Ecological context shapes hybridization dynamics

Gene exchange among oak species (Quercus) in Europe is known to be pervasive and to complicate population genetic studies of this species complex. A study in this issue of Molecular Ecology adds geographical and stand-level resolution to the patterns of genetic variation among four species and documents the relatively high frequency of hybrids (10.7-30.5% of trees in a population, including hybrids between all pairs of species; Lepais et al. 2009). In addition, the authors show that the relative abundance of parental species affects the genetic composition of hybrids and shifts the average direction of introgression. Variation in the relative abundance of parental species is one example of how the ecological context of hybridization can influence the dynamics and outcome of contact between species and represents an opportunity to investigate the components of reproductive isolation between species. This research raises several questions about the dynamics of hybridization in this well-studied species complex, and highlights methodological and conceptual issues associated with contemporary research on hybridization.