Evidence of selection in sympatry between two closely related species of rockfish

Ecological or reproductive barriers can maintain species boundaries by preventing introgression in closely related taxa when their distributions overlap (sympatry). In sympatry, sister-taxa may have greater genetic divergence compared to allopatric parts of their range. When analyzing populations of one of the sister-taxa, this divergence may translate to greater genetic differentiation between sympatric and allopatric populations of this species range. This genetic differentiation can be caused by either genetic drift or natural selection, depending on the evolutionary history of secondary contact. To identify a selective process, it is critical to find genes responsible for maintaining species barriers in sympatry. Here, the role of natural selection in genetic differentiation was examined within two recently diverged rockfish species, Sebastes diaconus and Sebastes mystinus. These species overlap along over 400 km of coastline in the eastern Pacific, with no evidence of hybridization in this sympatric region. This study found evidence of geographic genetic differentiation across a large span of the S. diaconus range, but not within the S. mystinus range. For both species, outlier loci were identified that were associated with regions of the genome under directional selection in allopatric versus sympatric populations. This study also found signals of directional selection in shared genomic regions of both species, including in genes involved in neurological function, photoreception, and gene regulation. This suggests that the evolutionary process of reinforcement maintains species boundaries when the two species come into secondary contact.