IMa estimates of effective population sizes, migration, and divergence time between regional groups of Littorina saxatilis.

θ is effective population size (θ = 4Nμ) for first (θ1) and second (θ2) populations in the model and their ancestral population (θA); migration rates m1→2/μ and m2→1/μ are from first to second and second to first population in the model, respectively; divergence time is converted to thousands years (ka) using mutation rates of 2 and 4% per Ma. Values in parentheses represent 90% HPD confidence intervals for each parameter (see Table S3 for probability analysis of significant differences between parameters).Groups of populations are as following (see map in Fig. 1):NEA – Northeast Atlantic; ISL - Northeast Atlantic islands; NWA – Northwest Atlantic; NEA(S) includes NEA sites grouped in SAMOVA with North Atlantic islands (Ireland-1, Norway, Barents Sea, White Sea, Sweden-2, Germany, Belgium and France); NWA(S) includes NWA populations grouped in SAMOVA with North Atlantic islands (Cape Breton in Nova Scotia and Newfoundland); NEA(MAIN) includes mainland sites in Europe; BI includes all British Isles sites; CAN includes all Maritime Canada sites; USSOUTH includes southern US sites below 43°N; EUROPE(S) includes European sites (British Isles and Sweden) that formed a group with Venice in SAMOVA analyses; SPAIN is the site in Galicia; VENICE is the Venetian lagoon site.