Koinophilia is a term used in biology, meaning that when sexual creatures seek a mate, they prefer that mate not to have any unusual, peculiar or deviant features. Stated differently, sexual creatures prefer mates with a preponderance of common or average features.

Natural selection results, over the course of generations, in beneficial (or "fit") features replacing their disadvantageous counterparts. This is the fundamental force which drives evolution, and is the major insight into Biology which immortalized Charles Darwin. Thus, natural selection causes beneficial features to 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 displaying a predominance of common or average features. This is termed "koinophilia". It has, as an important side effect, that mates displaying mutant features (the result of a genetic mutation) are also avoided. (The mutation causes the individual to 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, remains, on average, 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 thoroughly tried and tested features.

Koinophilia provides very simple and obvious explanations for such evolutionary puzzles as the process of speciation, evolutionary stasis and punctuated equilibria, sex and the affordability of males, and the evolution of cooperation, (see Contents 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 independently identified 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.

Sex is, fundamentally, the exchange of genetic material (DNA) between individuals. During such an exchange, it is obviously in a sexual creature's best interests for the DNA that it (or its offspring) gains through the exchange to be of at least the same quality as its own, or better. Sexual creatures would therefore be expected to be choosey about their sexual partners. They should prefer fit partners (with good DNA) over unfit partners (with poor DNA). However, fitness is something that can only be recognized in retrospect. It is, by definition, the relative propensity to produce offspring and grand-offspring. At any one moment it is therefore impossible to predict which inherited phenotypic features (i.e. the visible physical and behavioral effects of different parts of the DNA) are fitter than their alternatives.

Thus, if, in the highly schematic diagram on the right, the white, blue, yellow and red dots in generation 1 represent (imaginary) alternative nose colors in reindeer, then, in generation 1, it is totally impossible to even hazard a guess as to which color is the best for a reindeer's nose. Even if you knew every nose's dimensions, and physical and functional properties in full, it would still not indicate which one is the fittest of the four. That will only be revealed in several generations' time, because, regardless of how well the reindeer can smell, or breathe, or nuzzle in the snow with a particular nose, the ultimate and only test of fitness is whether that nose allows the bearer of that nose to breed better than the bearers of alternative nose colors. Thus, after 4 generations there can be no doubt that red is selectively more advantageous than any of the other nose colors (even if, for instance, a red nose does not function as a nose at all, but appears, instead, as "cute" to humans, with the result that red-nosed reindeers are never slaughtered for food, but are pampered, and protected by humans.)

Thus, it is only possible to tell which phenotypic features have proven themselves to be fit over the past several generations. If a given feature is common, and its alternatives are rare, then it is a fairly good bet that the common feature is fitter than any of the others (or, has been up until now). This is the basis on which a koinophilic choice of a sexual mate is made.

There are, however, two other ways in which a feature can become more common than its alternatives. The first comes about through genetic drift. This only happens in small populations (e.g. on a small island, in a totally isolated mountain valley, or following a mass extinction) 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 then tend to maintain that feature. Koinophilia cannot distinguish between commonness caused by natural selection and commonness caused by genetic drift, with the result that modern humans would not be sexually attracted to a person covered in a thick coat of fur (like a bear or chimpanzee), regardless of what caused humans to become the "Naked Ape" in the first place.

The second way in which a feature can become common without being fitter than its alternative forms 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 (see the "Handicap principle" summarised below). However, the preference that the females have for the gaudy tails has to have an advantage or it would not have become the norm in that species. In fact, the combination of the preference in the one gender and its disadvantageous 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 commonness of the peacock's tail among male peacocks is due to the simple operation of natural selection making fit features (or, in this case, a fit combination of features) more common than their less fit counterparts. It is therefore, in reality, not an alternative method of attaining commonness.

(An alternative, and possibly simpler, explanation for the peacock's extraordinary, and probably disadvantageous tail, is that it could have arisen through genetic drift, and subsequently been maintained by koinophilia. That does not imply that the peacock's tail is devoid of function. Consider the "Naked Ape", whose lack of fur demanded its replacement with clothing. That clothing, apart from providing warmth, has also become a major signal in human society, indicating, in addition to the individual's gender, his or her age, status, current activity, mood and desires. The human's lack of fur might therefore have engendered craftsmanship, leading to tool making, and, eventually, the ingenuity that characterizes this species today.)

If natural selection is, overall, a more pervasive force than genetic drift, then commonness will denote fitness very much more often than it does not. That, in turn, implies that koinophilia is likely to be widespread amongst sexual creatures. It is, in fact, the only form of mate choice which relies on the direct effects of fitness itself, instead of on a surrogate indicator of fitness. For instance, a gaudy peacock's tail might originally have provided proof (to the peahens) that the bearer of such a wasteful, metabolically costly, and aerodynamically disastrous tail must be very fit if he can remain healthy despite his handicap. However the gaudy tail is a surrogate indicator of fitness. If females prefer males with gaudy tails then that is what they will get: males with gaudy tails. Natural selection will then tend, over time, to reduce the effect of the handicap as males with beautiful tails which cause them a minimum of inconvenience replace the males with tails that are genuine handicaps. The link between the gaudy tail and fitness is then broken. In other words, the tail has become a fake indicator of fitness. Commonness can not be faked in this way. It is natural selection's ultimate, and indeed only, stamp of fitness. It would, therefore, be extraordinary if sexual creatures were not, in the main, dedicated koinophiliacs.

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 on 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.

In sexual species, the canalization of the entire phenotype (i.e. the uniformity of all the individuals' appearances within a species) is both striking and extraordinary. Most people if their bungalow in an African game reserve was invaded by a vervet monkey, and then again a day or two later, would not know whether they were seeing the same animal again, or, in fact, another member of the same species. Even the camp attendants, who are seeing these animals on a daily basis, would not be able to distinguish between the individual members of the troop. Similarly, an ornithologist studying behavior in a flock of birds can only distinguish between the individuals if they have been fitted with individualized combinations of colored bands 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 just a single individual (or of a single male and a single female) in a field guide or encyclopedia usually 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 either already latently present in the original domestic dog population, or that viable mutations (particularly for changes in body proportions) are very common. This, in turn, implies that evolution has an enormous amount of raw material, on hand as it were, 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, major climate changes, and mass extinctions. 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, unfamiliar, fringe, or extreme 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 interbreeding 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.), and stabilizing selection.

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 would always be 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 tic-tac-toe level of pointlessness. (The Game Theory prediction is that tic-tac-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.

(For a full discussion of these Game Theory strategies and their evolutionary implications, see the Prisoner's dilemma, Chicken (game), and Evolutionarily stable strategy articles in Wikipedia.)

In summary, Game Theory, and all the ancillary theories dealing with the evolution of cooperation, while providing piece-meal answers, do not fit into a single, all encompassing theory which explains the prevalence and evolutionary stability of all aspects of cooperation, in its numerous guises. There is, for instance, no satisfactory explanation as to why individuals should show fear, submission, or embarrassment. A selfish individual would be expected to bluff about such inner feelings, with Game Theory predicting that it is probably best to remain "poker faced" at all times. Yet the expressions of the emotions are essential in a cooperative, sociable community, as are hierarchies, conventions, and rituals, which remain evolutionary mysteries.

The Koinophilic solution

The problem with cooperation is that although a group of cooperative individuals is fitter than an equivalent group of selfish individuals, selfish individuals interspersed amongst a community of cooperators are fitter than their hosts. This means they raise, on average, more offspring than their hosts, and will therefore ultimately replace them.

If, however, the selfish individuals are ostracized, and rejected as mates, because of their deviant and unusual behavior, then their evolutionary advantage becomes an evolutionary liability. Cooperation in all of its very many forms then becomes evolutionarily stable. Sociability, social conventions, ritualistic behavior, expressions of the emotions, and other forms of communication between individuals, all essential ingredients for full cooperativity, can all be similarly evolutionarily stabilized by koinophilia.

Thus, whether it is a matter of joining the hunting pack, respecting the rules governing contests over territories, or gannets adhering to a convention which permits breeding on only one of two adjacent cliffs, koinophilia vigorously defends all of these practices against extinction at the hands of selfish, antisocial or nonconformist mutants.

If all of these practices are rendered evolutionarily stable by koinophilia, then the purposes they might serve need not always be explained only in terms of selfish interests.

Thus, for example, it becomes, surprisingly, a Darwinian feasibility that, as David Crews and V.C Wynne-Edwards have suggested, the ceremonial challenging of territory ownership, or the massed formation flying and dusk choruses of starlings (and many other species) at their roosts might serve to control population size. When there is a mismatch between population size and the number of available territories the frequency of contests is likely to increase, or the dusk choruses to become excessively raucous. When this occurs, depending on the species, the females' secretion of pituitary gonadotropins is inhibited, inducing infertility and thus a reduction in population size.

External links

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This article is based on "Koinophilia" from the free encyclopedia Wikipedia (http://en.wikipedia.org). It is licensed under the terms of the GNU Free Documentation Licencse. In the Wikipedia you can find a list of the authors by visiting the following address: http://en.wikipedia.org/w/index.php?title=Koinophilia&action=history