

Genetic change

Genetic change occurs in a population when natural selection and mutations act on its genetic variability. The first pathways of enzyme-based metabolism may have been parts of purine nucleotide metabolism, with previous metabolic pathways being part of the ancient RNA world. By this means, the population adapts genetically to its circumstances. Genetic changes may result in visible structures, or may adjust physiological activity in a way that suits the habitat.

Habitats and biota do frequently change. Therefore, it follows that the process of adaptation is never finally complete. Over time, it may happen that the environment changes little, and the species comes to fit its surroundings better and better. On the other hand, it may happen that changes in the environment occur relatively rapidly, and then the species becomes less and less well adapted. Seen like this, adaptation is a genetic tracking process, which goes on all the time to some extent, but especially when the population cannot or does not move to another, less hostile area. Given enough genetic change, as well as specific demographic conditions, an adaptation may be enough to bring a population back from the brink of extinction in a process called evolutionary rescue. It should be noted that adaptation does affect, to some extent, every species in a particular ecosystem.

Leigh Van Valen thought that even in a stable environment, competing species constantly had to adapt to maintain their relative standing. This became known as the Red Queen hypothesis, as seen in host-parasite interaction.

Co-adaptation

Pollinating insects are co-adapted with flowering plants.

In coevolution, where the existence of one species is tightly bound up with the life of another species, new or 'improved' adaptations which occur in one species are often followed by the appearance and spread of corresponding features in the other species. These co-adaptational relationships are intrinsically dynamic, and may continue on a trajectory for millions of years, as has occurred in the relationship between flowering plants and pollinating insects.

Mimicry

A and B show real wasps; the rest are Batesian mimics: three hoverflies and one beetle.

Bates' work on Amazonian butterflies led him to develop the first scientific account of mimicry, especially the kind of mimicry which bears his name: Batesian mimicry. This is the mimicry by a palatable species of an unpalatable or noxious species, gaining a selective advantage. A common example seen in temperate gardens is the hoverfly, many of which—though bearing no sting—mimic the warning coloration of hymenoptera (wasps and bees). Such mimicry does not need to be perfect to improve the survival of the palatable species.

Bates, Wallace and Fritz Müller believed that Batesian and Müllerian mimicry provided evidence for the action of natural selection, a view which is now standard amongst biologists.

Trade-offs

It is a profound truth that Nature does not know best; that genetical evolution... is a story of waste, makeshift, compromise and blunder.

— Peter Medawar, The Future of Man

All adaptations have a downside: horse legs are great for running on grass, but they can't scratch their backs; mammals' hair helps temperature, but offers a niche for ectoparasites; the only flying penguins do is under water. Adaptations serving different functions may be mutually destructive. Compromise and makeshift occur widely, not perfection. Selection pressures pull in different directions, and the adaptation that results is some kind of compromise.

Since the phenotype as a whole is the target of selection, it is impossible to improve simultaneously all aspects of the phenotype to the same degree.

— Ernst Mayr, The Growth of Biological Thought: Diversity, Evolution, and Inheritance

Consider the antlers of the Irish elk, (often supposed to be far too large; in deer antler size has an allometric relationship to body size). Obviously, antlers serve positively for defense against predators, and to score victories in the annual rut. But they are costly in terms of resource. Their size during the last glacial period presumably depended on the relative gain and loss of reproductive capacity in the population of elks during that time. As another example, camouflage to avoid detection is destroyed when vivid coloration is displayed at mating time. Here the risk to life is counterbalanced by the necessity for reproduction.

Stream-dwelling salamanders, such as Caucasian salamander or Gold-striped salamander have very slender, long bodies, perfectly adapted to life at the banks of fast small rivers and mountain brooks. Elongated body protects their larvae from being washed out by current. However, elongated body increases risk of desiccation and decreases dispersal ability of the salamanders; it also negatively affects their fecundity. As a result, fire salamander, less perfectly adapted to the mountain brook habitats, is in general more successful, have a higher fecundity and broader geographic range.

An Indian peacock's train
in full display

The peacock's ornamental train (grown anew in time for each mating season) is a famous adaptation. It must reduce his maneuverability and flight, and is hugely conspicuous; also, its growth costs food resources. Darwin's explanation of its advantage was in terms of sexual selection: "This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction." The kind of sexual selection represented by the peacock is called 'mate choice,' with an implication that the process selects the more fit over the less fit, and so has survival value. The recognition of sexual selection was for a long time in abeyance, but has been rehabilitated.

The conflict between the size of the human foetal brain at birth, (which cannot be larger than about 400 cm³, else it will not get through the mother's pelvis) and the size needed for an adult brain (about 1400 cm³), means the brain of a newborn child is quite immature. The most vital things in human life (locomotion, speech) just have to wait while the brain grows and matures. That is the result of the birth compromise. Much of the problem comes from our upright bipedal stance, without which our pelvis could be shaped more suitably for birth. Neanderthals had a similar problem.

As another example, the long neck of a giraffe is a burden and a blessing. The neck of a giraffe can be up to 2 m (6 ft 7 in) in length. This neck can be used for inter-species competition or for foraging on tall trees where shorter herbivores cannot reach. However, as previously stated, there is always a trade-off. This long neck is heavy and it adds to the body mass of a giraffe, so the giraffe needs an abundance of nutrition to provide for this costly adaptation.

Shifts in function

Adaptation and function are two aspects of one problem.

— Julian Huxley, Evolution: The Modern Synthesis

Pre-adaptation

Pre-adaptation occurs when a population has characteristics which by chance are suited for a set of conditions not previously experienced. For example, the polyploid cordgrass Spartina townsendii is better adapted than either of its parent species to their own habitat of saline marsh and mud-flats. Among domestic animals, the White Leghorn chicken is markedly more resistant to vitamin B₁ deficiency than other breeds; on a plentiful diet this makes no difference, but on a restricted diet this preadaptation could be decisive.

Pre-adaptation may arise because a natural population carries a huge quantity of genetic variability. In diploid eukaryotes, this is a consequence of the system of sexual reproduction, where mutant alleles get partially shielded, for example, by genetic dominance. Microorganisms, with their huge populations, also carry a great deal of genetic variability. The first experimental evidence of the pre-adaptive nature of genetic variants in microorganisms was provided by Salvador Luria and Max Delbrück who developed the Fluctuation Test, a method to show the random fluctuation of pre-existing genetic changes that conferred resistance to bacteriophages in Escherichia coli.

Co-option of existing traits: exaptation

The feathers of Sinosauropteryx, a dinosaur with feathers, were used for insulation, making them an exaptation for flight.

Features that now appear as adaptations sometimes arose by co-option of existing traits, evolved for some other purpose. The classic example is the ear ossicles of mammals, which we know from paleontological and embryological evidence originated in the upper and lower jaws and the hyoid bone of their synapsid ancestors, and further back still were part of the gill arches of early fish. The word exaptation was coined to cover these common evolutionary shifts in function. The flight feathers of birds evolved from the much earlier feathers of dinosaurs, which might have been used for insulation or for display.

Non-adaptive traits

Some traits do not appear to be adaptive, that is, they have a neutral or deleterious effect on fitness in the current environment. Because genes have pleiotropic effects, not all traits may be functional: they may be what Stephen Jay Gould and Richard Lewontin called spandrels, features brought about by neighbouring adaptations, like the triangular areas under neighbouring arches in architecture which began as functionless features.

Another possibility is that a trait may have been adaptive at some point in an organism's evolutionary history, but a change in habitats caused what used to be an adaptation to become unnecessary or even maladapted. Such adaptations are termed vestigial. Many organisms have vestigial organs, which are the remnants of fully functional structures in their ancestors. As a result of changes in lifestyle the organs became redundant, and are either not functional or reduced in functionality. Since any structure represents some kind of cost to the general economy of the body, an advantage may accrue from their elimination once they are not functional. Examples: wisdom teeth in humans; the loss of pigment and functional eyes in cave fauna; the loss of structure in endoparasites.

Extinction and coextinction

If a population cannot move or change sufficiently to preserve its long-term viability, then obviously, it will become extinct, at least in that locale. The species may or may not survive in other locales. Species extinction occurs when the death rate over the entire species exceeds the birth rate for a long enough period for the species to disappear. It was an observation of Van Valen that groups of species tend to have a characteristic and fairly regular rate of extinction.

Just as there is co-adaptation, there is also coextinction, the loss of a species due to the extinction of another with which it is coadapted, as with the extinction of a parasitic insect following the loss of its host, or when a flowering plant loses its pollinator, or when a food chain is disrupted.

Philosophical issues

Adaptation raises philosophical issues concerning how biologists speak of function and purpose, as this carries implications of evolutionary history – that a feature evolved by natural selection for a specific reason – and potentially of supernatural intervention – that features and organisms exist because of a deity's conscious intentions. In his biology, Aristotle introduced teleology to describe the adaptedness of organisms, but without accepting the supernatural intention built into Plato's thinking, which Aristotle rejected. Modern biologists continue to face the same difficulty. On the one hand, adaptation is obviously purposeful: natural selection chooses what works and eliminates what does not. On the other hand, biologists want to deny conscious purpose in evolution. The dilemma gave rise to a famous joke by the evolutionary biologist Haldane: "Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public.'" David Hull commented that Haldane's mistress "has become a lawfully wedded wife. Biologists no longer feel obligated to apologize for their use of teleological language; they flaunt it."