Despite the enormous economic and ecological importance of marine organisms, the spatial scales of adaptation and biocomplexity remain largely unknown. Yet, the preservation of local stocks that possess adaptive diversity is critical to the long-term maintenance of productive stable fisheries and ecosystems. Here, we document genomic evidence of range-wide adaptive differentiation in a broadcast spawning marine fish, Atlantic cod (Gadus morhua), using a genome survey of single nucleotide polymorphisms. Of 1641 gene-associated polymorphisms examined, 70 (4.2%) tested positive for signatures of selection using a Bayesian approach. We identify a subset of these loci (n ¼ 40) for which allele frequencies show parallel temperature-associated clines (p , 0.001, r 2 ¼ 0.89) in the eastern and western north Atlantic. Temperature associations were robust to the statistical removal of geographic distance or latitude effects, and contrasted 'neutral' loci, which displayed no temperature association. Allele frequencies at temperature-associated loci were significantly correlated, spanned three linkage groups and several were successfully annotated supporting the involvement of multiple independent genes. Our results are consistent with the evolution and/or selective sweep of multiple genes in response to ocean temperature, and support the possibility of a new conservation paradigm for non-model marine organisms based on genomic approaches to resolving functional and adaptive diversity.

As populations diverge, genomic regions associated with adaptation display elevated differentiation. These genomic islands of adaptive divergence can inform conservation efforts in exploited species, by refining the delineation of management units, and providing genomic tools for more precise and effective population monitoring and the successful assignment of individuals and products. We explored heterogeneity in genomic divergence and its impact on the resolution of spatial population structure in exploited populations of Atlantic cod, Gadus morhua, using genome wide expressed sequence derived single nucleotide polymorphisms in 466 individuals sampled across the range. Outlier tests identified elevated divergence at 5.2% of SNPs, consistent with directional selection in onethird of linkage groups. Genomic regions of elevated divergence ranged in size from a single position to several cM. Structuring at neutral loci was associated with geographic features, whereas outlier SNPs revealed genetic discontinuities in both the eastern and western Atlantic. This fine-scale geographic differentiation enhanced assignment to region of origin, and through the identification of adaptive diversity, fundamentally changes how these populations should be conserved. This work demonstrates the utility of genome scans for adaptive divergence in the delineation of stock structure, the traceability of individuals and products, and ultimately a role for population genomics in fisheries conservation.

Fishes possess both the largest and smallest vertebrate genomes, but the evolutionary significance of this variation is unresolved. The present study provides new genome-size estimates for more than 500 species, with a focus on the cartilaginous and ray-finned fishes. These results confirm that genomes are smaller in ray-finned than in cartilaginous fishes, with the exception of polyploids, which account for much genome-size variation in both groups. Genome-size diversity in ray-finned fishes is not related to metabolic rate, but is positively correlated with egg diameter, suggesting linkages to the evolution of parental care. Freshwater and other eurybiotic fishes have larger genomes than their marine and stenobiotic counterparts. Although genome-size diversity among the fishes appears less clearly linked to any single biological correlate than in the birds, mammals, or amphibians, this study highlights several particularly variable taxa that are suitable for further study.

The nature of species at the extremes of their ranges impinges fundamentally on diverse biological issues, including species’ range dynamics, population variability, speciation and conservation biology. We review the literature concerning genetic and ecological variation at species’ range edges, and discuss historical and contemporary forces that may generate observed trends, as well as their current and future implications. We discuss literature which shows how environmental, ecological and evolutionary factors act to limit species’ ranges, and how these factors impose selection for adaptation or dispersal in peripheral populations exposed to extreme and stochastic biotic and abiotic stressors. When conditions are sufficiently harsh such that local extinction is certain, peripheral populations may represent temporary offshoots from stable core populations. However, in cases where peripheral populations persist at the range edge under divergent or extreme conditions, biologically significant differences can arise from historical and contemporary ecological and evolutionary forces. In many such cases reviewed herein, peripheral populations tended to diverge from the species’ core, and to display lower genetic diversity or greater stress-adaptation. We conclude that while such populations may be of particular conservation value as significant components of intraspecific biodiversity or sources of evolutionary innovation and persistence during environmental change, small and greatly variable population size, especially combined with low genetic variability, can result in elevated extinction risk in harsh and stochastic peripheral environments. As a result, while peripheral populations should not be dismissed as evolutionary dead-ends destined for local extinction, neither should they be uncritically granted inherently superior significance based only on their peripheral position alone.

Cytological and organismal characteristics associated with cellular DNA content underpin most adaptionist interpretations of genome size variation. Since fishes are the only group of vertebrate for which relationships between genome size and key cellular parameters are uncertain, the cytological correlates of genome size were examined in this group. The cell and nuclear areas of erythrocytes showed a highly significant positive correlation with each other and with genome size across 22 cartilaginous and 201 ray-finned fishes. Regressions remained significant at all taxonomic levels, as well as among different fish lineages. However, the results revealed that cartilaginous fishes possess higher cytogenomic ratios than ray-finned fishes, as do cold-water fishes relative to their warm-water counterparts. Increases in genome size owing to ploidy shifts were found to influence cell and nucleus size in an immediate and causative manner, an effect that persists in ancient polyploid lineages. These correlations with cytological parameters known to have important influences on organismal phenotypes support an adaptive interpretation for genome size variation in fishes.