FOR 1078: Natural Selection in Structered Populations

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The overarching goal of this Research Unit is the study of natural selection in realistic population settings. Because of its role as a driving force of adaptation, understanding natural selection is undoubtedly one of the most important objectives of evolutionary biology. Although we have generally learned a lot about natural selection in the past two decades, very recent investigations have shown that reliable inference and detailed analysis of selective events depend critically on our knowledge of the population or species in which selection occurred. For example, the effects of natural selection and of population subdivision and demography on the genome are difficult to distinguish if only single (or a small number of) genes are analysed. This difficulty arises because the effects of positive directional selection and population size expansion on the patterns of polymorphism are very similar, as are those of balancing selection and population subdivision with limited gene flow. However, the situation has drastically improved with the advent of population genomics methods. The notion is that (strong) natural selection usually occurs at individual genes, while the structure of a population (subdivision or demography) affects the entire genome. Thus, our ability to survey a large number of loci from the same organism greatly facilitates the study of selection in natural populations. The model underlying population genetic analyses is usually the standard equilibrium model (the so-called Wright-Fisher model). Yet, most natural populations deviate in some way from the standard equilibrium model of population genetics. One important assumption of this model is random mating (panmixia). As Gillespie emphasises, however, "a species whose range exceeds the distance an individual moves in its lifetime cannot possibly mate at random. Departures from random mating can have profound consequences on the evolutionary dynamics of a species." There are several ways in which a population might deviate from random mating. In this Research Unit, we will concentrate on population subdivision. That is, we take into account that individuals within a local population (deme) mate randomly but that mating between demes may be restricted if migration is limited. Another important assumption of the Wright-Fisher model is that natural populations are stable over long periods of time. However, numerous investigations have shown that this assumption is often violated. Thus, in this Research Unit we focus on two important departures from the Wright-Fisher model: population subdivision and demographic history.