Dynamics of genetic differentiation in experimental <i>Drosophila</i> metapopulations:Unravelling the interplay of selection and migration

As a result of the fragmentation of their habitats, many organisms live in small and relatively isolated populations. In my thesis, I investigate the interplay of migration and local adaptation. Migration promotes genetic exchange among populations, thus contributing to the maintenance of genetic variation. Moreover, migration is a homogenising process that reduces the differences between local populations. Local adaptation (the adaptation of populations to their local circumstances) occurs when the direction of natural selection differs between habitats (diversifying selection). Hence, local adaptation results in the diversification of populations. By means of Drosophila experiments and computer simulations, I surveyed the interrelation between migration and local adaptation. I found that diversifying selection strongly reduces the efficiency of migration. In the presence of local adaptation, migrants are not well-adapted to their new environment and therefore less able to spread their genes to future generations. This applies to all their genes, and not only to the genes involved in local adaptation. Hence, despite considerable migration, local populations become genetically isolated (‘isolation-by-adaptation’). In the presence of regular extinction and recolonisation, migration is more efficient in preventing genetic isolation. However, efficient migration is often associated with the loss of local adaptation and therefore may reduce population viability.Already under the highly standardized conditions of my lab experiments and my simulations the interplay between migration and local adaptation turned out to be very intricate, with many surprising facets. This pleads for caution when making inferences and predictions in natural settings