Widespread yet heterogeneous genomic divergence.

Genetic differentiation during adaptive divergence and speciation is heterogeneous among genomic regions. Some regions can be highly differentiated between populations, for example, because they harbour genes under divergent selection or those causing reproductive isolation and thus are resistant to gene flow. Other regions might be homogenized by gene flow and thus weakly differentiated. Debates persist about the number of differentiated regions expected under divergence with gene flow, and their causes, size, and genomic distribution. In this issue of Molecular Ecology, a study of freshwater stickleback used next-generation sequencing to shed novel insight into these issues (Roesti et al. 2012). Many genomic regions distributed across the genome were strongly differentiated, indicating divergence with gene flow can involve a greater number of loci than often thought. Nonetheless, differentiation of some regions, such as those near the centre of chromosomes where recombination is reduced, was strongly accentuated over others. Thus, divergence was widespread yet highly heterogeneous across the genome. Moreover, different population pairs varied in patterns of differentiation, illustrating how genomic divergence builds up across stages of the speciation process. The study demonstrates how variation in different evolutionary processes, such as selection and recombination rate, can combine to result in similar genomic patterns. Future work could focus on teasing apart the contributions of different processes for causing differentiation, a task facilitated by experimental manipulations.

Divergent host-plant use promotes reproductive isolation among cynipid gall wasp populations.

Ecological speciation occurs when reproductive isolation evolves as a consequence of divergent natural selection among environments. A direct prediction of this process is that ecologically divergent pairs of populations will exhibit greater reproductive isolation than ecologically similar pairs of populations. By comparing allopatric populations of the cynipid gall wasp Belonocnema treatae infesting Quercus virginiana and Quercus geminata, we tested the role that divergent host use plays in generating ecological divergence and sexual isolation. We found differences in body size and gall structure associated with divergent host use, but no difference in neutral genetic divergence between populations on the same or different host plant. We observed significant assortative mating between populations from alternative host plants but not between allopatric populations on the same host plant. Thus, we provide evidence that divergent host use promotes speciation among gall wasp populations.

Functional Genomics Offers New Tests of Speciation Hypotheses.

Speciation is a fundamental process shaping biodiversity. However, existing empirical methods often cannot provide key genetic and functional details required to validate speciation theory. New gene modification technologies can verify the causal functionality of genes with astonishing accuracy, helping resolve questions about how reproductive isolation evolves during speciation.

Transitions between phases of genomic differentiation during stick-insect speciation.

Speciation can involve a transition from a few genetic loci that are resistant to gene flow to genome-wide differentiation. However, only limited data exist concerning this transition and the factors promoting it. Here, we study phases of speciation using data from >100 populations of 11 species of Timema stick insects. Consistent with early phases of genic speciation, adaptive colour-pattern loci reside in localized genetic regions of accentuated differentiation between populations experiencing gene flow. Transitions to genome-wide differentiation are also observed with gene flow, in association with differentiation in polygenic chemical traits affecting mate choice. Thus, intermediate phases of speciation are associated with genome-wide differentiation and mate choice, but not growth of a few genomic islands. We also find a gap in genomic differentiation between sympatric taxa that still exchange genes and those that do not, highlighting the association between differentiation and complete reproductive isolation. Our results suggest that substantial progress towards speciation may involve the alignment of multi-faceted aspects of differentiation.