Genetics and Evolution in Structured Populations

Abstract Structured populations, or metapopulations, are sets of small populations tied together by migration, extinction, and recolonization. Because of the importance of population structure, the genetics of metapopulations cannot be modeled as simple extensions of standard single population systems. Genetic evolution, typically defined as changes in allele frequency (Dobzhansky 1937; see Futuyma 1998 for a more complete discussion), is normally considered to occur as a result of exactly four forces: selection, mutation, migration, and genetic drift. Within a single population only the first two, selection and mutation, are the primary forces of interest. Migration necessarily involves more than a single population, and becomes the cohesive force that distinguishes a metapopulation from a set of independently evolving populations. Genetic drift also takes on special meaning in a metapopulation. Whereas in a single population the consequences of small population size are limited to the effects on variation within demes, in metapopulations the effects of small deme size on the variation among demes also becomes an important area of study. Although generally considered as a force within single populations, selection also needs to be considered specifically within the context of metapopulations since selection, especially when it is coupled with genetic drift, may have very different consequences in a metapopulation compared with the well-understood effects it has in unstructured populations.