Isolation by Distance in the Atlantic Cod, Gadus Morhua, at Large and Small Geographic Scales

Abstract

Abstract. Genetic isolation by distance (IBD) has rarely been described in marine species with high potential for dispersal at both the larval and adult life‐history stages. Here, we report significant relationships between inferred levels of gene flow and geographic distance in the Atlantic cod, Gadus morhua, at 10 nuclear restriction‐fragment‐length‐polymorphism (RFLP) loci at small regional scales in the western north Atlantic region (< 1600 km) that mirror those previously detected over its entire geographic range (up to 7300 km). Highly significant allele frequency differences were observed among eight northwestern Atlantic populations, although the mean FST for all 10 loci was only 0.014. Despite this weak population structuring, the distance separating populations explained between 54% and 62% of the variation in gene flow depending on whether nine or 10 loci were used to estimate Nm. Across the species' entire geographic range, highly significant differences were observed among six regional populations at nine of the 10 loci (mean FST= 0.068) and seven loci exhibited significant negative relationships between gene flow and distance. At this large geographic scale, natural selection 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 populations from the analysis and was found to play a minor role in contributing to the relationship between gene flow and distance at either geographic scale. The correlation between gene flow and distance detected in G. morhua at small and large spatial scales suggests that dispersal distances and effective population sizes are much smaller than predicted for the species and that the recent age of populations, rather than extensive gene flow, may be responsible for its weak population structure. Our results suggest that interpreting limited genetic differences among populations as reflecting high levels of ongoing gene flow should be made with caution.

Limits to gene flow in a cosmopolitan marine planktonic diatom

“…Hence, our data strongly suggest that isolation by large distances allows for genetic divergence, even in high-dispersal organisms like planktonic diatoms. This is in line with other marine organisms, where similar patterns of significant IBD were observed from regional to global scales (23,(50)(51)(52).…”

Disruption of historical patterns of isolation by distance in coastal steelhead

“…salmonids (Palstra et al 2007), including California steelhead (Garza et al 2004;Pearse et al 2007), cod (Pogson et al 2001), oysters (Peterson and Denno 1998), insects (Rose et al 2006), and plants (CruseSanders and Hamrick 2004), but see Pearse et al (2006) for an exception in turtles). However, without a historical reference point, there is no way to know if the populations are in migration-drift equilibrium or representative of their natural state.…”

Population genetic structure of coral reef species Plectorhinchus flavomaculatus in South China Sea

“…Tropical Pacific sea urchins also show a pattern of isolation by distance, in which F ST measured with mitochondrial sequence data increases markedly with geographic distance between pairs of populations (Palumbi et al, 1997). Isolation by distance was also detected in Atlantic cod (Gadus morhua) and coral reef species Australian barramundi (Lates calcarifer) (Pogson et al, 2001;Chenoweth et al 1998).…”