Koinophilia
Koinophilia is a term used in genetics, meaning that sexual creatures prefer mates with a preponderance of common or average features. Stated differently, sexual creatures avoid mates with unusual, uncommon or deviant features.
Because natural selection results in beneficial (or "fit") features replacing their disadvantageous counterparts, the beneficial features become increasingly more common with each generation, while the disadvantageous features become increasingly rare. A sexual creature, therefore, wishing  to mate with a fit partner, would be expected to avoid individuals sporting unusual features, while being especially attracted to those individuals with a predominance of common or average features. Koinophilia has, as an important side effect, that mates displaying mutant features (the result of a genetic mutations) are also avoided. (The mutation causes the individual too look odd, or different.) This, in itself, is also advantageous, because the vast majority of mutations are disadvantageous. Because it is impossible to judge whether a new mutation is beneficial or not, koinophilic creatures will avoid them all with equal determination, even if this means avoiding the very occasional beneficial mutation. Thus, koinophilia, although not perfect or infallible in its ability to distinguish fit from unfit mates, on average, remains the best strategy when choosing a mate: it will be right far more often than it will be wrong. Even when it is wrong, a koinophilic choice always ensures that the offspring will inherit a suite of functional features.

# Introduction
Koinophilia provides very simple and obvious explanations for such evolutionary conundrums as the process of speciation, evolutionary stasis and punctuated equilibria, sex and the affordability of males, and the evolution of cooperation (see Contents above, and External links below).
This mating strategy, was first referred to as koinophilia by Johan H. Koeslag, from the Greek, koinos, meaning "the usual" or "common", and philos, meaning "fondness" or "love". It was tested in humans by Judith Langlois, who found that the average of two human faces was more attractive than either of the faces from which that average was derived. The more faces (of the same gender and age) that were used in the averaging process the more attractive and appealing the average face became.
The koinophilia model assumes that sexual creatures have senses with which they evaluate the various individual features of potential mates; and that they are capable of judging which features are common or usual, and which are unusual. Commonness is the only universally appropriate sign of fitness, or, at the very least, of an evolutionarily tried and tested phenotype. The link between all other mate attractants (e.g. a peacock's tail) and fitness is both cirumstance-dependent, and ultimately "fakable" (i.e., the link between the mate attractant and fitness fades, or might even reverse). The commonness of a feature cannot be faked, because the only way it can be achieved, through natural selection, is by that feature being more beneficial than any of its alternative forms. Stated differently, if a feature is "fit" then that "fitness" is revealed, or expressed, by its becoming increasingly more common through the generations than any of its less fit counterparts. If it does not become more common then it is, by definition, not as fit as its competitors.
Koinophilia is also not circumstance-dependent in the way that the other mate attractants are. Thus, rats and peacocks can both use commoness to judge the desirability of a potential mate, but a gaudy tail cannot be used likewise. Koinophilia is therefore evolutionarily robust and likely to be widespread amongst sexual creatures.
It should, however, be remembered that there are two other ways in which a feature can become more common than its alternatives. The first comes about when one gender (e.g. the females of the species) develops a preference for, say, a gaudy tail in the other gender. That will cause gaudy tails to become more common in the males, even though gaudy tails are not to their advantage. However, the preference that the females have for the gaudy tails has to have an advantage or it would not have become common in that species. In fact, the combination of the preference in the one gender and its target in the other gender has to be advantageous overall for it to persist. If it is not, natural selection will eliminate it. Viewed in this light the commoness of the peacock's tail among male peacocks is due to the simple operation of natural selection making fit features more common than their less fit counterparts. It is therefore, in reality, not an alternative method of attaining commoness.
The second way in which a feature can become common without being more fit than its alternative forms, is through genetic drift. This only happens in small populations where random events can cause the loss of a fit feature, leaving a less fit alternative to become the dominant phenotype. Genetic drift is an important evolutionary force which might, for instance, explain why humans have lost their fur. The lack of fur does not seem to impart any advantage over the other mammals, and is therefore unlikely to have arisen through natural selection. But once it became established as the norm (through genetic drift), koinophilia would tend to maintain that feature. Koinophilia cannot distinguish between commoness caused by natural selection and commoness caused by genetic drift.
Koinophilia will therefore only persist, and be a powerful biological force, if natural selection is, overall, a more pervasive force than genetic drift, causing commoness to reflect fitness more often than it does not.

# Physical attractiveness
In keeping with these theoretical considerations, humans clearly find young average faces the most attractive. However, Perrett et al. found that both men and women found a slightly off-average female face the most attractive from a wide range of women's faces with neutral expressions and identical hairstyles. When the non-average features were slightly exaggerated the face was judged more attractive still. Close examination of the photos in Perrett, May and Yoshikawa's paper shows, in fact, that the exaggerated face looks younger than the average face (composed of women's faces aged 22-46 years). The differences are, however, very small, and, to many people, not immediately obvious. Since the same results were obtained with Japanese subjects, these findings are probably culture independent, and would indicate that people generally find young  average female faces sexually the most attractive, as expected.

# Speciation and "punctuated equilibria"

## The evolutionary problem
A major evolutionary problem has been how the continuous process of evolution produces the morphologically discontinuous groups labeled species. Thus, there are lions, leopards, cheetahs and lynxes in the African savannah, each markedly different from the other, with no intermediate forms, or gradations in appearance going from, for instance, lions at one extreme to leopards at the other. Although various processes of speciation have long been recognized (e.g. allopatric, peripatric, parapatric, sympatric), they do not explain why the process is almost universal and very prominent among sexual creatures, while it is often conspicuously absent among asexual creatures. Asexual organisms very frequently show the continuous variation in form (often in many different directions) that evolution is expected to produce, making their classification into "species" (more correctly "morphospecies") very difficult.  The above named processes also do not explain the uniformity in outward appearance of all the members of a particular species, regardless of each individual trait's relative contribution to fitness. Thus, a deer's tail does not contribute nearly as much to the animal's fitness as does its fur coat, the shape of its ears or the position of its eyes, yet all vary as little in form and appearance as do the others, as if selection were acting equally strongly on all of them. (It is a general principle in evolution that features that are neither particularly advantageous, or disadvantageous, can be expected to be subject to greater individual variation than, say, the camouflage coloring of the coat which is under strong selective pressure causing it to be the same in all individuals in the same environment.)
This canalization of the entire phenotype (i.e. the uniformity of all the individuals' appearances within a species) is extraordinary. Most people if they saw an Okapi, for instance, and then, a few days later a second Okapi in the same general vicinity, would not know whether they were seeing the same animal again, or, in fact, another member of the same species. Similarly, an ornithologist studying individual behavior in a flock of birds can only distinguish between the individuals if they have been fitted with individualized combinations of colored rings on their legs. In fact, for most persons, if you have seen one member of a species, then you have seen them all. So much so, that an illustration of a just single individual (or of a single male and a single female) in a field guide or encyclopedia suffices to describe, in minute and absolute detail, all the adult members of a species. Consider, on the other hand, the wide variety of dogs that humans have bred over the past 100 -200 years or so. There are breeds with no fur at all, and others with extra thick coats; there are long-legged greyhounds and stubby-legged dachshund; there are long snouted Afghan hounds and stub nosed pugs and bull dogs; and so it goes on - great danes and chihuahuas; loose skinned dogs and tight skinned dogs. That all of these breeds could have been derived in such a short space of time implies that all of this enormous variation was already latently present in the original domestic dog population. This, in turn, implies that evolution has an enormous amount of raw material on hand to work with. The slightest change of circumstances would, therefore, be expected to produce a change in phenotype. This is indeed what happens in asexual creatures. Sexual creatures, however, seem to vigorously resist these changes, down to even the most trivial of phenotypic features.
This is, however, only one aspect of what is almost certainly a two-dimensional problem. The "horizontal" dimension refers to the almost complete absence of transitional forms between present-day species (e.g. lions, leopards, cheetahs and lynxes). The "vertical" dimension concerns the fossil record. Palaeontological species are frequently remarkably stable over extremely long periods of geological time, despite continental drift and major climate changes. Extreme examples of evolutionary conservatism (or "evolutionary stasis") include the Coelacanth which has been in place since the middle of the Devonian, 410 million years ago, the Horseshoe crab which has hardly changed in the past 350-400 million years, cockroaches which have existed for 300-350 million years, crocodiles which have changed very little since the time of the dinosaurs, and the Xenopus frog of which 90 million year-old fossil remains have been found. When phenotypic change does occur, it tends to be abrupt in geological terms, again producing phenotypic gaps, but now between successive species, which then often co-exist for considerable periods of time. Standard evolutionary theory predicts "gradualism", meaning that creatures are expected to evolve gradually, and more or less continuously, from one species into another. The fossil record, though open to different interpretations (see Evolution), does not seem to support this prediction. It suggests that evolution occurs in bursts, interspersed by long periods of stasis (i.e. by means of punctuated equilibria). Why this is so, has been one of evolution's great mysteries.

## The solution
Koinophilia could explain both the horizontal and vertical manifestations of speciation, and why it usually involves the entire external phenotype. Since, by definition, fit traits replace less fit traits, each fit trait tends to become more common, and ultimately the dominant phenotype, while the maladaptive traits become increasingly rare. Sexual creatures would therefore be expected to prefer mates sporting predominantly common features, while avoiding mates with unusual or unfamiliar attributes. This is termed koinophilia. It causes common features to become more common still, and at a rate that exceeds that which would be driven by natural selection alone. Since it affects the entire external phenotype, the members of an interbreeding group will soon all begin to look alike, and noticeably different from other interbreeding groups. Any individual from one interbeeding group who wanders into another interbreeding group will now be immediately recognizable as morphologically different, and will, therefore, be discriminated against during the mating season. This koinophilia-induced reproductive isolation might thus be the first crucial step in the development of, ultimately, molecular biological, physiological, behavioral, and anatomical barriers to hybridization, and thus, ultimately, full specieshood. Koinophilia will thereafter defend that species phenotype against invasion by unusual or unfamiliar forms (which might arise by immigration or mutation), and thus be a paradigm of punctuated equilibria (or the "vertical" aspect of the speciation problem.).

# The evolution of cooperation

## Definition of cooperation
Cooperation is any group behavior that benefits the individuals more than if they were to act as independent agents. There is however a second, very important, corollary to cooperation: it can always be exploited by selfish individuals who benefit even more by not taking part in the group activity, yet reaping its benefits. It is for this reason that cooperation poses an evolutionary problem. An extreme example was described by Wynne-Edwards  in the 1960s.  He described a gannetry on Cape St Mary on the Newfoundland coast. It consisted of two adjacent cliffs on which the gannets roosted at night. The birds that roosted on Cliff 1 mated, built nests and raised chicks. The birds that roosted on Cliff 2, despite being adult, and of both sexes, did not mate, build nests  or raise chicks, unless a vacancy arose on Cliff 1. A pair of birds would then move from Cliff 2 to Cliff 1, and start breeding. The obvious benefit of this behavior was that it limited population size, thereby ensuring that everyone had enough to eat, even in the long-term. It had the additional benefit that should an epidemic or inclement weather  wipe out a large portion of the colony, there were always enough spare birds to fill the vacancies created by the disaster on Cliff 1. The population could thus be restored within a very short space of time.

## The evolutionary problem
This observation caused a major controversy in biological circles because it seemed evolutionarily impossible. Imagine a mutant bird on Cliff 2 which, because of its mutation, was blind to the convention that (s)he should not breed on Cliff 2. Imagine that it found a mate equally unconcerned about the convention, and that the two of them set about raising chicks either on Cliff 2, or on a convenient ledge nearby. Since this unusual behavior is due to a genetic mutation, the resultant chicks would feel equally unbound by the convention that only birds that can be accommodated on Cliff 1 may breed.  Thus the grand-offspring of the first mutant pair would behave similarly, as would the great-grand-offspring. If, for ease of calculation, only half of the colony is normally accommodated on Cliff 1, then each normal bird has, on average, a 50% chance of breeding. The mutants, on the other hand, have a 100% chance of breeding. They are therefore twice as "fit" as the normal birds, meaning that the mutation will spread extremely rapidly. Within a very short space of time, it would replace its normal counterpart. In biological jargon, the normal cooperative behavior is "evolutionarily unstable"; it has no evolutionary defense against selfish mutants.
Although this is an extreme example, it illustrates the evolutionary conundrum posed by any form of cooperative behavior. The selfish individuals who do not join the hunting pack and its incumbent dangers but nevertheless share the spoils, might be only 5-10% fitter than the cooperative individuals, but that does not matter. That extra 5% fitness will eventually result in the selfish mutation replacing its cooperative counterpart. Once the mutation has taken over from the normal gene, everyone is at a disadvantage compared to the original (cooperative) condition. Though this might seem extraordinary, it is nevertheless an inevitable evolutionary trap from which there is no obvious way of escape.

## Game Theory solutions
The problem has been widely discussed in biology, because, despite these theoretical predictions (that cooperation is evolutionarily unstable), cooperative behavior is widespread in nature. The problem is most commonly addressed in "Game Theory" terms, which have led to important insights and understanding of  cooperative behavior and how it might be evolutionarily stabilized. An example is the following. The males of many species defend and fight for territories during the mating season. If all these encounters were full scale battles, then it might turn out that, for the average male, the loss in fitness due to injuries is greater than the gains in fitness from securing a territory and therefore a mate. If this is the case, then Game Theory predicts that inter-male squabbles should be settled through the observance of one or other non-violent convention, while reserving full scale battles only for those occasions when the convention is broken by one or other party. In other words it is tit-for tat. The convention then becomes an "Evolutionarily Stable Strategy" because the individuals who disregard the convention suffer, on average, heavier losses in fitness than the individuals who adhere to the convention.
This is indeed what is observed in nature. The rule that is most commonly followed in most species is that squabbles are settled by the convention that the current owner of the territory always wins, usually without a physical fight.  This, indeed, solves the original problem, but raises a new one. If the owner always wins when challenged, then why challenge? The "game" has now degenerated into the tick-tack-toe level of pointlessness. (The Game Theory prediction is that tick-tack-toe is a game not worth pursuing as it has only one stable end-point: an endless series of draws.)
A "tit-for-tat" strategy does not always lead to pointless "games". In some situations, where the same pair of individuals regularly meet, it confers evolutionarily stability to cooperation, unless one of the players makes a mistake (e.g. mistakenly rewards cooperation with a selfish response), in which case a long series of selfish tit-for-tat acts ensues, which can only be broken by a second mistake (e.g. a cooperative response to a selfish act). Different variations, however, on the tit-for-tat theme can outperform pure tit-for-tat, and therefore take over from it, but some are, in turn, unstable in the presence of selfishness. Thus, if the different strategies arise by random mutations, then the affected population will cycle through all of the strategies in an endless series of chaotic cycles. Game Theory, on its own, therefore does not seem to provide an answer to why cooperation is as common as it is (and apparently evolutionarily stable). It certainly does not provide an explanation as to why Wynne-Edwards' gannets behaved the way he observed them to behave.

## The Koinophilic solution
Koinophilia, because it discriminates against any rare or unusual form of behavior is capable of stabilizing any strategy in any of the cooperation versus selfishness games. Any individual who behaves abnormally (as a result of a mutation, or through immigration) will not easily find a mate, and will thus not be able to pass that mutation on into the next generation. Koinophilia therefore has the effect of increasing the fitness of whatever happens to be the common strategy. Different groups, practicing different strategies will thus become evolutionarily trapped in these different behaviors. A group that happens, by chance, to follow a cooperative strategy will, by definition, be fitter, as a group, than groups that consist of selfish individuals. Competition between such groups will ultimately result in the replacement of the selfish groups by cooperative groups. Thus, Wynne-Edwards' gannets can be expected to continue practicing population control well into the future, without fear of being driven into extinction by selfishly reproducing mutants, because those mutants will experience difficulties in finding mates. And, even if one of them did happen to find a mate, then the offspring would be discriminated against, as their behavior would still stand out as being unusual, or deviant, in a large colony of several hundred birds.

# Koinophilia in popular culture
- The concept is poignantly summarized in a poem by Richard Fein entitled Koinophilia.