Constraints on ecological speciation?

“Constraints on speciation suggested by comparing lake-stream stickleback divergence across two continents” – just appeared in Molecular Ecology (19:4963-4978). This paper tests whether the striking phenotypic (foraging morphology) and neutral genetic (microsatellite) divergence that characterizes incipient speciation across lake-stream transitions in Canadian (Vancouver Island) stickleback fish can also be found in some very young (150 years or less) European lake-stream pairs. The main findings are that, first, morphological divergence is generally much lower in the European population pairs, and there are striking overall phenotypic differences between the continents. Although alternative explanations are possible, it seems likely that there are some genetic constraints. Alleles that serve in adaptive lake-stream divergence (e.g. in body shape) in Canada appear to be absent in Europe. Limited time for morphological adaptation might also play a role. Second, there is only trivial divergence in microsatellite frequencies in the European pairs, contrary to the very high divergence found in some Canadian watersheds. This result suggests high gene flow and no evolved reproductive barriers in the young European systems. However, individual-based simulations tailored to these systems reveal that, even if reproductive barriers were absolute, it would be unlikely to see them with neutral markers. Those markers just evolve too slowly. It is thus possible that despite weak morphological divergence, reproductive isolation in Europe pairs could be strong, and hence we are simply dealing with the earliest stages of speciation. This could be possible if isolation is mediated by other traits not studied (e.g. behavior). Follow-up work is examining genome-wide divergence in these lake-stream pairs. This post was contributed by Daniel Berner.