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.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.