Shared and nonshared genomic divergence in parallel ecotypes of Littorina saxatilis at a local scale

Parallel speciation occurs when selection drives repeated, independent adaptive divergence that reduces gene flow between ecotypes. Classical examples show parallel speciation originating from shared genomic variation, but this does not seem to be the case in the rough periwinkle (Littorina saxatilis) that has evolved considerable phenotypic diversity across Europe, including several distinct ecotypes. Small wave' ecotype snails inhabit exposed rocks and experience strong wave action, while thick-shelled, crab' ecotype snails are larger and experience crab predation on less exposed shores. Crab and wave ecotypes appear to have arisen in parallel, and recent evidence suggests only marginal sharing of molecular variation linked to evolution of similar ecotypes in different parts of Europe. However, the extent of genomic sharing is expected to increase with gene flow and more recent common ancestry. To test this, we used de novo RAD-sequencing to quantify the extent of shared genomic divergence associated with phenotypic similarities amongst ecotype pairs on three close islands (<10km distance) connected by weak gene flow (Nm similar to 0.03) and with recent common ancestry (<10000years). After accounting for technical issues, including a large proportion of null alleles due to a large effective population size, we found similar to 8-28% of positive outliers were shared between two islands and similar to 2-9% were shared amongst all three islands. This low level of sharing suggests that parallel phenotypic divergence in this system is not matched by shared genomic divergence despite a high probability of gene flow and standing genetic variation.