| Summary: | Phylogeographic studies provide critical insight into the evolutionary histories of model organisms; yet, to date, range-wide data are lacking for the rough periwinkle Littorina saxatilis, a classic example of marine sympatric speciation. I utilized mitochondrial DNA (mtDNA) sequence data to demonstrate that L. saxatilis is not monophyletic, but is comprised of two mtDNA lineages (I and II) that are shared with its sister-species L. arcana and L. compressa. Bayesian coalescent dating and phylogeographic patterns indicate that both L. saxatilis lineages (I and II) originated in the northeastern Atlantic, around the British Isles, approximately 0.64 Ma. Both lineages are now distributed broadly across the eastern, central, and western North Atlantic, and show strong phylogeographic structure among regions. The Iberian Peninsula is genetically distinct, suggesting prolonged isolation from northeastern Atlantic populations. Western Atlantic populations of L. saxatilis Lineages I and II both predate the last glacial maximum and have been isolated from eastern Atlantic populations since that time. The existence of two broadly distributed cryptic mtDNA lineages further complicate observed patterns of repeated incipient ecological speciation in L. saxatilis, because the sympatric origins of distinct ecotype pairs on northeastern Atlantic shores may be confounded by admixture of divergent lineages.
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