Myopia as a latent phenotype of a pleiotropic gene positively selected for facilitating neurocognitive development, and the effects of environmental factors in its expression

Abstract

Myopia has become an almost pandemic problem in many populations. There are compelling evidence to suggest that myopia is a hereditary condition. However, myopia would constitute a definite selection disadvantage during most stages of human evolution, which is incompatible with its moderate to high prevalence in most modern populations. The rapid upsurge of myopia over just a few decades also implies that its inheritance does not follow any of the usual patterns, and environmental factors may have an important role in precipitating its occurrence in those who are genetically predisposed. Previous studies showed that myopes were, on average, more intelligent than non-myopes, and this association had been attributed to a biological link between eye growth and brain development. We propose a pleiotropic genetic model to explain the atypical epidemiologic and inheritance pattern of myopia and its relationship with neurocognitive development. This pleiotropic gene was positively selected for its facilitation of human intelligence. The myopic component is a latent phenotype; myopia will not be expressed unless some novel external factors are encountered (i.e. a "quirk" phenomenon). Therefore, the myopic component was selectively neutral in our ancestral environment. The net gain in Darwinian fitness enables the pleiotropic gene to attain a high frequency in the human population, as reflected by our current prevalence of myopia. © 2005 Elsevier Ltd. All rights reserved.