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Determinants of genetic diversity
Hans Ellegren1 and Nicolas Galtier2
Abstract | Genetic polymorphism varies among species and within genomes, and has important implications for the evolution and conservation of species. The determinants of this variation have been poorly understood, but population genomic data from a wide range of organisms now make it possible to delineate the underlying evolutionary processes, notably how variation in the effective population size (Ne) governs genetic diversity. Comparative population genomics is on its way to providing a solution to Lewontins paradox the discrepancy between the many orders of magnitude of variation in population size and the much narrower distribution of diversity levels. It seems that linked selection plays an important part both in the overall geneticdiversity of a species and in the variation in diversity within the genome. Genetic diversityalso seems to be predictable from the life history of a species.
The distinct individuals of a given species are not genet ically identical. Their DNA sequences differ to some extent, and these differences form the genetic diversity, known as polymorphism, of a species. Ever since the earliest empirical population genetic studies were able to demonstrate a wealth of polymorphism in many species1,2, thus settling a midtwentieth century debate about the extent of genetic variation in natural popu lations (BOX1), the variable nature of gene and genome sequences has been identified as a key aspect of biologi cal evolution. In particular, genetic diversity contributes to the ability of a species to respond to environmental changes, with implications in terms of, for example, human health3,4, breeding strategies in cropsand farm animals5, management of infectious diseases6 and conservation of endangered species7,8.
It was recognized early from data on allozymes that genetic diversity differs considerably among species, and this observation was subsequently confirmed by DNA sequence data (BOX2). For instance, approximately 3% of the genome of a fruitfly, Drosophila simulans, is vari able9,10, whereas only 0.1% of a human genome shows allelic variation11,12. Remarkably, the reasons for such a variation in diversity have in large part been mysterious
(BOX2). Polymorphism also varies substantially across loci and chromosomes, as demonstrated in various species of plants13,14, fungi15 and animals9,16,17.
From a theoretical viewpoint, genetic diversity can be thought of as...
