Koinophilia

From Wikipedia, the free encyclopedia

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.

Since natural selection causes beneficial (or "fit") features to replace 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 mutation) are also avoided. This, in itself, is also advantageous, since the vast majority of mutations are disadvantageous. Since 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, while not perfect or infallible in its ability to distinguish fit from unfit mates, remains far and away the best bet strategy when choosing a mate: it will be right far more often than it will be wrong. And, even when it is wrong, a koinophilic choice always ensures that the offspring will inherit a suite of tried and tested features.

Contents 1 Introduction 2 Physical attractiveness 3 Speciation and "punctuated equilibria" 3.1 The evolutionary problem 3.2 Koinophilia 4 Sex and the affordability of males 4.1 Definition of sex 4.2 The evolutionary problem 4.3 Koinophilia 4.4 Koinophilia explains when sex can outcompete asex and vice versa 5 The evolution of cooperation 5.1 Definition of cooperation 5.2 The evolutionary problem 5.3 Game Theory solutions 5.4 Koinophilia 6 References

[edit] Introduction

Koinophilia provides very simple and obvious explanations for such evolutionary conundrums as the process of speciation,^[1] evolutionary stasis and punctuated equilibria,^[1][2] sex and the affordability of males,^[3][4] and the evolution of cooperation^[5] (see Contents, above). Most of the answers to these evolutionary problems stem from the evolutionary conservatism (i.e. resistance to change) that is the inevitable result of koinophilia. Evolutionary change can still take place, but generally not in large groups. It is presumably only when small groups of individuals become isolated from the main group that genetic drift will induce changes, which, as the isolate grows, become the new set of norms according to which the desirability of potential mates will be judged. If the new set of norms is inherently disadvantageous the isolate will die out. If, on the other hand, the isolate survives and comes into contact with other isolates with other norms, competition between the groups will determine which set of norms survives, and therefore what the new "species" will look like,^[1] and how it will behave.^[5]

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".^[2] It was tested in humans by Judith Langlois,^[6] 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 and fitness is both cirumstance-dependent, and ultimately fakable (i.e. the link between the mate attractant and fitness fades, or might even reverse). Commonness cannot be faked, nor is it 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.) Fitness, by definition, causes commonness, and drives less fit traits into rarity. Koinophilia is therefore evolutionarily robust and likely to be widespread amongst sexual creatures.

[edit] Physical attractiveness

Main article: Physical attractiveness

In keeping with these theoretical considerations, humans clearly find young average faces the most attractive.^[6][7][8][9] However, Perrett et al.^[9] 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^[9] 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,^[6] as expected.

[edit] Speciation and "punctuated equilibria"

Main article: Speciation

[edit] The evolutionary problem

A major evolutionary problem has been how the continuous process of evolution produces the morphologically discontinuous groups labeled species. Although various processes of speciation have long been recognized (e.g. allopatric, peripatric, parapatric, sympatric),^[10] 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.^{[2][11][12][13]} The above named processes also do not explain the phenotypic canalization of the entire outward appearance of both genders, 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 is extraordinary. Consider the wide variety of dogs that humans have bred over the past 100 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.^[11][12][13] 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 tigers). 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. When phenotypic change does occur, it tends to be abrupt in geological terms, again producing phenotypic gaps, but now between successive species, which often co-exist for considerable periods of time.

[edit] Koinophilia

Koinophilia could explain both the horizontal and vertical manifestations of speciation, and why it usually involves the entire external phenotype.^[1] 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 (so much so, that the entire species can often be depicted by means of a single illustration in a wild-life field-guide), 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 equilibrium (or the "vertical" aspect of the speciation problem.^[1]).

[edit] Sex and the affordability of males

[edit] Definition of sex

Sex is the exchange of genetic material between individuals. Gender and reproduction are widespread but incidental associations with sex. Unicellular organisms can exchange genetic material more or less at any time during their life cycles. For multicellular organisms the best, and most practically feasible time to exchange genetic material is at the one cell stage - hence the almost universal association of sex with reproduction in higher organisms.

[edit] The evolutionary problem

Having two genders is, however, a major unsolved evolutionary problem. The reason for this is that among genderless creatures all individuals can produce offspring. When there are males, then only half of the members of the community can produce offspring. In technical jargon, genderless and asexual creatures have an inherent twofold selective advantage over sexual creatures that produce male and female offspring. Despite this major selective disadvantage, bi-gender sex is extremely widespread among multicellular organisms.

Primitive sex may have arisen for any number of possible reasons. Hickey and Rose^[14] have, for instance, suggested that sex resulted from DNA parasites that induced their hosts (i.e. the infected individuals) to exchange genetic material with their neighbors to promote the parasite's horizontal spread through the population. Once sex became established it conveyed a small selective advantage over asex because, whereas an asexual creature will pass all of its mutations on to everyone of its offspring, in sexual creatures the mutations are spread unevenly between the offspring. Some inherit none, while others get all of the mutations from both parents. Sex is therefore a mechanism for shedding mutations. This advantage is, however, not great enough to compensate for the twofold disadvantage of producing males.

[edit] Koinophilia

If creatures exchange genetic material, it is obviously in their best interests for the DNA they gain through the exchange to be of at least the same quality as their own, or better. Sexual creatures can therefore be expected to be choosey about their sexual partners; they should prefer fit partners over unfit partners. 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 phenotypic features are fitter than their alternatives. Many generations have to pass before that evaluation can be made with any degree of certainty. Thus, it is only possible to tell which phenotypic features have proved themselves to be fit over the past several generations. If a given feature is common, and its alternative is rare, then it is a fairly good bet that the common feature is the fitter of the two (or, has been up until now). Sexual creatures would therefore be expected to prefer common features over unusual or rare features. This is known as koinophilia.

Koinophilia has, as one of its inevitable side effects, the effect that sexual creatures will avoid mates sporting mutant features. Since most mutations are disadvantageous, this, in itself, would probably make sexual creatures obligatory koinophiliacs. In computer simulations to study competition between asexual and sexual reproduction, with and without koinophilia,^[3][4] it could be shown that bi-gender, koinophilic sex could indeed outcompete asex if mutations with random fitnesses occurred randomly. The advantage of koinophilia was greatest when populations were large, the incidence of mildly disadvantageous mutations was high, and beneficial mutations were rare. Under these circumstances, koinophilic bi-gender sex could resist replacement by asexuals for over 10 000 generations (when the simulations were voluntarily stopped), even when one sexual mutated into a parthenogen every generation.

The reason that koinophilia provides its greatest advantage when the mutations are only mildly disadvantageous is that these are the mutations that are, by definition, cleared only very slowly by natural selection. They tend therefore to accumulate in large numbers in asexual creatures. (Highly disadvantageous mutations, which kill early in the life cycle, are weeded out by natural selction and therefore tend not to accumulate in the way that mildly disadvantageous mutations do.) Mild mutations obviously also tend to accumulate in sexual creatures, but koinophilia weeds them out with the same vigor that natural selection reserves for lethal and near-lethal mutations.

[edit] Koinophilia explains when sex can outcompete asex and vice versa

Thus, the koinophilia hypothesis explains not only how bi-gender sex can resist asex, but also the reasons for the exceptions, which are not randomly distributed in nature. Asex is more common among unicellular organisms than among the more complex creatures such as the vertebrates. This is almost certainly due to the relative rates at which mutations appear in the two types of creature. Koinophilia is at its most effective when mutations occur frequently, which they are more likely to do in megagenomic creatures than in organisms with relatively fewer genes. Furthermore, asex tends to be more common in marginal environments such as high latitudes, mountains and deserts than elsewhere.^[15][16] In these environments, the choice of mates is not only limited, but the effectiveness of koinophilia as a genetic error detecting devise may be severely compromised. Genetic drift confounds the very principle on which it operates: commonness loses its association with fitness. In a computer simulation of such low population density conditions, with otherwise identical parameters (mutation rates etc.) as the before, the population was always replaced by asex within a few dozen generations.

It should be noted, however, that these considerations only indicate that koinophilic sexual creatures can afford to produce male offspring. They do not explain what evolutionary pressures cause them to do so (i.e. we do not know the advantage of producing males, and, in particular, not why the practice of producing males is so widespread in nature).

[edit] The evolution of cooperation

Main article: Co-operation (evolution)

[edit] 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.^[17] in 1962. He described a gannetry on Cape St Mary's 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.

[edit] 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.

[edit] 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".^[18] In some situations it confers evolutionarily stability to cooperation, unless a player 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).^[19] Different variations, however, on the tit-for-tat theme^[20][21] 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.^[22] 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.

[edit] Koinophilia

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 the mutation on into the next generation. Koinophilia therefore has the effect of increasing the fitness of whatever happens to be the common strategy.^[20] 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.^[5] 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.

[edit] References

1. ^ ^{a b c d e} KOESLAG, J.H. (1995). On the engine of speciation. J. theor. Biol. 177, 401-409
2. ^ ^{a b c} KOESLAG, J.H. (1990). Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour. J. theor. Biol. 144, 15-35
3. ^ ^{a b} KOESLAG, P.D., KOESLAG, J.H. (1994). Koinophilia stabilizes bi-gender sexual reproduction against asex in an unchanging environment. J. theor. Biol. 166, 251-260
4. ^ ^{a b} KOESLAG, J.H., KOESLAG, P.D. (1993). Evolutionarily stable meiotic sex. J. Heredity 84, 396-399
5. ^ ^{a b c} KOESLAG, J.H. (2003). Evolution of cooperation: cooperation defeats defection in the cornfield model. J. theor. Biol. 224, 399-410
6. ^ ^{a b c} LANGLOIS, J.H. & ROGGMAN, L. (1990). Attractive faces are only average. Psychol. Sci. 1, 115-121
7. ^ ETCOFF, N. (1994). Beauty and the beholder. Nature (Lond) 368, 186-187
8. ^ ENQUIST , M & GHIRLANDA, S. (1998). The secret of faces. Nature (Lond) 394, 826-827
9. ^ ^{a b c} PERRETT D.I. et al. (1998). Effects of sexual dimorphism on facial attractiveness. Nature (Lond) 394, 884-887
10. ^ COYNE, J.A. (1992). Genetics and speciation. Nature (Lond) 335, 511-515
11. ^ ^{a b} MAYNARD SMITH, J. (1983) The genetics of stasis and punctuation. Ann. rev. Genet. 17, 11-25
12. ^ ^{a b} CLAPHAM, A.R., TUTIN, T.G., WARBURG, E.F. (1952). Flora of the British Isles. Cambridge: Cambridge University Press.
13. ^ ^{a b} GRANT, V. Plant speciation New York:Columbia University Press
14. ^ HICKEY, D.A., ROSE, M.R. (1988) The role of gene transfer in the evolution of eukaryotic sex. In The Evolution of Sex Ed: MICHOD, R.E., LEVIN, B.R. Sunderland, Massachusetts: Sinauer. pp. 161-175
15. ^ BELL, G. (1982). The Masterpiece of Nature: the Evolution and Genetics of Sexuality. Berkley: University of California Press
16. ^ LEWIS, W.M. (1987). The cost of sex. In The Evolution of Sex and its Consequences. Ed: STEARN, S.C. Basel: Birkhäuser. pp. 33-57.
17. ^ WYNNE-EDWARDS, V.C. (1962). Animal Dispersion in Relation to Social Behaviour Edinburgh:Oliver & Boyd
18. ^ AXELROD, R. HAMILTON, W.D. (1981). The evolution of cooperation. Science 211, 1390-1396
19. ^ SIGMUND, K. (1993). Games of Life. Oxford: Oxford University Press
20. ^ ^{a b} KOESLAG, J.H. (1997). Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation. J. theor. Biol. 189, 53--61
21. ^ NOVAK, M. SIGMUND, K. (1993). A strategy of win stay, lose-shift that outperforms tit-for-tat in the Prisoner's Dilemma Game. Nature (Lond.) 364, 56-58
22. ^ BOYD, R. LORBERBAUM, J.P. (1987). No pure strategy is evolutionarily stable in the repeated Prisoner's Dilemma Game. Nature (Lond.) 327, 58-59

Simply put, koinophilia is a love of the norm.