Abstract
BackgroundHuman populations and breeds of domestic animals are composed of individuals with a multiplicity of eye (= iris) colorations. Some wild birds and mammals may have intraspecific eye color variability, but this variation seems to be due to the developmental stage of the individual, its breeding status, and/or sexual dimorphism. In other words, eye colour tends to be a species-specific trait in wild animals, and the exceptions are species in which individuals of the same age group or gender all develop the same eye colour. Domestic animals, by definition, include bird and mammal species artificially selected by humans in the last few thousand years. Humans themselves may have acquired a diverse palette of eye colors, likewise in recent evolutionary time, in the Mesolithic or in the Upper Paleolithic.Presentation of the hypothesisWe posit two previously unrecognized hypotheses regarding eye color variation: 1) eye coloration in wild animals of every species tends to be a fixed trait. 2) Humans and domestic animal populations, on the contrary, have eyes of multiple colors. Sexual selection has been invoked for eye color variation in humans, but this selection mode does not easily apply in domestic animals, where matings are controlled by the human breeder.Testing the hypothesisEye coloration is polygenic in humans. We wish to investigate the genetics of eye color in other animals, as well as the ecological correlates.Implications of the hypothesisInvestigating the origin and function of eye colors will shed light on the reason why some species may have either light-colored irises (e.g., white, yellow or light blue) or dark ones (dark red, brown or black). The causes behind the vast array of eye colors across taxa have never been thoroughly investigated, but it may well be that all Darwinian selection processes are at work: sexual selection in humans, artificial selection for domestic animals, and natural selection (mainly) for wild animals.
Highlights
Human populations and breeds of domestic animals are composed of individuals with a multiplicity of eye (= iris) colorations
Sexual selection has been invoked for eye color variation in humans, but this selection mode does not apply in domestic animals, where matings are controlled by the human breeder
Animal coloration has fascinated evolutionary ecologists ever since Darwin and Wallace debated about the contribution of natural and sexual selection to the evolution of ornamental characters
Summary
Animal coloration has fascinated evolutionary ecologists ever since Darwin and Wallace debated about the contribution of natural and sexual selection to the evolution of ornamental characters. We will contend that this variation in humans and our domestic animals is recent in evolutionary time This is evident with domestic animals whose ancestors underwent artificial selection in the Mesolithic or the Neolithic less than 10,000 years ago. The genetic variant for blue eyes has been retrieved from DNA in hunter-gatherer remains at the Motala site in Sweden and Loschbour in Luxembourg, dated to 8000 years BP [6,7,8], and in Mesolithic Europeans who lived in the Iberian Peninsula 7000 years ago [9] All this evidence based on DNA suggests that multi-colored eyes emerged in Homo sapiens after their westward expansion in Europe [10], where they replaced the Neanderthals about 40,000 years ago [11]. That in those species for which the ancestor is extinct, including cattle and horses, we can chart the diversification of eye color over time only by retrieving ancient DNA from remains
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