The role of natural selection in adaptation and differentiation

Studies of genetic differentiation of populations are of critical importance for understanding evolutionary divergence and speciation. The causes of differentiation are generally thought to be natural selection, random genetic drift and restricted migration. However, there is little empirical evidence showing how and to what extent these forces drive population differentiation. We analyze the relative impact of these processes on levels and patterns of DNA sequence variation and differentiation in Drosophila ananassae, a highly subdivided species whose geographic center is in Southeast Asia. In particular, we test the hypothesis that selection for locally favored alleles (i.e. local adaptation) is primarily responsible for population differentiation in D. ananassae. The second part of the talk will about the detection of footprints of natural selection ("selective sweeps") based on a population genomics approach. A newly developed likelihood ratio test will be used to identify selective sweeps along chromosomes. This test allows us to estimate the strength and frequency of selected substitutions as well as the "target sites" of selection in the genome.