EVOLUTION OF COALESCENCE TIMES, GENETIC DIVERSITY AND STRUCTURE DURING COLONIZATION

We consider the impact of a colonization process on the genetic divers ity and spatial structure of a geographically subdivided population. A stepping-stone model combined with coalescence theory is used to pred ict the evolution of sequence divergence and genetic parameters. We fi rst derive analytical results for coalescence times in a population un dergoing logistic growth, We next consider a stepping-stone model in w hich demes are successively colonized, starting from a first deme at o ne of the borders of the metapopulation. We use recurrence equations t o calculate coalescence times for two genes chosen either inside the s ame deme or in different demes. This allows us to obtain the distribut ion and the expectation of the coalescence times, and to deduce from t hem the distribution of the average pairwise differences and the evolu tion of F-st. Our results reflect the impact of the founder effect, wh ich becomes stronger as the distance of the deme from the first deme i ncreases. An increase in migration rate or growth rate generally leads to a decrease of the founder effect. F-st (i) increases during the be ginning of the colonization, (ii) decreases when migration creates hom ogenization and (iii) increases again towards an equilibrium value. Th e distributions of pairwise coalescence times or differences between s equences show a peak corresponding to the colonization period. These r esults could help detect former colonization events in natural populat ions. (C) 1997 Academic Press.