Gene flow, adaptive population divergence and comparative population structure across loci

Many recent studies have sought to identify targets of diversifying selection by testing the neutral expectation that all loci will show similar levels of population divergence (F-st). Contrasts between quantitative traits (Q(st)) and molecular markers (F-st) suggest that quantitative traits typically diverge in response to local selection pressures more than do individual genes. Coalescence theory makes it possible to simulate the distribution of F-st and Q(st) expected under neutrality for many situations, including nonequilibrium conditions. Such simulations show that a very high variance of F-st and Q(st) are expected under neutrality, making it difficult to draw firm conclusions about the action of selection on individual loci or traits. Recent quantitative genetic theory shows that, under diversifying selection on quantitative traits, covariances (linkage disequilibrium) among allele frequencies at underlying additive loci contribute a substantial fraction of the among-population trait variance. Thus, adaptive trait divergence can be accomplished, with limited divergence of allele frequencies. However, the contribution of covariances among loci to the divergence of traits depends upon there being multiple loci underlying quantitative trait variation. (C) New Phytologist (2003).