Introgression as a source of adaptive variation

The capacity for natural populations to adapt to rapid environmental change will depend on standing variation. One often-overlooked process that contributes to standing variation is introgression from other populations or species. For this reason, we investigate the processes of introgression and speciation across diverse taxa. For Swainson’s Thrush (Catharus ustulatus), a migratory songbird, we used demographic analysis of genomic data to study a hybrid zone between two subspecies. We found unexpectedly high levels of unidirectional gene flow at divergent loci, suggesting adaptive alleles that arose during allopatry are leaking across species boundaries (Bay and Ruegg 2017 Proc B). In three-spine stickleback we used morphological and genomic analyses to document coupling of genes encoding mate choice with those under divergent natural selection in a sympatric species pair (Bay et al. 2017 Curr Biol). Finally, we am examining introgression between native cutthroat trout and introduced rainbow trout, looking for parallel signatures of introgression across distant watersheds, which might suggesting a subset of introduced alleles are adaptive in the native species.

Sharing of alleles via introgression could provide the raw material for adaptation to future climate change scenarios. We documented thermal tolerance differences between cryptic species of the coral A. hyacinthus (Rose et al. 2018 Evolution). We are now examining the interaction between large-scale differences in gene expression between the species and differences in allele frequencies. As moderate levels of gene flow still exist between these young species, introgression of key alleles provides opportunity for adaptation to warming.


. Polygenic evolution drives species divergence and climate adaptation in corals. Evolution, 2018.

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. Genetic coupling of female mate choice with polygenic ecological divergence facilitates stickleback speciation. Current Biology, 2017.

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. Genomic islands of divergence or opportunities for introgression?. Proceedings of the Royal Society B, 2017.

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