Highly heterogenous patterns of genetic differentiation between two Towhee species.
Take two closely related species that are hybridizing. Collect some samples and sequence their genomes (which is relatively cheap nowadays). Next, compare those genomes. You will notice that some genomic regions are drastically different between both species. Why is this? There are multiple possible explanations. Perhaps, the species are living in distinct habitats and the genomic differences are the outcome of different selection pressures. Or could it be that the genomic regions are involved in reproductive isolation. In hybrids, a particular genomic region might lead to infertility. So, this region is not exchanged between the species and becomes different over time. Or could it be a combination of both?
Sarah Kingston and her colleagues explored this conundrum in two Towhee species: Spotted Towhee (Pipilo maculatus) and Collared Towhee (P. ocai). Why these two species? Because they interbreed in not one, but two Mexican hybrid zones! One zone runs from north to south along the Tezuitlán gradient, while the other zone is orientated east-west along the Transvolcanic gradient. On the one hand, genetic differences that occur in both hybrid zones probably reflect historical divergence and reproductive isolation. On the other hand, genetic differences that are specific to a hybrid zone point to stochastic processes or environment-dependent selection.
There are various ways to characterize genetic differentiation between two species. This study, published in Journal of Evolutionary Biology, relied on a fixation index (Fst) and genomic cline analyses. The latter method should not be confused with geographical cline analysis (you can read more about that method here). I will not go into the details of genomic cline analyses, but interested readers can check out this paper. In short, you compare numerous genes against a model that describes the genomic background of the interbreeding species. Two parameters (alfa and beta) can be estimated to find genes that deviate from this null model. These genes are likely under divergent selection or involved in reproductive isolation.
A Complex History
Applying these two methods (Fst and genomic clines), Sarah Kingston and her colleagues found several genomic regions (6-20 percent) that are different between the Towhee species in both hybrid zones. But there were also many regions that are specific to each hybrid zone. They conclude that “these results are consistent with a history in which reproductive isolation has been influenced by a common set of loci in both hybrid zones, but where local environmental and stochastic factors also lead to genomic differentiation.” A similar pattern was uncovered by a study comparing several crow hybrid zones (see here). I guess the Towhees (and the crows) can change their Facebook-status to ‘it is complicated…’
Kingston, S. E., T. L. Parchman, Z. Gompert, C. A. Buerkle and M. J. Braun (2017). Heterogeneity and concordance in locus-specific differentiation and introgression between species of towhees. Journal of Evolutionary Biology 30(3): 474-485.
This paper has been added to the Emberizidae page.