Genomic evidence for ecological divergence against a background of population homogeneity in the marine snail Chlorostoma funebralis

The balance between natural selection, gene flow and genetic drift is difficult to resolve in marine invertebrates with extensive dispersal and fluctuating population sizes. The intertidal snail Chlorostoma funebralis has planktonic larvae and previous work using mtDNA polymorphism reported no genetic population structure. Nevertheless, recent studies have documented differences in thermal tolerance and transcriptomic responses to heat stress between northern and southern California, USA, populations. To gain insight into the dynamics influencing adaptive divergence, we used double-digest restriction site-associated DNA (ddRAD) sequencing to identify 1861 genomewide, quality-filtered single-nucleotide polymorphism (SNP) loci for C. funebralis collected from three northern and three southern California sites (15 individuals per population). Considering all SNPs, there was no evidence for genetic differentiation among populations or regions (average FST = 0.0042). However, outlier tests r...