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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 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 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, a very good 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.
Koinophilia must be distinguished from assortative mating, which means “like prefers like”. If like prefers like it would mean that albino animals, for instance, would be sexually attracted to one another, and, since albinism is not an excessively rare mutation, an albino subspecies would rapidly come into being, as would similar variations of the parent species. Koinophilia predicts that this is unlikely to occur because an albino animal is attracted to the population average in the same way that all the other members of the species are attracted to that average. But none of the other members of the species are attracted to the albino, because of its unusual appearance. Few albinos therefore find mates. This means that they are very unlikely to form albino lineages that might lead to the creation of a new subspecies, or species.
According to Koeslag, koinophilia provides very simple and obvious explanations for such evolutionary puzzles as the process of speciation, evolutionary stasis and punctuated equilibria, and the evolution of cooperation. Koinophilia might also contribute, possibly substantially, to the maintenance of sexual reproduction, preventing its reversion to the much simpler and inherently more advantageous asexual form of reproduction.
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". It was independently identified in humans by Judith Langlois and her coworkers, 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.
Francis Galton, a half cousin of Charles Darwin, created composite portraits of a number 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 rather inexplicable finding in 1878, concluding that average features combine to create good-looking faces.
Despite of the novelty of this finding, Galton’s observations were forgotten until Judith Langlois and Lori Roggman created computer generated composite images in the late 1980s. 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 and face profiles, have subsequently shown that this is a general principle: average faces are consistently more attractive than the faces used to generate them.                      
This principle transcends culture. For instance, Coren Apicella and her co-workers from Harvard University created average faces of an isolated hunter-gatherer tribe in Tanzania in Africa, the Hadza people. Hadza people rated the average Hadza faces as more attractive than the actual faces in the tribe. While Europeans also rated average Hadza faces as attractive, the Hadza people did not express any preference for average European faces. Apicella attributes this difference to the visual experiences of the Europeans and the Hadza tribespeople. While the Hadza had never been exposed to human races outside their immediate environment, the Europeans 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. 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.
That the preference for the average is biological rather than cultural has been supported by a number of studies on babies. Neonates and infants gaze longer at attractive faces than at unattractive faces.       Furthermore, Mark Stauss reported that 10-month-old children respond to average faces in the same way as they respond to attractive faces, and that these infants are able to extract the average from simply drawn faces consisting of only 4 features. Adam Rubenstein and coworkers 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.
Despite these findings, David Perrett and his colleagues at St Andrews University in Scotland 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. However, 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 sexually the most attractive. (European viewers saw no differences between the three female Japanese faces created by David Perrett.)
Speciation and "punctuated equilibria"
A major evolutionary problem has been how the continuous process of evolution produces groups of individuals, labeled 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 inhabiting the same general environment, and hunting much the same prey, in much the same way, but they look extraordinarily different, and would not be confused one for the other even by the most unsophisticated observer. There would seem to be no obvious evolutionary reason which suggests that lion-leopard intermediates are likely to be less successful hunters than either of the two distinct species that inhabit the African savanna today. Why then do they not exist? What evolutionary force drives these intermediate forms to extinction, leaving only highly uniform and distinctive lions on the one hand and highly uniform and distinctive leopards on the other?
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, 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 appearance or form does occur, 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 considerable periods of time. Thus the fossil record, though open to different interpretations, suggests that evolution occurs in bursts, interspersed by long periods of evolutionary stagnation (i.e. by means of 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 creatures concerned. If sexual creatures prefer mates sporting predominantly common features, and avoid mates with unusual, unfamiliar, fringe, or extreme attributes, then common features will tend to become more common still, and at a rate and to an extent that natural selection on its own is unlikely to achieve. Since koinophilia affects the entire external appearance, the members of an interbreeding group will soon all begin to look astoundingly alike, both with regard to important or essential features (e.g. the jaws, dentition, and claws of a lion) and trivial features (e.g. the black furry tuft at the tip of the lion’s tail, or the lion's “beard”). It is almost inevitable that each interbreeding group will, in this way, very quickly develop its own characteristic appearance. An individual from one group who wanders into another group will consequently be recognized as being 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, 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).
Rate of evolution
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 only about 10 thousand years ago. In the light of this extraordinarily rapid rate of evolution, through artificial selection, George C. Williams and others, 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.
If, as a rule, 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 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.)
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, or during the aftermath of a mass extinction. 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. 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. 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 and gorillas.
The evolution of cooperation
Cooperation is any group behavior that benefits the individuals more than if they were to act as independent agents. An inevitable consequence of cooperation is, however, the fact that it can always be exploited by selfish individuals who benefit even more by not taking part in the group activity, but still enjoy 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. 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.
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- 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.