The influence of migration on the maintenance of assortative mating

Rapid speciation has been shown to be plausible without the need for extreme founder events, complete geographical isolation, the existence of distinct adaptive peaks or selection for local adaptation. However, standard theory predicts that extremely low migration rates are enough to hinder divergence between populations, and thus speciation. In this study we asked how low migration rates need to be for divergence to occur and hence for speciation to be possible. We experimentally transferred individual seed beetles, Callosobruchus maculatus, between populations in the laboratory, thus mimicking different rates of migration, and used deviations from Hardy–Weinberg equilibrium as an indicator of assortative mating. We found that assortative mating was upheld for several generations in populations experiencing immigration rates of up to 8% or 13–15 immigrants per generation, despite the lack of adaptive divergence and trade-offs between the exchanging populations. However, after some generations of extensive gene flow and in the absence of selection against hybrids, the system of assortative mating faltered. Based on our results, we conclude that selection is likely to be an important factor in speciation in the face of gene flow and that without it divergence will simply come to a halt.