Elucidating the role of genetic drift and natural selection in cork oak differentiation regarding drought tolerance

Drought is the main selection agent in Mediterranean ecosystems and it is been suggested as an important evolutionary force responsible for population diversification in these environments. However, population divergence in quantitative traits can be driven by either directional natural selection, genetic drift or both. To investigate the roles of these forces on among-population divergence in ecophysiological traits related to drought tolerance (Δ13C, SLA, leaf size, Nmass), we compared molecular and quantitative genetic differentiation among thirteen cork oak (Quercus suber L.) populations spanning over a gradient of rainfall and temperature. Population differentiation for height, specific leaf area and leaf nitrogen content far exceeded the molecular differentiation measured by six nuclear microsatellites and was associated with the climatic gradient in among-year rainfall variation. These results suggest (1) these traits are subjected to directional selection and (2) the genetic dif...