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Koinophilia is a term used by biologist Johan Koeslag, meaning that when sexual creatures seek a mate, they prefer that mate not to have any unusual, peculiar or deviant features.

Natural selection results, over the course of generations, in beneficial (or "fit") features replacing their disadvantageous counterparts. 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. 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.

According to Koeslag, Koinophilia provides very simple and obvious explanations for such evolutionary puzzles 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][6].



This mating strategy, was first referred to as koinophilia by Johan H. Koeslag[2], from the Greek, koinos, meaning "the usual" or "common", and philos, meaning "fondness" or "love". It was independently identified in humans by Judith Langlois,[7 ][8][9][10][11][12][13] 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.

Physical attractiveness

In keeping with these theoretical considerations, some studies have found that subjects find young average faces the most attractive.[7 ][14][15][16 ][17] However, Perrett et al.[14] 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[14] 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 youthful average female faces sexually the most attractive,[7 ] as expected.

Speciation and "punctuated equilibria"

The striking uniformity of the outward appearances of all the adult members of a species is a great evolutionary mystery.

A major evolutionary problem has been how the continuous process of evolution produces the morphologically discontinuous groups labeled species, whose adult members look extraordinarily similar, and distinctively different from the members of other species.

Speciation poses a "2-dimensional" problem. The phenotypic discontinuities between existing species represent the "horizontal dimension" of the problem. The succession of fossil species represent the "vertical dimension".

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. 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. Thus the fossil record, though open to different interpretations, suggests that evolution occurs in bursts, interspersed by long periods of stasis (i.e. by means of punctuated equilibria[18]). Why this is so, has been one of evolution's great mysteries.

Koinophilia could explain both the horizontal and vertical manifestations of speciation, and why it usually involves the entire external phenotype.[1][2] If sexual creatures prefer mates sporting predominantly common features, and avoid mates with unusual, unfamiliar, fringe, or extreme attributes, then common features tend 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.[1][2]), and stabilizing selection.

The evolution of cooperation

Cooperative hunting by wolves allows them to tackle much larger and more nutritious prey than any individual wolf could handle. However, such cooperation could, potentially, be exploited by selfish individuals who do not expose themselves to the dangers of the hunt, but nevertheless share in the spoils.

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. For instance, a selfish individual who does not join the hunting pack and its incumbent dangers but nevertheless shares in the spoils has a fitness advantage over the other members of the pack. Thus, although a group of cooperative individuals is fitter than an equivalent group of selfish individuals, selfish individuals interspersed amongst a community of cooperators are always fitter than their hosts. This means they raise, on average, more offspring and grandoffspring 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. [5][6] Sociability, social conventions, ritualistic behavior, the 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.



  1. ^ a b c d Koeslag, J.H. (1995). On the engine of speciation. J. theor. Biol. 177, 401-409
  2. ^ a b c d Koeslag, J.H. (1990). Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour. J. theor. Biol. 144, 15-35
  3. ^ 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. ^ Koeslag, J.H., Koeslag, P.D. (1993). Evolutionarily stable meiotic sex. J. Heredity 84, 396-399
  5. ^ a b Koeslag, J.H. (1997). Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation. J. theor. Biol. 189, 53--61
  6. ^ a b Koeslag, J.H. (2003). Evolution of cooperation: cooperation defeats defection in the cornfield model. J. theor. Biol. 224, 399-410
  7. ^ a b c LANGLOIS, J.H. & ROGGMAN, L. (1990). Attractive faces are only average. Psychol. Sci. 1, 115-121
  8. ^ LANGLOIS, J.H., ROGGMAN, L.A., MUSSELMAN, L., ACTON, S. (1991). A picture is worth a thousand words: Reply to "On the difficulty of averaging faces." Psychological Science 2, 354-357.
  9. ^ LANGLOIS, J.H., ROGGMAN, L.A., MUSSELMAN, L. (1994). What is average and what is not average about attractive faces? Psychological Science 5, 214-220
  10. ^ LANGLOIS, J.H., MUSSELMAN, L. (1995). The myths and mysteries of beauty. In D.R. Calhoun (Ed.), 1996 Yearbook of Science and the Future , pp. 40-61. Chicago: Encyclopedia Britannica, Inc.
  11. ^ KALICK, S.M., ZEBROWITZ, L.A., LANGLOIS, J.H., JOHNSON, R.M. (1998). Does human facial attractiveness honestly advertise health? Longitudinal data on an evolutionary question. Psychological Science,9, 8-13
  12. ^ RUBENSTEIN, A.J., LANGLOIS, J.H., ROGGMAN, L.A. (2002). What makes a face attractive and why: The role of averageness in defining facial beauty. In G. Rhodes & L.A. Zebrowitz (Eds.), Facial attractiveness: Evolutionary, cognitive, and social perspectives: Westport, CT: Ablex
  13. ^ HOSS, R.A., LANGLOIS, J.H. (2003). Infants prefer attractive faces. In O. Pascalis & A. Slater (Eds.), The development of face processing in infancy and early childhood: Current perspectives pp. 27-38. New York: Nova Science Publishers.
  14. ^ a b c PERRETT D.I. MAY, K.A. & YOSHIKAWA S. (1994) .Facial shape and judgments of female attractiveness. Nature (Lond) 368, 239-242
  15. ^ ETCOFF, N. (1994). Beauty and the beholder. Nature (Lond) 368, 186-187
  16. ^ ENQUIST , M & GHIRLANDA, S. (1998). The secret of faces. Nature (Lond) 394, 826-827
  17. ^ PERRETT D.I. et al. (1998). Effects of sexual dimorphism on facial attractiveness. Nature (Lond) 394, 884-887
  18. ^ ELDREDGE, N., GOULD, S.J., (1972). "Punctuated equilibria: an alternative to phyletic gradualism" In SCHOPF, T.J.M., ed., Models in Paleobiology. San Francisco: Freeman Cooper. pp. 82-115. Reprinted in ELDREDGE, N. (1985) Time frames. Princeton: Princeton Univ. Press.

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