Cumulative effects of founding events during colonisation on genetic diversity and differentiation in an island and stepping-stone model

This paper investigates the cumulative effect of founding events on the genetic differentiation and the within-population heterozygosity in a metapopulation increasing its size by colonisation. Two contrasting models are considered: first, an island model, where migrants and colonists are taken at random from the entire metapopulation, and second, a linear stepping-stone model, where migrants and colonists are sampled from a limited neighbourhood. The genetic consequences of a range expansion depend on the relative magnitudes of the number of colonists and migrants, in a way similar to extinction and colonisation processes (Wade and McCauley, 1988). The cumulative effect of founding events, resulting most often in a transient increase in genetic differentiation and a gradual loss of within-population heterozygosity, also depends on the age-structure that is established during colonisation. It is the highest when colonists are sampled from recently founded populations and migrants are exchanged among populations of similar ages. The genetic consequences of a range expansion are therefore far more pronounced and lasting in the linear stepping-stone model than in the island model. These two models, however, represent the two extremes between which real populations will fall.