CUMULATIVE EFFECTS OF FOUNDING EVENTS DURING COLONIZATION ON GENETIC DIVERSITY AND DIFFERENTIATION IN AN ISLAND AND STEPPING-STONE MODEL

This paper investigates the cumulative effect of founding events on th e 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 colo nists are taken at random from the entire metapopulation, and second, a linear stepping-stone model, where migrants and colonists are sample d from a limited neighbourhood. The genetic consequences of a range ex pansion depend on the relative magnitudes of the number of colonists a nd 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 differentiatio n and a gradual loss of within-population heterozygosity, also depends on the age-structure that is established during colonisation. It is t he highest when colonists are sampled from recently founded population s and migrants are exchanged among populations of similar ages. The ge netic consequences of a range expansion are therefore far more pronoun ced and lasting in the linear stepping-stone model than in the island model. These two models, however, represent the two extremes between w hich real populations will fall.