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, 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.
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.
In keeping with these theoretical considerations, one study on an isolated human population, as opposed to Western subjects, has suggested that preferences for averageness appear to be universal.[14] The isolated people preferred average faces from their own race, but did not show any preference for average faces of other races. This makes sense since they are not exposed to outside races and thus have no knowledge of what an average face from another race looks like. This suggests that it is averageness alone, that is making a face attractive rather than some other artifact that results from averaging techniques [14]. Many studies have confirmed that subjects find young average faces the most attractive.[7][14][15][16][17][18] However, Perrett et al.[15] found that both men and women considered that a face averaged from a set of attractive faces was more attractive 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. Although the three faces look remarkably similar, close examination of the photos in Perrett, May and Yoshikawa's paper[15] shows, in fact, that the exaggerated face looks younger than the average face (composed of women's faces aged 22–46 years). 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.
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[19]. 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?
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] 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”) [28]. 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.[1][2]), and stabilizing selection.
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.
Co-operation or co-operative behaviours are terms used to describe behaviours by organisms which are beneficial to other organisms, and are selected for on that basis.[1] Under this definition, altruism is a form of co-operation in which there is no direct benefit to the actor (the organism carrying out the behaviour).[1] Co-operative behaviour in which there is a direct benefit to the actor as well as the recipient can be termed "mutually beneficial".[1] There are several theories which help to explain why natural selection favours some types of co-operative behaviour. They are not mutually exclusive, however, and more than one of the theories discussed below may contribute to explaining a particular case of co-operative behaviour. Note: This article uses British English. Standard U.S. English drops the hyphen resulting in cooperation or cooperative.
One well accepted explanation for altruistic behaviour (that is, co-operative behaviour which lacks a direct benefit for the actor) is the theory of kin selection. This theory suggests that individuals act co-operatively in order to help others which are genetically similar. Genes for such co-operative behaviour are preserved, because they help to perpetuate their own existence. The classic example is the social insects, such as bees and ants. Worker insects never reproduce, but instead, they work to allow the (genetically similar) queen to reproduce.
The theory of reciprocity suggests that individuals carry out co-operative behaviours because they get something in return. In order for such behaviours to be favoured, there needs to be some perception of external physical markers that the other individual will recognise (otherwise, there is no selective pressure to maintain the behaviour). Much research about reciprocity as leading to co-operation has concentrated on the 'prisoner's dilemma' known from game theory.
One theory suggesting a mechanism that could lead to the evolution of co-operation is the "market effect" as suggested by Noe and Hammerstein.[2] The mechanism relies on the fact that in many situations there exists a trade-off between efficiency obtaining a desired resource and the amount of resources one can actively obtain. In that case, each partner in a system could benefit from specializing in producing one specific resource and obtaining the other resource by trade. When only two partners exist, each can specialize in one resource, and trade for the other. Trading for the resource requires co-operation with the other partner and includes a process of bidding and bargaining.
This mechanism can be relied to both within a species or social group and within species systems. It can also be applied to a multi-partner system, in which the owner of a resource has the power to choose its co-operation partner. This model can be applied in natural systems (examples exist in the world of apes, cleaner fish, and more). Easy for exemplifying, though, are systems from international trading. Arabic countries control vast amounts of oil, but seek technologies from western countries. These in turn are in need of Arab oil. The solution is co-operation by trade.
Multi-level selection theory suggests that selection operates on more than one level: for example, it may operate at an atomic and molecular level in cells, at the level of cells in the body, and then again at the whole organism level, and the community level, and the species level. Any level which is not competitive with others of the same level will be eliminated, even if the level below is highly competitive. A classic example is that of genes which prevent cancer. Cancer cells divide uncontrollably, and at the cellular level, they are very successful, because they are (in the short term) reproducing very well and out competing other cells in the body. However, at the whole organism level, cancer is often fatal, and so may prevent reproduction. Therefore, changes to the genome which prevent cancer (for example, by causing damaged cells to act co-operatively by destroying themselves) are favoured. Multi-level selection theory contends that similar effects can occur, for example, to cause individuals to co-operate to avoid behaviours which favour themselves short-term, but destroy the community (and their descendants) long term.
