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

Koinophilia is an evolutionary hypothesis which proposes that mate-seeking animals preferentially chose individuals with a minimum of unusual features. Koinophilia intends to explain the clustering of organisms into species and other issues described by Darwins 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 to replace their disadvantageous counterparts. A sexual creature 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 what constitutes a beautiful face,[8][9][10][11][12][13][14] how the individuality of a face is recognized,[3][4] speciation (in particular Darwin's Dilemma),[1][15] evolutionary stasis and punctuated equilibria,[1][2][15] and the evolution of cooperation.[16][17] 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.[18][19]

As a mating strategy it was identified in humans by Judith Langlois and her coworkers,[8][9][11][13] who found that the average of two human faces[20] 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.[21] In this context “koinophilia” is generally known as Averageness.

Physical attractiveness[edit]

Main article: Averageness

Francis Galton, a half cousin of Charles Darwin, created composite portraits of convicted criminals, hoping to generate a prototypical criminal face. Surprisingly, the composite portrait became more and more attractive with the addition of each new face. Galton published this finding in 1878, concluding that average features combined to create good-looking faces.[22]

A University of Toronto study found that the facial proportions of celebrities including Jessica Alba were close to the average of all female profiles.[23]

Despite the novelty of this finding, Galton's observations were forgotten until Judith Langlois and Lori Roggman[8] created computer generated composite images in 1990. They found that facial attractiveness increased in proportion to the number of faces that went into creating the composite. Many studies, using different averaging techniques, including the use of line drawings[24] and face profiles,[25] have subsequently shown that this is a general principle: average faces are consistently more attractive than the faces used to generate them.[26][27][28][29] Furthermore if a female composite (averaged) face made of 32 different faces is overlain with the face of an extremely attractive female model, the two images often line up closely, indicating that the model's facial configuration is very similar to the composite's.[30] See, for example, the illustration of Jessica Alba on the left.[27][28]

Transcending culture: Hadza people rated averaged Hadza faces as more attractive than actual faces from the tribe.

This principle transcends culture. For instance, Coren Apicella and her co-workers from Harvard University[31] created average faces of an isolated hunter-gatherer tribe of 1,000 in Tanzania, Africa, the Hadza people. Hadza people rated the averaged Hadza faces as more attractive than the actual faces in the tribe. While Europeans also rated average Hadza faces as attractive, the Hadza people expressed no preference for average European faces. Apicella[31] attributes this difference to the wider visual experiences of the Europeans, as they had been exposed to both Western and African faces. Thus the indifference of the Hadza towards average European faces could have been the result of lacking the European norm in their visual experience.[3] These results suggest that the rules for extracting attractive faces are culture-independent and innate, but the results of applying the rules depend on the environment and cultural experience.[32][28]

That the preference for the average is biological rather than cultural has been supported by studies on babies, who gaze longer at attractive faces than at unattractive ones.[33][34][35] Furthermore, Mark Stauss[36] reported that 10-month-old children respond to average faces in the same way as they respond to attractive faces, and that these infants can extract the average from simply drawn faces consisting of only 4 features. Adam Rubenstein and coworkers[37] showed that already at six months of age, children not only treat average faces the same as they treat attractive faces, but they are also able to extract the central tendency (i.e. the average) from a set of complex, naturalistic faces presented to them (i.e. not just the very simple 4-features faces used by Strauss). Thus the ability to extract the average from a set of realistic facial images operates from an early age, and is therefore almost certainly instinctive.[36][37]

Despite these findings, David Perrett and his colleagues[32] found that both men and women considered that a face averaged from a set of attractive faces was more appealing than one averaged from a wide range of women's faces. When the differences between the first face and the second face were slightly exaggerated the new face was judged, on average, to be more attractive still. The three faces are difficult to distinguish one from the other, although close examination shows that the so-called "exaggerated face" looks slightly younger than the average face (composed of women's faces aged 22–46 years). Since the same results were obtained using Japanese subjects and viewers, these findings are probably culture-independent, indicating that people generally find youthful average faces[38] sexually the most attractive.[32]

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,[39] and a great evolutionary mystery.[40][41] 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. [42][43]

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.[40][41]

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 [44][45], referring to the almost complete absence of transitional or intermediate forms between present-day species (e.g., between lions, leopards, cheetahs and lynxes).[40][46][47] 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.[48][49] 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.[48] Why this is so, has been one of evolution's great mysteries.[49]

Koinophilia could explain both the horizontal and vertical manifestations of speciation, and why it usually involves the entire external appearance of the creatures concerned.[1][2][15] Since koinophilia affects the entire external appearance, the members of an interbreeding group are driven to look alike in every detail.[50][51] 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. This koinophilia-induced reproductive isolation 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][15]), 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]

Rate of evolution[edit]

Some domestic animals that are significantly different from their wild ancestors
Pekin Ducks.
Domestic turkeys
Zebu cattle
Domestic rabbit
Domestic pigeons
Gold fish

Humans have created a wide range of new species, and varieties within those species, of both domesticated animals and plants in a very short geological period of time, spanning only a few tens of thousands of years, and sometimes less. Maize, Zea mays, for instance, is estimated to have been created in what is now known as Mexico in only a few thousand years, starting between about 7 000 and 12 000 years ago, from still uncertain origins.[52] In the light of this extraordinarily rapid rate of evolution, through (prehistoric) artificial selection, George C. Williams[53] and others,[48][54][55] have remarked the following:

The question of evolutionary change in relation to available geological time is indeed a serious theoretical challenge, but the reasons are exactly the opposite of that inspired by most people’s intuition. Organisms in general have not done nearly as much evolving as we should reasonably expect. Long term rates of change, even in lineages of unusual rapid evolution, are almost always far slower than they theoretically could be. The basis for such expectation is to be found most clearly in observed rates of evolution under artificial selection, along with the often high rates of change in environmental conditions that must imply rapid change in intensity and direction of selection in nature.[53]

Evolution is imposed on species or groups. It is not planned or striven for in some Lamarckist way.[56] The mutations on which the process depends are random events, and, except for the "silent mutations" which do not affect the functionality or appearance of the carrier, are thus usually disadvantageous, and their chance of proving to be useful in the future is vanishingly small. Therefore, while a species or 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 who have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the capability to evolve is a group adaptation, a concept discredited by (for example) George C. Williams,[57] John Maynard Smith[58] and Richard Dawkins[59][60][61][62] as selectively disadvantageous to the individual. .

If sexual creatures avoid mates with strange or unusual characteristics,[1][2][5][8] 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 halted or slowed down considerably. 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, and are, used to trace the relatedness and age of populations and species.[1][63])

This 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,[64] or during the aftermath of a mass extinction.[63] 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.[64] Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. 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.[63][65][66] 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.[66]

Hominin species distributed through time edit

Homo Australopithecus Ardipithecus Paranthropus Homo sapiens Homo neandertalensis Homo heidelbergensis Homo erectus Paranthropus robustus Paranthropus boisei Paranthropus aethiopicus Homo ergaster Homo habilis Australopithecus sediba Australopithecus garhi Australopithecus africanus Australopithecus bahrelghazali Australopithecus afarensis Australopithecus anamensis Orrorin tugenensis Sahelanthropus Pleistocene Pliocene Miocene

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

Evolution of cooperation[edit]

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. In consequence, selfish individuals who benefit even more by not taking part in the group activity, but still enjoy its benefits, can exploit the cooperativeness 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 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. They will raise, on average, more offspring than their hosts, and will ultimately replace them.[59][60][61][62]

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.[16][17]

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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  21. ^ averaging process the more attractive and appealing the average face became
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External links[edit]

  • Beauty Check includes example blended faces and discusses why average face shapes are more attractive.
  • Averaging faces shows how the average of two faces looks more attractive than either of the faces used in the averaging process.