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Koinophilia is an evolutionary hypothesis concerning sexual selection which proposes that animals seeking mate preferentially choose individuals with a minimum of unusual features. Koinophilia intends to explain the clustering of organisms into species and other issues described by Darwin's Dilemma. The term derives from the Greek, koinos, "the usual", and philos, "fondness".
Natural selection causes beneficial inherited features to become more common and eventually replace their disadvantageous counterparts. A sexually-reproducing animal would be expected to avoid individuals with unusual features, and to prefer to mate with individuals displaying a predominance of common or average features. This means that mates displaying mutant features are also avoided. This is advantageous because most mutations that manifest themselves as changes in appearance, functionality or behavior, are disadvantageous. Because it is impossible to judge whether a new mutation is beneficial or not, koinophilic animals avoid them all, at the cost of avoiding the occasional beneficial mutation. Thus, koinophilia, although not infallible in its ability to distinguish fit from unfit mates, is a good strategy when choosing a mate. A koinophilic choice ensures that offspring are likely to inherit features that have been successful in the past.
Koinophilia differs from assortative mating, where "like prefers like". If like preferred like, leucistic animals (such as white peacocks) would be sexually attracted to one another, and a leucistic subspecies would come into being. Koinophilia predicts that this is unlikely because leucistic animals are attracted to the average in the same way as other animals. Since non-leucistic animals are not attracted by leucism, few leucistic individuals find mates, and leucistic lineages will rarely form.[a]
Koinophilia provides simple explanations for the rarity of speciation (in particular Darwin's Dilemma), evolutionary stasis, punctuated equilibria, and the evolution of cooperation. Koinophilia might also contribute to the maintenance of sexual reproduction, preventing its reversion to the much simpler and inherently more advantageous asexual form of reproduction.
The koinophilia hypothesis is supported by research into the physical attractiveness of human faces by Judith Langlois and her co-workers. They 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. This work into averageness supports koinophilia as an explanation of what constitutes a beautiful face, and how the individuality of a face is recognized.
Speciation and punctuated equilibria
Biologists from Darwin onwards have puzzled over how evolution produces species whose adult members look extraordinarily similar, and distinctively different from the members of other species. Lions and leopards are, for instance, both large carnivores that inhabit the same general environment, and hunt much the same prey, but look quite different. The question is why intermediates do not exist.
This is the "horizontal" dimension of a two-dimensional problem, referring to the almost complete absence of transitional or intermediate forms between present-day species (e.g., between lions, leopards, cheetahs and lynxes). The "vertical" dimension concerns the fossil record. Fossil species are frequently remarkably stable over extremely long periods of geological time, despite continental drift, major climate changes, and mass extinctions. When a change in form occurs, it tends to be abrupt in geological terms, again producing phenotypic gaps (i.e., an absence of intermediate forms), but now between successive species, which then often co-exist for long periods of time. Thus the fossil record suggests that evolution occurs in bursts, interspersed by long periods of evolutionary stagnation in so-called punctuated equilibria. 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 appearance of the animals concerned. Since koinophilia affects the entire external appearance, the members of an interbreeding group are driven to look alike in every detail. Each interbreeding group will rapidly develop its own characteristic appearance. An individual from one group which wanders into another group will consequently be recognized as different, and will be discriminated against during the mating season. Reproductive isolation induced by koinophilia might thus be the first crucial step in the development of, ultimately, physiological, anatomical and behavioral barriers to hybridization, and thus, ultimately, full specieshood. Koinophilia will thereafter defend that species' appearance and behavior 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 an explanation for the existence of many "living fossils" (i.e., creatures that have remained almost unchanged in appearance for, sometimes, hundreds of millions of years, surviving mass extinctions, alternating periods of global warming and glaciation, as well as extensive remodeling of the earth's geography through continental drift).
Effect on rate of evolution
Evolution can be extremely rapid, as shown in the creation of domesticated animals and plants in a very short space of time geologically, spanning only a few tens of thousands of years. Maize, Zea mays, for instance, was created in Mexico in only a few thousand years, starting about 7 000 to 12 000 years ago. This raises the question of why the long term rate of evolution is far slower than is theoretically possible. and others. While a group might benefit from being able to adapt to a new environment by accumulating a wide range of genetic variation, this is to the detriment of the individuals which have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the capability to evolve would have to be by the discredited mechanism of Group selection. John Maynard Smith and Richard Dawkins.
However, if, through koinophilia, sexual creatures avoid mates with strange or unusual characteristics, then mutations that affect the external appearance of their carriers will seldom be passed on to the next and subsequent generations. They will therefore seldom be tested by natural selection. Evolution is, therefore, effectively slowed down or stopped. The only mutations that can accumulate in a population are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "neutral mutations", which can be used to trace the relatedness and age of populations and species.
This brake on evolution through koinophilia implies that evolution can only occur if mutant mates cannot be avoided, as a result of a severe scarcity of potential mates. This is most likely to occur in small, isolated communities. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves, or during the aftermath of a mass extinction, when sudden bursts of evolution can be expected. Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition. If an isolated population such as this survives its genetic upheavals, and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come in being. In geological terms this will be an abrupt event. A resumption of avoiding mutant mates will, thereafter, result, once again, in evolutionary stagnation. Thus the fossil record typically consists of species that suddenly appear and remain unchanged in external appearance until they ultimately disappear.
Evolution of co-operation
Co-operation is any group behavior that benefits the individuals more than if they were to act as independent agents. In consequence, selfish individuals who benefit even more by not taking part in the group activity, but still enjoy its benefits, can exploit the co-operativeness of others. For instance, a selfish individual which does not join the hunting pack and share its risks, but nevertheless shares in the spoils, has a fitness advantage over the other members of the pack. Thus, although a group of co-operative individuals is fitter than an equivalent group of selfish individuals, selfish individuals interspersed amongst a community of co-operators are always fitter than their hosts. They will raise, on average, more offspring than their hosts, and will 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 then becomes evolutionarily stable. Sociability, social conventions, ritualistic behavior, the expressions of the emotions, and other forms of communication between individuals can all be similarly evolutionarily stabilized by koinophilia.
William B. Miller, in an extensive recent (2013) review of koinophilia theory, notes that while it provides parsimonious explanations for the grouping of sexual animals into species, their unchanging persistence in the fossil record over long periods of time, and the phenotypic gaps between species, both fossil and extant, it represents a major departure from the widely accepted view that beneficial mutations spread to, ultimately, all the members of a population (i.e. they become “fixed”, causing that population to evolve). Koinophilia would prevent that happening because all unusualness is discriminated against.
He also notes that koinophilia provides no explanation as to how the physiological, anatomical and genetic causes of reproductive isolation come about. It is only the behavioral reproductive isolation that is addressed by koinophilia. It is furthermore difficult to see how koinophilia might apply to plants, and certain marine creatures that discharge their gametes into the environment to meet up and fuse, it seems, entirely randomly.
The apparent preference of the females of certain, particularly bird, species for exaggerated male ornaments, such as the peacock’s tail, is not easily reconciled with the concept of koinophilia.
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