Isolation by distance in the Atlantic cod, Gadus morhua, at large and small geographic scales

Genetic isolation by distance (IBD) has rarely been described in marine spe cies with high potential for dispersal at both the larval and adult life-hi story stages. Here, we report significant relationships between inferred le vels of gene flow and geographic distance in the Atlantic cod, Gadus morhua , at 10 nuclear restriction-fragment-length-polymorphism (RFLP) loci at sma ll regional scales in the western north Atlantic region (< 1600 km) that mi rror those previously detected over its entire geographic range (up to 7300 km). Highly significant allele frequency differences were observed among e ight northwestern Atlantic populations, although the mean F-ST for all 10 l oci was only 0.014. Despite this weak population structuring, the distance separating populations explained between 54% and 62% of the variation in ge ne flow depending on whether nine or 10 loci were used to estimate Nm. Acro ss the species' entire geographic range, highly significant differences wer e observed among six regional populations at nine of the 10 loci (mean F-ST = 0.068) and seven loci exhibited significant negative relationships betwe en gene flow and distance. At this large geographic scale, natural selectio n acting in the vicinity of one RFLP locus (GM798) had a significant effect on the correlation between gene flow and distance, and eliminating it from the analysis caused the coefficient of determination to increase from 17% to 62%. The role of vicariance was assessed by sequentially removing popula tions from the analysis and was found to play a minor role in contributing to the relationship between gene flow and distance at either geographic sca le. The correlation between gene flow and distance detected in G. morhua at small and large spatial scales suggests that dispersal distances and effec tive population sizes are much smaller than predicted for the species and t hat the recent age of populations, rather than extensive gene flow, may be responsible for its weak population structure. Our results suggest that int erpreting limited genetic differences among populations as reflecting high levels of ongoing gene how should be made with caution.