Neurohacking is a subclass of biohacking,
focused specifically on the brain. Neurohackers seek to better
themselves or others by "hacking the brain" to improve reflexes, learn
faster, or treat psychological disorders. The modern neurohacking movement has been around since the 1980s.
However, herbal supplements have been used to increase brain function
for hundreds of years. After a brief period marked by a lack of research
in the area, neurohacking started regaining interest in the early
2000s. Currently, most neurohacking is performed via do-it-yourself (DIY) methods by in-home users.
Simple uses of neurohacking include the use of chemical supplements to increase brain function. More complex medical devices can be implanted to treat psychological disorders and illnesses.
History
The use of mind-altering substances derived from plants dates back to ancient history. Neurohackers use a class of chemical substances that improve higher
order brain functions called nootropics. The term nootropics was first
proposed in 1972 by Corneliu Giurgea, a Romanian chemist from University of Bucharest.
In his study, he classified Piracetam as a nootropic and determined that nootropics should fit the following criteria:
Enhance learning
Resist impairing agents
Augment informational transfer between the two hemisphere of the brain
Heighten the brain's resistance against various forms of "aggressions"
Improved "tonic, cortico-subcortical 'control'"
Lack of pharmacological effects of other common psychoactive drugs.
Today, various nootropics are available via prescription and over the counter.
The 2000 study by Michael A. Nitsche and Walter Paulus at the University of Goettingen
is considered to be one of the first device-oriented attempts at
influencing the brain non-invasively. The study found that the motor
cortex of the brain responds to weak electrical stimuli in the form of transcranial direct current stimulation (tDCS). A later study in 2003 by Branislav Savic and Beat Meier found that (tDCS) improves motor sequence learning. More recent studies have concluded that tDCS may alleviate neuropathic
pain, depression, schizophrenia, and other neurological disorders. Methods of non-invasive brain stimulation (NIBS) have been found to enhance human performance. In 2019, a study funded by the US Department of Defense
found that cognition and motor performance could be improved by tDCS.
This investigation showed that tDCS could be used to enhance the
abilities of military personnel. However, side effects such as "itching,
tingling, and headaches" were noted. The study concluded that more research into adequate safety regulations is needed before it can be properly implemented.
A resurgence in the popularity of at-home and DIY neurohacking started in 2011. The recent availability of brain stimulation devices contributed to the rise in the home neurohacking movement. Individuals applied weak electrical currents to their brain in hopes of improving performance and productivity. Since 2017, neurohacking devices have been available to the general
public for unsupervised use. However, these methods of neurohacking have
yet to gain widespread acceptance from the general public, and user
retention rate for the devices remains low.
In 2018, Marom Bikson and his colleagues at the City College of New York released a report to aid consumers in making an informed choice regarding the purchase of tDCS devices. In particular, Bikson stated that the report hoped to educate consumers
on the reasons why a significant price differentiation existed across
the various devices on the market.
Technology
There are three main categories of neurohacking methods: oral
supplements or ingestibles, procedural training exercises, and the
transmission of electrical currents through the brain.
Oral supplements and ingestibles
Nootropics are any chemical compounds that cause an improvement in brain function. Although many are naturally produced by the body, ingestible
supplements are often required to artificially raise the concentration
of these compounds in the bloodstream to produce a significant effect.
Nootropics can be further classified into two categories: synthetics
nootropics and natural nootropics.
Synthetic nootropics
Synthetic nootropics refer to any lab-produced nootropics, including Piracetam. Synthetic nootropics can act at three different junctions:
Natural, or herbal, nootropics, include food-based antioxidants and vitamin supplements. There are three main mechanisms by which natural nootropics affect brain activity:
Popular supplements such as Ginkgo biloba and Panax quinquefolius (American ginseng) are characterized as natural and herbal nootropics. Few studies have been conducted regarding the safety and long-term
effects of prescribing these herbal supplements as a means of mitigating
age-related cognitive decline. However, current research has indicated
that these methods have the potential to alleviate the mental
deterioration in older individuals.
Procedural training exercises
Procedural training methods strengthen the connections between
neurons. For example, brain training games have been around since the
2000s. Companies such as PositScience, Lumosity, and CogniFit created video games designed to improve the user's brain function. These brain-training games improve neural capacity by adding game-like features to comprehension skills.
DBS involves implanting an electrical device, or neurostimulator, into the brain. The neurostimulator is a thin wire with electrodes at its tip. Low
levels of electric current are transmitted through the brain. The
location where the electrodes are implanted depends on the neurological
disorder being treated. The company Neuralink
hopes that their DBS device will include "as many as 3072 electrodes
distributed along 96 threads", and that the procedure to implant the
threads would be as non-invasive as LASIK eye surgery.
Transcranial magnetic stimulation (TMS)
TMS sends short bursts of magnetic energy to the left frontal cortex through a small electromagnetic coil. Some studies have found that TMS improves cognition and motor performance. Other studies have investigated the relation between TMS and its ability to recover lost memories.
Transcranial direct current stimulation (tDCS)
Brain cells, or neurons, emit chemical signals across the gaps, or synapses, between neurons. When learning a new skill or topic, the neurons involved in
understanding that particular subject are then primed to emit signals
more readily. Less electrical current is required to signal the neurons
to secrete the chemicals for transport across the synapse. tDCS involves
running a very low current (less than 2mA) through an anode and a
cathode placed on the head. The research shows that brain function
improves around the anode, with no change or reduced function around the
cathode.
Applications
Many applications of neurohacking center around improving quality of life.
Mental health
Bettering people's mental health is one primary application of neurohacking.
Virtual reality exposure therapy
is one application of neurohacking, and is being used to treat post
traumatic stress. The USC Institute for Creative Technologies has been
working on exposure therapy techniques since 2005, and exposure therapy
is now an evidence based treatment for post traumatic stress.
Exposure therapy retrains the mind of the patient to reduce the
fear associated with feeling a certain way or experiencing certain
triggering stimuli. By confronting situations in a safe and controlled virtual reality
environment, the patient is able to reduce the anxiety associated with
those circumstances.
Enhancing the human experience is another application of
neurohacking. Methods include simple brain-training games, chemical
enhancers, and electrical brain stimulation.
Caffeine
is an effective method for enhancing human performance in everyday
life. Caffeine is the most popular drug in the world (humans drink a
collective 1.6 billion cups per day) and is also the most popular method
by which people are neurohacking. Caffeine improves memory, sociability, and alertness.
Information retrieval
The third primary application of neurohacking is information retrieval from the brain. This typically involves the use of a brain-machine interface (BMI) – an apparatus to measure electrical signals in the brain.
In 2016, researchers modeled an individual's interest in digital content by monitoring their EEG
(electroencephalogram). The researchers asked the user to read
Wikipedia articles. From data in the EEG, they could predict which
article the user would want to read next based on the individual's
expressed interest in each topic. The researchers claim this paradigm
can be used to "recommend information without any explicit user
interaction".
In July 2019, Neuralink
– a company developing implantable brain-machine interfaces – presented
their research on their high bandwidth BMI. Neuralink claims to have
developed an implantable BMI device that is capable of recording and
delivering full bandwidth data from the brain. The company hopes to use
this technology to create a high-speed connection between the brain and
digital technology, bypassing the need to type search queries or read
the results.
Legal and ethical aspects
Advertisement of brain-training software
The neurohacking trend has been heavily commercialized, with companies such as Lumosity
and CogniFit marketing games that allegedly optimize the performances
of the brain as well as alleviate the symptoms of senescence-related
cognitive decline and other neurodegenerative disorders. Several studies have called into question the effectiveness of these softwares. The Federal Trade Commission (FTC) has filed claims against some companies producing brain training software for misleading marketing. Claims against Lumosity for misleading advertisement are over $2 million. Conclusive evidence regarding the effectiveness of brain training software has yet to be presented. Despite this uncertainty, the public demand for such products is
rising. Sales in 2015 reached $67 million in the United States and
Canada.
Unfair advantages
No governing organizations responsible for overseeing athletics and
education have policies regulating neurohacking. Athletes and students
can use neurohacking to gain an unfair advantage in sporting events and
academic settings. Studies have indicated that neurohacking can improve memory, creativity, learning speed, muscle gain, and athletic performance. However, there are no well-developed tests or instruments capable of
detecting neurohacking. Students and athletes may utilize neurohacking
techniques and never be detected.
Side effects and potential risks
Most manufacturers fail to disclose the potential side effects of
neurohacking devices, including significant changes to the user's
self-identity and decreased reasoning skills. Affordable neurohacking devices are available online with prices
ranging from $99 to $800, making them easily accessible to consumers.
For instance, a "brain stimulator" device produced by the "Brain
Stimulator" company that utilizes tDCS is priced $127 to $179. However, these devices are rarely regulated by the government. Using these unapproved devices with no medical supervision could cause devastating side effects. Cases have been cited where individuals physically harm others as a side effect of neurohacking.
Insurance claims
The Vercise DBS System produced by Boston Scientific Corporation is the only neurohacking medical device for sale that is approved by the Food and Drug Administration (FDA), Code of Federal Regulations (CFR), and Good Practices in Clinical Research. With the rise of DIY neurohacking, many individuals self-treat without proper supervision by a medical professional. Insurance companies deny medical insurance compensation for users who
are injured using unapproved medical-grade neurohacking devices. Most neurohacking devices are uncertified and unregulated.
This illustration compares the somewhat larger and hotter Sun (left) to the relatively inactive star Tau Ceti.
Solar-type stars, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type. Observations of these stars are important for understanding better the
properties of the Sun in relation to other stars and the habitability of
planets.
By similarity to the Sun
Defining the three categories by their similarity to the Sun reflects
the evolution of astronomical observational techniques. Originally,
solar-type was the closest that similarity to the Sun could be defined.
Later, more precise measurement techniques and improved observatories
allowed for greater precision of key details like temperature, enabling
the creation of a solar analog category for stars that were particularly
similar to the Sun. Later still, continued improvements in precision
allowed for the creation of a solar-twin category for near-perfect
matches.
Similarity to the Sun allows for checking derived quantities—such
as temperature, which is derived from the color index—against the Sun,
the only star whose temperature is confidently known. For stars that are
not similar to the Sun, this cross-checking cannot be done.
Solar-type
These stars are broadly similar to the Sun. They are main-sequence stars with a B−V color between 0.48 and 0.80, the Sun having a B−V color of 0.65. Alternatively, a definition based on spectral type can be used, such as F8V through K2V, which would correspond to B−V color of 0.50 to 1.00. This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.
Solar-type stars show highly correlated behavior between their
rotation rates and their chromospheric activity (e.g. Calcium H & K
line emission) and coronal activity (e.g. X-ray emission) Because solar-type stars spin down during their main-sequence lifetimes due to magnetic braking, these correlations allow rough ages to be derived. Mamajek & Hillenbrand (2008) have estimated the ages for the 108 solar-type (F8V–K2V) main-sequence
stars within 52 light-years (16 parsecs) of the Sun based on their
chromospheric activity (as measured via Ca, H, and K emission lines).
The following table shows a sample of solar-type stars within 50
light years that nearly satisfy the criteria for solar analogs (B−V
color between 0.48 and 0.80), based on current measurements (the Sun is
listed for comparison):
These stars are photometrically similar to the Sun, having the following qualities:
Temperature within 500 K from that of the Sun (5278 to 6278 K)
Metallicity of 50–200% (± 0.3 dex) of that of the Sun, meaning the star's protoplanetary disk would have had similar amounts of dust from which planets could form
No close companion (orbital period of ten days or less), because such a companion stimulates stellar activity
Solar analogs not meeting the stricter solar twin criteria include, within 50 light years and in order of increasing distance (The Sun is listed for comparison.):
To date no solar twin that exactly matches the Sun has been found. However, there are some stars that come very close to being identical
to the Sun, and are such considered solar twins by members of the
astronomical community. An exact solar twin would be a G2V star with a
5,778 K surface temperature, be 4.6 billion years old, with the correct
metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity, radius, chemical composition, rotation, magnetic activity, and size are also very important to low luminosity variation.
Morgan-Keenan
spectral classification of stars. Most common star type in the universe
are M-dwarfs, 76%. The Sun is a 4.6 billion year-old G-class (G2V) star
and is more massive than 95% of all stars. Only 7.6% are G-class stars
The stars below are more similar to the Sun and having the following qualities:
Temperature within 50 K from that of the Sun (5728 to 5828 K) (within 10 K of sun (5768–5788 K)).
Metallicity of 89–112% (± 0.05 dex) of that of the Sun, meaning the star's proplyd would have had almost exactly the same amount of dust for planetary formation
No stellar companion, because the Sun itself is a solitary star
An age within 1 billion years from that of the Sun (3.6 to 5.6 Ga)
Other Sun parameters:
Sun rotates on its axis once in about 27 days or 1.997 kilometres per second (1.241 mi/s)
The following are the known stars that come closest to satisfying the
criteria for a solar twin. The Sun is listed for comparison. Highlighted
boxes are out of range for a solar twin. The star may have been noted
as solar twin in the past, but are more of a solar analog.
Some other stars are sometimes mentioned as solar-twin candidates such as: Beta Canum Venaticorum; however it has too low metallicities (−0.21) for solar twin. 16 Cygni B is sometimes noted as twin, but is part of a triple star system and is very old for a solar twin at 6.8 Ga.
By potential habitability
Another way of defining solar twin is as a "habstar"—a star with
qualities believed to be particularly hospitable to a life-hosting
planet. Qualities considered include variability, mass, age,
metallicity, and close companions.
The requirement that the star remain on the main sequence for at
least 0.5–1 Ga sets an upper limit of approximately 2.2–3.4 solar
masses, corresponding to a hottest spectral type of A0-B7V. Such stars can be 100x as bright as the Sun.Tardigrade-like
life (due to the UV flux) could potentially survive on planets orbiting
stars as hot as B1V, with a mass of 10 M☉, and a temperature of 25,000
K, a main-sequence lifetime of about 20 million years.
Non-variability is ideally defined as variability of less than
1%, but 3% is the practical limit due to limits in available data.
Variation in irradiance in a star's habitable zone due to a companion
star with an eccentric orbit is also a concern.
Terrestrial planets in multiple star
systems, those containing three or more stars, are not likely to have
stable orbits in the long term. Stable orbits in binary systems take one
of two forms: S-Type (satellite or circumstellar) orbits around one of
the stars, and P-Type (planetary or circumbinary) orbits around the
entire binary pair. Eccentric Jupiters may also disrupt the orbits of planets in habitable zones.
Metallicity of at least 40% solar ([Fe/H] = −0.4) is required for
the formation of an Earth-like terrestrial planet. High metallicity
strongly correlates to the formation of hot Jupiters, but these are not absolute bars to life, as some gas giants end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons.
One example of such a star is HD 70642 , a G5V, at temperature of 5533 K, but is much younger than the Sun, at 1.9 billion years old.
Another such example would be HIP 11915, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet Jupiter does in the Solar System. To strengthen the similarities, the star is class G5V, has a
temperature of 5750 K, has a Sun-like mass and radius, and is only 500
million years younger than the Sun. As such, the habitable zone would
extend in the same area as the zone in the Solar System, around 1 AU.
This would allow an Earth-like planet to exist around 1 AU.
Sexually reproducinganimals, plants, fungi and protists are thought to have evolved from a common ancestor that was a single-celled eukaryoticspecies.Sexual reproduction is widespread in eukaryotes, though a few
eukaryotic species have secondarily lost the ability to reproduce
sexually, such as Bdelloidea, and some plants and animals routinely reproduce asexually (by apomixis and parthenogenesis) without entirely having lost sex. The evolution of sexual reproduction contains two related yet distinct themes: its origin and its maintenance. Bacteria and Archaea (prokaryotes) have processes that can transfer DNA from one cell to another (conjugation, transformation, and transduction), but it is unclear if these processes are evolutionarily related to sexual reproduction in Eukaryotes. In eukaryotes, true sexual reproduction by meiosis and cell fusion is thought to have arisen in the last eukaryotic common ancestor, possibly via several processes of varying success, and then to have persisted.
Since hypotheses for the origin of sex are difficult to verify experimentally (outside of evolutionary computation),
most current work has focused on the persistence of sexual reproduction
over evolutionary time. The maintenance of sexual reproduction
(specifically, of its dioecious
form) by natural selection in a highly competitive world has long been
one of the major mysteries of biology, since both other known mechanisms
of reproduction – asexual reproduction and hermaphroditism
– possess apparent advantages over it. Asexual reproduction can proceed
by budding, fission, or spore formation and does not involve the union
of gametes, which accordingly results in a much faster rate
of reproduction compared to sexual reproduction, where 50% of offspring
are males and unable to produce offspring themselves. In hermaphroditic
reproduction, each of the two parent organisms required for the
formation of a zygote can provide either the male or the female gamete, which leads to advantages in both size and genetic variance of a population.
Sexual reproduction therefore must offer significant fitness
advantages because, despite the two-fold cost of sex (see below), it
dominates among multicellular forms of life, implying that the fitness
of offspring produced by sexual processes outweighs the costs. Sexual
reproduction derives from recombination, where parent genotypes are reorganised and shared with the offspring. This stands in contrast to single-parent asexual replication, where the offspring is always identical to the parents (barring mutation). Recombination supplies two fault-tolerance mechanisms at the molecular level: recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time) and complementation (also known as heterosis, hybrid vigour or masking of mutations).
Historical perspective
Reproduction, including modes of sexual reproduction, features in the writings of Aristotle; modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin (1731–1802) in the 18th century. August Weismann picked up the thread in 1885, arguing that sex serves to generate genetic variation, as detailed in the majority of the explanations below. On the other hand, Charles Darwin (1809–1882) concluded that the effect of hybrid vigor (complementation) "is amply sufficient to account for the ... genesis of the two sexes". This is consistent with the repair and complementation hypothesis, described below. Since the emergence of the modern evolutionary synthesis in the 20th century, numerous biologists including W. D. Hamilton, Alexey Kondrashov, George C. Williams,
Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and
Richard E. Michod – have suggested competing explanations for how a
vast array of different living species maintain sexual reproduction.
Advantages of sex and sexual reproduction
The concept of sex includes two fundamental phenomena: the sexual
process (fusion of genetic information of two individuals) and sexual
differentiation (separation of this information into two parts).
Depending on the presence or absence of these phenomena, all of the
existing forms of reproduction can be classified as asexual,
hermaphrodite or dioecious. The sexual process and sexual
differentiation are different phenomena, and, in essence, are
diametrically opposed. The first creates (increases) diversity of
genotypes, and the second decreases it by half.
Reproductive advantages of the asexual forms are in quantity of
the progeny, and the advantages of the hermaphrodite forms are in
maximal diversity. Transition from the hermaphrodite to dioecious state
leads to a loss of at least half of the diversity. So, the primary
challenge is to explain the advantages given by sexual differentiation,
i.e. the benefits of two separate sexes compared to hermaphrodites
rather than to explain benefits of sexual forms (hermaphrodite +
dioecious) over asexual ones. It has already been understood that since
sexual reproduction is not associated with any clear reproductive
advantages over asexual reproduction, there should be some important
advantages in evolution.
Advantages due to genetic variation, DNA repair and genetic complementation
For the advantage due to genetic variation, there are three possible
reasons this might happen. First, sexual reproduction can combine the
effects of two beneficial mutations
in the same individual (i.e. sex aids in the spread of advantageous
traits) without the mutations having to have occurred one after another
in a single line of descendants.Second, sex acts to bring together currently deleterious mutations to
create severely unfit individuals that are then eliminated from the
population (i.e. sex aids in the removal of deleterious genes). However,
in organisms containing only one set of chromosomes, deleterious
mutations would be eliminated immediately, and therefore removal of
harmful mutations is an unlikely benefit for sexual reproduction.
Lastly, sex creates new gene combinations that may be more fit than
previously existing ones, or may simply lead to reduced competition
among relatives.
For the advantage due to DNA repair, there is an immediate large benefit of removing DNA damage by recombinational DNA repair during meiosis (assuming the initial mutation rate is higher than optimal), since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation
to each sexual partner is avoidance of the bad effects of their
deleterious recessive genes in progeny by the masking effect of normal
dominant genes contributed by the other partner.
The classes of hypotheses based on the creation of variation are
further broken down below. Any number of these hypotheses may be true in
any given species (they are not mutually exclusive),
and different hypotheses may apply in different species. However, a
research framework based on creation of variation has yet to be found
that allows one to determine whether the reason for sex is universal for
all sexual species, and, if not, which mechanisms are acting in each
species.
On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species.
Protection from major genetic mutation
In contrast to the view that sex promotes genetic variation, Heng, and Gorelick and Heng reviewed evidence that sex actually acts as a constraint on genetic
variation. They consider that sex acts as a coarse filter, weeding out
major genetic changes, such as chromosomal rearrangements, but
permitting minor variation, such as changes at the nucleotide or gene
level (that are often neutral) to pass through the sexual sieve.
Novel genotypes
This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous alleles A and B
occur at random. The two alleles are recombined rapidly in a sexual
population (top), but in an asexual population (bottom) the two alleles
must independently arise because of clonal interference.
Sex could be a method by which novel genotypes are created. Because
sex combines genes from two individuals, sexually reproducing
populations can more easily combine advantageous genes than can asexual
populations. If, in a sexual population, two different advantageous alleles
arise at different loci on a chromosome in different members of the
population, a chromosome containing the two advantageous alleles can be
produced within a few generations by recombination.
However, should the same two alleles arise in different members of an
asexual population, the only way that one chromosome can develop the
other allele is to independently gain the same mutation, which would
take much longer. Several studies have addressed counterarguments, and
the question of whether this model is sufficiently robust to explain the
predominance of sexual versus asexual reproduction remains.
Ronald Fisher
suggested that sex might facilitate the spread of advantageous genes by
allowing them to better escape their genetic surroundings, if they
should arise on a chromosome with deleterious genes.
Supporters of these theories respond to the balance argument that
the individuals produced by sexual and asexual reproduction may differ
in other respects too – which may influence the persistence of
sexuality. For example, in the heterogamouswater fleas of the genus Cladocera, sexual offspring form eggs which are better able to survive the winter versus those the fleas produce asexually.
Increased resistance to parasites
One of the most widely discussed theories to explain the persistence
of sex is that it is maintained to assist sexual individuals in
resisting parasites, also known as the Red Queen hypothesis.
When an environment changes, previously neutral or deleterious
alleles can become favourable. If the environment changed sufficiently
rapidly (i.e. between generations), these changes in the environment can
make sex advantageous for the individual. Such rapid changes in
environment are caused by the co-evolution between hosts and parasites.
Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and one gene in hosts with two alleles h and H, conferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele h, and P to H. Such a situation will lead to cyclic changes in allele frequency – as p increases in frequency, h will be disfavoured.
In reality, there will be several genes involved in the
relationship between hosts and parasites. In an asexual population of
hosts, offspring will only have the different parasitic resistance if a
mutation arises. In a sexual population of hosts, however, offspring
will have a new combination of parasitic resistance alleles.
In other words, like Lewis Carroll's Red Queen, sexual hosts are continually "running" (adapting) to "stay in one place" (resist parasites).
Evidence for this explanation for the evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins. The genes coding for immune system proteins evolve considerably faster.
Further evidence for the Red Queen hypothesis was provided by
observing long-term dynamics and parasite coevolution in a "mixed"
(sexual and asexual) population of snails (Potamopyrgus antipodarum).
The number of sexuals, the number of asexuals, and the rates of
parasite infection for both were monitored. It was found that clones
that were plentiful at the beginning of the study became more
susceptible to parasites over time. As parasite infections increased,
the once plentiful clones dwindled dramatically in number. Some clonal
types disappeared entirely. Meanwhile, sexual snail populations remained
much more stable over time.
However, Hanley et al. studied mite infestations of a parthenogenetic
gecko species and its two related sexual ancestral species. Contrary to
expectation based on the Red Queen hypothesis, they found that the
prevalence, abundance and mean intensity of mites in sexual geckos was
significantly higher than in asexuals sharing the same habitat.
In 2011, researchers used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens
to generate a host-parasite coevolutionary system in a controlled
environment, allowing them to conduct more than 70 evolution experiments
testing the Red Queen hypothesis. They genetically manipulated the mating system of C. elegans,
causing populations to mate either sexually, by self-fertilization, or a
mixture of both within the same population. Then they exposed those
populations to the S. marcescens parasite. It was found that the self-fertilizing populations of C. elegans
were rapidly driven extinct by the coevolving parasites while sex
allowed populations to keep pace with their parasites, a result
consistent with the Red Queen hypothesis. In natural populations of C. elegans, self-fertilization is the predominant mode of reproduction, but infrequent out-crossing events occur at a rate of about 1%.
Other hypotheses
Critics of the Red Queen hypothesis question whether the constantly
changing environment of hosts and parasites is sufficiently common to
explain the evolution of sex; an alternative is the court jester hypothesis, which emphasises abiotic factors including climate. Otto and Nuismer presented results showing that species interactions (e.g. host vs
parasite interactions) typically select against sex. They concluded
that, although the Red Queen hypothesis favors sex under certain
circumstances, it alone does not account for the ubiquity of sex. Otto
and Gerstein further stated that "it seems doubtful to us that strong selection per
gene is sufficiently commonplace for the Red Queen hypothesis to explain
the ubiquity of sex". Parker reviewed numerous genetic studies on plant disease resistance and
failed to uncover a single example consistent with the assumptions of
the Red Queen hypothesis.
Disadvantages of sex and sexual reproduction
The paradox of the existence of sexual reproduction is that though it
is ubiquitous in multicellular organisms, there are ostensibly many
inherent disadvantages to reproducing sexually when weighed against the
relative advantages of alternative forms of reproduction, such as
asexual reproduction. Thus, because sexual reproduction abounds in
complex multicellular life, there must be some significant benefit(s) to
sex and sexual reproduction that compensates for these fundamental
disadvantages.
Population expansion cost of sex
Among the most limiting disadvantages to the evolution of sexual
reproduction by natural selection is that an asexual population can grow
much more rapidly than a sexual one with each generation.
For example, assume that the entire population of a theoretical
species has 100 total organisms consisting of two sexes (i.e. males and
females), with 50:50 male-to-female representation, and that only the
females of this species can bear offspring. If all capable members of
this population procreated once, a total of 50 offspring would be
produced (the F1 generation). Contrast this outcome
with an asexual species, in which each and every member of an equally
sized 100-organism population is capable of bearing young. If all
capable members of this asexual population procreated once, a total of
100 offspring would be produced – twice as many as produced by the
sexual population in a single generation.
This diagram illustrates the two-fold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation.
This idea is sometimes referred to as the two-fold cost of sexual reproduction. It was first described mathematically by John Maynard Smith.[36][page needed]
In his manuscript, Smith further speculated on the impact of an asexual
mutant arising in a sexual population, which suppresses meiosis and allows eggs to develop into offspring genetically identical to the mother by mitotic division.The mutant-asexual lineage would double its representation in the population each generation, all else being equal.
Technically the problem above is not one of sexual reproduction
but of having a subset of organisms incapable of bearing offspring.
Indeed, some multicellular organisms (isogamous) engage in sexual reproduction but all members of the species are capable of bearing offspring.The two-fold reproductive disadvantage assumes that males contribute
only genes to their offspring and sexual females spend half their
reproductive potential on sons. Thus, in this formulation, the principal cost of sex is that males and females must successfully copulate,
which almost always involves expending energy to come together through
time and space. Asexual organisms need not expend the energy necessary
to find a mate.
Selfish cytoplasmic genes
Sexual reproduction implies that chromosomes and alleles segregate
and recombine in every generation, but not all genes are transmitted
together to the offspring.There is a chance of spreading mutants that cause unfair transmission
at the expense of their non-mutant colleagues. These mutations are
referred to as "selfish" because they promote their own spread at the
cost of alternative alleles or of the host organism; they include
nuclear meiotic drivers and selfish cytoplasmic genes. Meiotic drivers
are genes that distort meiosis to produce gametes containing themselves
more than the 50% of the time expected by chance. A selfish cytoplasmic
gene is a gene located in an organelle, plasmid or intracellular
parasite that modifies reproduction to cause its own increase at the
expense of the cell or organism that carries it.
Genetic heritability cost of sex
A sexually reproducing organism only passes on ~50% of its own
genetic material to each L2 offspring. This is a consequence of the fact
that gametes from sexually reproducing species are haploid.
Again, however, this is not applicable to all sexual organisms. There
are numerous species which are sexual but do not have a genetic-loss
problem because they do not produce males or females. Yeast, for
example, are isogamous sexual organisms which have two mating types
which fuse and recombine their haploid genomes. Both sexes reproduce
during the haploid and diploid stages of their life cycle and have a
100% chance of passing their genes into their offspring.
Some species avoid the 50% cost of sexual reproduction, although they have "sex" (in the sense of genetic recombination). In these species (e.g., bacteria, ciliates, dinoflagellates and diatoms), "sex" and reproduction occur separately.
DNA repair and complementation
As discussed in the earlier part of this article, sexual reproduction
is conventionally explained as an adaptation for producing genetic
variation through allelic recombination. As acknowledged above, however,
serious problems with this explanation have led many biologists to
conclude that the benefit of sex is a major unsolved problem in
evolutionary biology.
An alternative "informational" approach to this problem has led to the view that the two fundamental aspects of sex, genetic recombination and outcrossing,
are adaptive responses to the two major sources of "noise" in
transmitting genetic information. Genetic noise can occur as either
physical damage to the genome (e.g. chemically altered bases of DNA or
breaks in the chromosome) or replication errors (mutations). This alternative view is referred to as the repair and complementation
hypothesis, to distinguish it from the traditional variation hypothesis.
The repair and complementation hypothesis assumes that genetic recombination is fundamentally a DNA repair process, and that when it occurs during meiosis
it is an adaptation for repairing the genomic DNA which is passed on to
progeny. Recombinational repair is the only repair process known which
can accurately remove double-strand damages in DNA, and such damages are
both common in nature and ordinarily lethal if not repaired. For
instance, double-strand breaks in DNA occur about 50 times per cell
cycle in human cells (see naturally occurring DNA damage).
Recombinational repair is prevalent from the simplest viruses to the
most complex multicellular eukaryotes. It is effective against many
different types of genomic damage, and in particular is highly efficient
at overcoming double-strand damages. Studies of the mechanism of
meiotic recombination indicate that meiosis is an adaptation for
repairing DNA. These considerations form the basis for the first part of the repair and complementation hypothesis.
In some lines of descent from the earliest organisms, the diploid stage of the sexual cycle, which was at first transient, became the predominant stage, because it allowed complementation – the masking of deleterious recessive mutations (i.e. hybrid vigor or heterosis). Outcrossing, the second fundamental aspect of sex, is maintained by the advantage of masking mutations and the disadvantage of inbreeding (mating with a close relative) which allows expression of recessive mutations (commonly observed as inbreeding depression). This is in accord with Charles Darwin, who concluded that the adaptive advantage of sex is hybrid vigor; or as
he put it, "the offspring of two individuals, especially if their
progenitors have been subjected to very different conditions, have a
great advantage in height, weight, constitutional vigor and fertility
over the self fertilised offspring from either one of the same parents."
However, outcrossing may be abandoned in favor of parthenogenesis
or selfing (which retain the advantage of meiotic recombinational
repair) under conditions in which the costs of mating are very high. For
instance, costs of mating are high when individuals are rare in a
geographic area, such as when there has been a forest fire and the
individuals entering the burned area are the initial ones to arrive. At
such times mates are hard to find, and this favors parthenogenic
species.
In the view of the repair and complementation hypothesis, the
removal of DNA damage by recombinational repair produces a new, less
deleterious form of informational noise, allelic recombination, as a
by-product. This lesser informational noise generates genetic variation,
viewed by some as the major effect of sex, as discussed in the earlier
parts of this article.
Deleterious mutation clearance
Mutations
can have many different effects upon an organism. It is generally
believed that the majority of non-neutral mutations are deleterious,
which means that they will cause a decrease in the organism's overall
fitness. If a mutation has a deleterious effect, it will then usually be removed from the population by the process of natural selection. Sexual reproduction is believed to be more efficient than asexual reproduction in removing those mutations from the genome.
There are two main hypotheses which explain how sex may act to remove deleterious genes from the genome.
While DNA is able to recombine to modify alleles,
DNA is also susceptible to mutations within the sequence that can
affect an organism in a negative manner. Asexual organisms do not have
the ability to recombine their genetic information to form new and
differing alleles. Once a mutation
occurs in the DNA or other genetic carrying sequence, there is no way
for the mutation to be removed from the population until another
mutation occurs that ultimately deletes the primary mutation. This is
rare among organisms.
Hermann Joseph Muller
introduced the idea that mutations build up in asexual reproducing
organisms. Muller described this occurrence by comparing the mutations
that accumulate as a ratchet.
Each mutation that arises in asexually reproducing organisms turns the
ratchet once. The ratchet is unable to be rotated backwards, only
forwards. The next mutation that occurs turns the ratchet once more.
Additional mutations in a population continually turn the ratchet and
the mutations, mostly deleterious, continually accumulate without
recombination. These mutations are passed onto the next generation because the offspring are exact genetic clones
of their parents. The genetic load of organisms and their populations
will increase due to the addition of multiple deleterious mutations and
decrease the overall reproductive success and fitness.
For sexually reproducing populations, studies have shown that
single-celled bottlenecks are beneficial for resisting mutation build-up. Passaging a population through a single-celled bottleneck involves the fertilization event occurring with haploid
sets of DNA, forming one fertilized cell. For example, humans undergo a
single-celled bottleneck in that the haploid sperm fertilizes the
haploid egg, forming the diploid zygote,
which is unicellular. This passage through a single cell is beneficial
in that it lowers the chance of mutations from being passed on through
multiple individuals. Instead, the mutation is only passed onto one
individual. Further studies using Dictyostelium discoideum
suggest that this unicellular initial stage is important for resisting
mutations due to the importance of high relatedness. Highly related
individuals are more closely related, and more clonal, whereas less
related individuals are less so, increasing the likelihood that an
individual in a population of low relatedness may have a detrimental
mutation. Highly related populations also tend to thrive better than
lowly related because the cost of sacrificing an individual is greatly
offset by the benefit gained by its relatives and in turn, its genes,
according to kin selection. The studies with D. discoideum
showed that conditions of high relatedness resisted mutant individuals
more effectively than those of low relatedness, suggesting the
importance of high relatedness to resist mutations from proliferating.
Removal of deleterious genes
Diagram illustrating different relationships between numbers of mutations and fitness. Kondrashov's model requires synergistic epistasis, which is represented by the red line – each subsequent mutation has a disproportionately large effect on the organism's fitness.
This hypothesis was proposed by Alexey Kondrashov, and is sometimes known as the deterministic mutation hypothesis. It assumes that the majority of deleterious mutations are only slightly
deleterious, and affect the individual such that the introduction of
each additional mutation has an increasingly large effect on the fitness
of the organism. This relationship between number of mutations and
fitness is known as synergistic epistasis.
By way of analogy, think of a car
with several minor faults. Each is not sufficient alone to prevent the
car from running, but in combination, the faults combine to prevent the
car from functioning.
Similarly, an organism may be able to cope with a few defects,
but the presence of many mutations could overwhelm its backup
mechanisms.
Kondrashov argues that the slightly deleterious nature of
mutations means that the population will tend to be composed of
individuals with a small number of mutations. Sex will act to recombine
these genotypes, creating some individuals with fewer deleterious
mutations, and some with more. Because there is a major selective
disadvantage to individuals with more mutations, these individuals die
out. In essence, sex compartmentalises the deleterious mutations.
There has been much criticism of Kondrashov's theory, since it
relies on two key restrictive conditions. The first requires that the
rate of deleterious mutation should exceed one per genome per generation
in order to provide a substantial advantage for sex. While there is
some empirical evidence for it (for example in Drosophila and E. coli), there is also strong evidence against it. Thus, for instance, for the sexual species Saccharomyces cerevisiae (yeast) and Neurospora crassa (fungus), the mutation rate per genome per replication are 0.0027 and 0.0030 respectively. For the nematode worm Caenorhabditis elegans, the mutation rate per effective genome per sexual generation is 0.036. Secondly, there should be strong interactions among loci (synergistic
epistasis), a mutation-fitness relation for which there is only limited
evidence. Conversely, there is also the same amount of evidence that mutations
show no epistasis (purely additive model) or antagonistic interactions
(each additional mutation has a disproportionally small effect).
Other explanations
Geodakyan's evolutionary theory of sex
Geodakyan suggested that sexual dimorphism provides a partitioning of
a species' phenotypes into at least two functional partitions: a female
partition that secures beneficial features of the species and a male
partition that emerged in species with more variable and unpredictable
environments. The male partition is suggested to be an "experimental"
part of the species that allows the species to expand their ecological
niche, and to have alternative configurations. This theory underlines
the higher variability and higher mortality in males, in comparison to
females. This functional partitioning also explains the higher
susceptibility to disease in males, in comparison to females and
therefore includes the idea of "protection against parasites" as another
functionality of male sex. Geodakyan's evolutionary theory of sex was
developed in Russia in 1960–1980 and was not known to the West till the
era of the Internet. Trofimova, who analysed psychological sex
differences, hypothesised that the male sex might also provide a
"redundancy pruning" function.
Speed of evolution
Ilan Eshel suggested that sex prevents rapid evolution. He suggests that recombination
breaks up favourable gene combinations more often than it creates them,
and sex is maintained because it ensures selection is longer-term than
in asexual populations – so the population is less affected by
short-term changes. This explanation is not widely accepted, as its assumptions are very restrictive.
It has recently been shown in experiments with Chlamydomonas algae that sex can remove the speed limit on evolution.
An information theoretic analysis using a simplified but useful
model shows that in asexual reproduction, the information gain per
generation of a species is limited to 1 bit per generation, while in
sexual reproduction, the information gain is bounded by , where is the size of the genome in bits.
The "libertine bubble theory" proposes that meiotic sex evolved in proto-eukaryotes to solve a problem that bacteria did not have, namely a large amount of DNA material, occurring in an archaic step of proto-cell
formation and genetic exchanges. So that, rather than providing
selective advantages through reproduction, sex could be thought of as a
series of separate events which combines step-by-step some very weak
benefits of recombination, meiosis, gametogenesis and syngamy. Therefore, current sexual species could be descendants of primitive
organisms that practiced more stable exchanges in the long term, while
asexual species have emerged, much more recently in evolutionary
history, from the conflict of interest resulting from anisogamy.
Parasites and Muller's ratchet
R. Stephen Howard and Curtis Lively were the first to suggest that the combined effects of parasitism and mutation
accumulation can lead to an increased advantage to sex under conditions
not otherwise predicted (Nature, 1994). Using computer simulations,
they showed that when the two mechanisms act simultaneously the
advantage to sex over asexual reproduction is larger than for either factor operating alone.
Many protists reproduce sexually, as do many multicellular plants, animals, and fungi. In the eukaryotic fossil record, sexual reproduction first appeared about 2.0 billion years ago in the Proterozoic Eon,although a later date, 1.2 billion years ago, has also been presented.Nonetheless, all sexually reproducing eukaryotic organisms likely derive from a single-celled common ancestor. It is probable that the evolution of sex was an integral part of the evolution of the first eukaryotic cell.There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants.
Diploidy
Organisms need to replicate their genetic material in an efficient
and reliable manner. The necessity to repair genetic damage is one of
the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a damaged section of their DNA via homologous recombination, since there are two copies of the gene in the cell and if one copy is damaged, the other copy is unlikely to be damaged at the same site.
A harmful damage in a haploid individual, on the other hand, is more likely to become fixed (i.e. permanent), since any DNA repair mechanism would have no source from which to recover the original undamaged sequence. The most primitive form of sex may have been one organism with damaged
DNA replicating an undamaged strand from a similar organism in order to
repair itself.
Meiosis
Sexual reproduction appears to have arisen very early in eukaryotic evolution, implying that the essential features of meiosis were already present in the last eukaryotic common ancestor. In extant organisms, proteins with central functions in meiosis are similar to key proteins in natural transformation in bacteria and DNA transfer in archaea. For example, recA recombinase, that catalyses the key functions of DNA homology search and strand exchange in the bacterial sexual process of transformation, has orthologs in eukaryotes that perform similar functions in meiotic recombination
Natural transformation in bacteria, DNA transfer in archaea, and meiosis
in eukaryotic microorganisms are induced by stressful circumstances
such as overcrowding, resource depletion, and DNA damaging conditions. This suggests that these sexual processes are adaptations for dealing
with stress, particularly stress that causes DNA damage. In bacteria,
these stresses induce an altered physiologic state, termed competence,
that allows active take-up of DNA from a donor bacterium and the
integration of this DNA into the recipient genome (see Natural competence) allowing recombinational repair of the recipients' damaged DNA.
If environmental stresses leading to DNA damage were a persistent
challenge to the survival of early microorganisms, then selection would
likely have been continuous through the prokaryote to eukaryote
transition, and adaptative adjustments would have followed a course in which
bacterial transformation or archaeal DNA transfer naturally gave rise to
sexual reproduction in eukaryotes.
Virus-like RNA-based origin
Sex might also have been present even earlier, in the hypothesized RNA world that preceded DNA cellular life forms. One proposed origin of sex in the RNA world was based on the type of
sexual interaction that is known to occur in extant single-stranded
segmented RNA viruses, such as influenza virus, and in extant
double-stranded segmented RNA viruses such as reovirus.
Exposure to conditions that cause RNA damage could have led to
blockage of replication and death of these early RNA life forms. Sex
would have allowed re-assortment of segments between two individuals
with damaged RNA, permitting undamaged combinations of RNA segments to
come together, thus allowing survival. Such a regeneration phenomenon,
known as multiplicity reactivation, occurs in the influenza virus and reovirus.
Parasitic DNA elements
Another theory is that sexual reproduction originated from selfish parasitic genetic elements
that exchange genetic material (that is: copies of their own genome)
for their transmission and propagation. In some organisms, sexual
reproduction has been shown to enhance the spread of parasitic genetic
elements (e.g. yeast, filamentous fungi).
Bacterial conjugation
is a form of genetic exchange that some sources describe as "sex", but
technically is not a form of reproduction, even though it is a form of horizontal gene transfer. However, it does support the "selfish gene" part theory, since the gene itself is propagated through the F-plasmid.
A similar origin of sexual reproduction is proposed to have evolved in ancient haloarchaea as a combination of two independent processes: jumping genes and plasmid swapping.
Partial predation
A third theory is that sex evolved as a form of cannibalism:
One primitive organism ate another one, but instead of completely
digesting it, some of the eaten organism's DNA was incorporated into the
DNA of the eater.
Vaccination-like process
Sex may also be derived from another prokaryotic process. A
comprehensive theory called "origin of sex as vaccination" proposes that
eukaryan sex-as-syngamy
(fusion sex) arose from prokaryan unilateral sex-as-infection, when
infected hosts began swapping nuclearised genomes containing coevolved,
vertically transmitted symbionts that provided protection against
horizontal superinfection by other, more virulent symbionts.
Consequently, sex-as-meiosis (fission sex) would evolve as a host
strategy for uncoupling from (and thereby render impotent) the acquired
symbiotic/parasitic genes.
Mechanistic origin of sexual reproduction
While theories positing fitness benefits that led to the origin of sex are often problematic,several theories addressing the emergence of the mechanisms of sexual reproduction have been proposed.
The viral eukaryogenesis (VE) theory proposes that eukaryotic cells arose from a combination of a lysogenic virus, an archaean, and a bacterium.
This model suggests that the nucleus originated when the lysogenic
virus incorporated genetic material from the archaean and the bacterium
and took over the role of information storage for the amalgam. The
archaeal host transferred much of its functional genome to the virus
during the evolution of cytoplasm, but retained the function of gene
translation and general metabolism. The bacterium transferred most of
its functional genome to the virus as it transitioned into a mitochondrion.
For these transformations to lead to the eukaryotic cell cycle,
the VE hypothesis specifies a pox-like virus as the lysogenic virus. A
pox-like virus is a likely ancestor because of its fundamental
similarities with eukaryotic nuclei. These include a double stranded DNA
genome, a linear chromosome with short telomeric
repeats, a complex membrane bound capsid, the ability to produce capped
mRNA, and the ability to export the capped mRNA across the viral
membrane into the cytoplasm.
The presence of a lysogenic pox-like virus ancestor explains the
development of meiotic division, an essential component of sexual
reproduction.
Meiotic
division in the VE hypothesis arose because of the evolutionary
pressures placed on the lysogenic virus as a result of its inability to
enter into the lytic cycle.
This selective pressure resulted in the development of processes
allowing the viruses to spread horizontally throughout the population.
The outcome of this selection was cell-to-cell fusion. (This is distinct
from the conjugation methods used by bacterial plasmids under
evolutionary pressure, with important consequences.) The possibility of this kind of fusion is supported by the presence of
fusion proteins in the envelopes of the pox viruses that allow them to
fuse with host membranes. These proteins could have been transferred to
the cell membrane during viral reproduction, enabling cell-to-cell
fusion between the virus host and an uninfected cell. The theory
proposes meiosis originated from the fusion between two cells infected
with related but different viruses which recognised each other as
uninfected. After the fusion of the two cells, incompatibilities between
the two viruses result in a meiotic-like cell division.
The two viruses established in the cell would initiate
replication in response to signals from the host cell. A mitosis-like
cell cycle would proceed until the viral membranes dissolved, at which
point linear chromosomes would be bound together with centromeres. The
homologous nature of the two viral centromeres would incite the grouping
of both sets into tetrads. It is speculated that this grouping may be
the origin of crossing over, characteristic of the first division in
modern meiosis. The partitioning apparatus of the mitotic-like cell
cycle the cells used to replicate independently would then pull each set
of chromosomes to one side of the cell, still bound by centromeres.
These centromeres would prevent their replication in subsequent
division, resulting in four daughter cells with one copy of one of the
two original pox-like viruses. The process resulting from combination of
two similar pox viruses within the same host closely mimics meiosis.
Neomuran revolution
An alternative theory, proposed by Thomas Cavalier-Smith, was labeled the Neomuran revolution.
The designation "Neomuran revolution" refers to the appearances of the
common ancestors of eukaryotes and archaea. Cavalier-Smith proposes that
the first neomurans emerged 850 million years ago. Other molecular
biologists assume that this group appeared much earlier, but
Cavalier-Smith dismisses these claims because they are based on the
"theoretically and empirically" unsound model of molecular clocks.
Cavalier-Smith's theory of the Neomuran revolution has implications for
the evolutionary history of the cellular machinery for recombination
and sex. It suggests that this machinery evolved in two distinct bouts
separated by a long period of stasis; first the appearance of
recombination machinery in a bacterial ancestor which was maintained for
3 Gy(billion years), until the neomuran revolution when the mechanics
were adapted to the presence of nucleosomes.
The archaeal products of the revolution maintained recombination
machinery that was essentially bacterial, whereas the eukaryotic
products broke with this bacterial continuity. They introduced cell
fusion and ploidy cycles into cell life histories. Cavalier-Smith argues
that both bouts of mechanical evolution were motivated by similar
selective forces: the need for accurate DNA replication without loss of
viability.
Seesaw effect
Schematic
illustration of the advantage of the first sexual individual resulting
from the seesaw effect. Possible combinations of the sex allele (S) and
non-sex allele (N) entering the clean genome (C) or dirty genome (D) are
shown. S (dominant over N) controls meiosis and fusion. α is the
deviation from equal division of dms (deleterious genes) over two genomes. The first automictic selfing event is successful with a 50% probability.
In 2022, Yasui and his colleague proposed
the "seesaw effect" hypothesis to explain the emergence of gametic
sexual reproduction. They suggested that an ancestral diploid asexual
eukaryote acquired a dominant sex allele (S), enabling meiosis and
gamete fusion. As genome size increased, deleterious mutations (dms) accumulated due to Muller's ratchet, approaching a lethal threshold (dmt :
e.g., 100 mutations). However, these mutations were asymmetrically
distributed between the two genomes—for example, 60 mutations in one
(dirty genome D) and 40 in the other (clean genome C). When the S allele
arises, the cell becomes SN and undergoes meiosis, producing four
gametes: two with S and two with N, split across the clean and dirty
genomes (e.g., CS, CS, DN, DN or CN, CN, DS, DS).
Because the first S-bearing individual had no partner,
self-fertilization (automixis) occurred. Terminal fusion of sister
chromatids created homozygous offspring: CS + CS produced a viable
zygote with 80 mutations (40+40), while DS + DS led to 120 mutations
(60+60), which exceeded dmt and resulted in death. Thus, harmful
mutations were purged, and the S allele became fixed, as only S-carrying
gametes could participate in fusion. DN and CN gametes (asexual types)
could not fuse and died.
From the second generation, each CSCS individual could produce four CS
gametes, yielding two viable offspring, matching the efficiency of
binary fission in asexual reproduction. Since S became fixed and all
gametes could reproduce sexually, the "twofold cost of meiosis" was
eliminated. This mechanism—concentrating mutational burden in some
gametes while preserving others—resembled a seesaw, hence the name
"seesaw effect." It explains both the origin of sex and its early
evolutionary advantage in purging deleterious mutations.
Questions
Some questions biologists have attempted to answer include:
Why does sexual reproduction exist, if in many organisms it has a
50% cost (fitness disadvantage) in relation to asexual reproduction?
Did mating types (types of gametes, according to their compatibility) arise as a result of anisogamy (gamete dimorphism), or did mating types evolve before anisogamy?
Why do most sexual organisms use a binary mating system?
Grouping itself offers a survival advantage. A binary recognition based
system is the most simple and effective method in maintaining species
grouping.
