Abstract
The interaction between selection and gene flow can determine to what
degree populations are able to adapt to local environmental conditions.
This presents a particular conundrum in marine systems, as many marine
species have high dispersal capacity resulting in nearly panmictic
populations. Increasingly, genomic studies find that even in systems
with little or no population structure divergence at particular loci may
indicate local adaptation in the presence of high gene flow. However we
are just beginning to understand which environmental variables might be
the strongest drivers of selection in marine systems and the functional
outcomes of genetic variants that are candidates for selection. Here, we
leverage fine-scale sampling across the California range of the Pacific
Purple Urchin (Strongylocentrutus purpuratus), a species with previous
evidence of both local adaptation and extremely high gene flow. We find
that despite complete absence of neutral population structure, sea
surface temperature and tidal height drive genetic differences among
populations, suggesting that balanced polymorphisms can lead to
adaptation across both large scale (latitudinal) and small scale
(subtidal v. intertidal) scales. Further, we find that genes that are
expressed at a single tissue or life history stage are more divergent
than expected across both latitudinal and tidal height comparisons,
suggesting that these genes have specific functions that might generate
phenotypic variation important for local adaptation. Together these
results suggest that even in panmictic populations genetic variation can
be sorted across even small spatial scales, potentially resulting in
local adaptation across a complex environmental mosaic.