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This leucistic peacock is unlikely to reproduce because it is unlikely to find a mate.

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.[1][2][3][4] 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.[5] 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.[1]

Koinophilia differs from assortative mating,[6][7] 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),[1][8] evolutionary stasis, punctuated equilibria,[1][2][8] and the evolution of cooperation.[9][10] 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.[11][12]

The koinophilia hypothesis is supported by research into the physical attractiveness of human faces by Judith Langlois and her co-workers.[13][14][15][16] They found that the average of two human faces[17] 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.[18] This work into averageness[13][14][19][15] supports koinophilia as an explanation of what constitutes a beautiful face,[20][16][21] and how the individuality of a face is recognized.[3][4]

Speciation and punctuated equilibria[edit]

The overwhelming impression of strict uniformity, involving all the external features of the adult members of a species, is illustrated by this herd of Springbok, Antidorcas marsupialis, in the Kalahari Desert. This homogeneity in appearance is typical, and virtually diagnostic, of almost all species,[22] and a great evolutionary mystery.[23][24] Darwin emphasized individual variation, which is unquestionably present in any herd such as this, but is extraordinarily difficult to discern, even after long-term familiarity with the herd.[25][26]

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.[23][24]

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

This is the "horizontal" dimension of a two-dimensional problem,[27][28] referring to the almost complete absence of transitional or intermediate forms between present-day species (e.g., between lions, leopards, cheetahs and lynxes).[23][29][30] 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.[31][32] 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.[31] Why this is so has been one of evolution's great mysteries.[32]

Koinophilia could explain both the horizontal and vertical manifestations of speciation, and why it usually involves the entire external appearance of the animals concerned.[1][2][8] Since koinophilia affects the entire external appearance, the members of an interbreeding group are driven to look alike in every detail.[33][34] Each interbreeding group will rapidly develop its own characteristic appearance.[2] 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[1][8]), 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).[1]

Rapid emergence of reproductive isolation on exposure of populations to different diets and environmental conditions[edit]

The best-documented creations of new species in the laboratory were performed in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.[35] The history of such attempts is described in Rice and Hostert (1993).[36][37]

Male Drosophila pseudoobscura

Diane Dodd used a laboratory experiment to show how reproductive isolation can evolve in Drosophila pseudoobscura fruit flies after several generations by placing them in different media, starch- and maltose-based media.[38]

Drosophila speciation experiment.svg

Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods.[39]

A map of Europe indicating the distribution of the carrion and hooded crows on either side of a contact zone (white line) separating the two species.

The carrion crow (Corvus corone) and hooded crow (Corvus cornix) are two closely related species whose geographical distribution across Europe is illustrated in the accompanying diagram. It is believed that this distribution might have resulted from the glaciation cycles during the Pleistocene, which caused the parent population to split into isolates which subsequently re-expanded their ranges when the climate warmed causing secondary contact.[40][41] Poelstra and coworkers sequenced almost the entire genomes of both species in populations at varying distances from the contact zone to find that the two species were genetically identical, both in their DNA in its expression (in the form of RNA), except for the lack of expression of a small portion (<0.28%) of the genome (situated on avian chromosome 18) in the hooded crow, which imparts the lighter plumage coloration on its torso.[40] Thus the two species can viably hybridize, and occasionally do so at the contact zone, but the all-black carrion crows on the one side of the contact zone mate almost exclusively with other all-black carrion crows, while the same occurs among the hooded crows on the other side of the contact zone. It is therefore clear that it is only the outward appearance of the two species that inhibits hybridization.[40][41] The authors attribute this to assortative mating, the advantage of which is not clear, and it would lead to the rapid appearance of streams of new lineages, and possibly even species, through mutual attraction between mutants. Unnikrishnan and Akhila[42] propose, instead, that koinophilia is a more parsimonious explanation for the resistance to hybridization across the contact zone, despite the absence of physiological, anatomical or genetic barriers to such hybridization.

Effect on rate of evolution[edit]

Main article: Rate of evolution
Plants and domestic animals and can differ markedly from their wild ancestors
Top: wild teosinte; middle: maize-teosinte hybrid; bottom: maize
More detailed version of diagram on left by geographical area.


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.[43] This raises the question of why the long term rate of evolution is far slower than is theoretically possible.[31][44][45][46]

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 involved a form of Group selection, which has been discredited by, among others, George C. Williams,[47] John Maynard Smith[48] and Richard Dawkins.[49][50][51][52]

However, if, through koinophilia, sexual creatures avoid mates with strange or unusual characteristics,[1][2][5][13] 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.[1][53]

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, caves,[54] or during periods of glaciation,[40] or following mass extinctions, when sudden bursts of evolution can be expected.[53] 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.[54] 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 of an evolutionary progression typically consists of species that suddenly appear, and ultimately disappear, in many cases close to a million years later, without any change in external appearance.[53][55][56] Graphically, these fossil species are represented by horizontal lines, whose lengths depict how long each of them existed. The horizontality of the lines illustrates the unchanging appearance of each of the fossil species depicted on the graph. During each species' existence new species appear at random intervals, each also lasting many hundreds of thousands of years before disappearing without a change in appearance. The exact relatedness of these concurrent species is generally impossible to determine. This is illustrated in the following diagram depicting the evolution of modern humans from the time that the Hominins separated from the line that led to the evolution of our closest living primate relatives, the chimpanzees.[56]

Distribution of Hominin species over time

For similar evolutionary time lines see, for instance, the paleontological list of African dinosaurs, Asian dinosaurs, the Lampriformes and Amiiformes.

Evolution of co-operation[edit]

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

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.[49][50][51][52]

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.[1] 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.[9][10]


William B. Miller,[2] 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,[57][58][59] is not easily reconciled with the concept of koinophilia.

See also[edit]


  1. ^ An exception could be where leucism offers a major selective advantage, as might happen in snow covered landscapes, where they might readily become a majority in a low density population.


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