Genetic variation across species’ geographical ranges: the central–marginal hypothesis and beyond

TLDR

Researchers are increasingly studying genetic structure across species’ ranges to explain stable limits and conservation value, yet most work relies on the untested assumption that peripheral populations have lower diversity and higher differentiation due to smaller effective sizes and isolation, and few studies have examined mechanisms, phylogeographic history, or trait variation underlying these patterns. The authors reviewed 134 studies on 115 species that examined within‑population diversity declines and among‑population differentiation increases toward range margins using nuclear markers. Across 134 studies, about two‑thirds found the predicted diversity decline and 70% of those also showed increased differentiation, with a positive correlation between the two, but the magnitude of diversity differences was generally small and taxonomic/biogeographic biases limit broad generalization.

Abstract

There is growing interest in quantifying genetic population structure across the geographical ranges of species to understand why species might exhibit stable range limits and to assess the conservation value of peripheral populations. However, many assertions regarding peripheral populations rest on the long-standing but poorly tested supposition that peripheral populations exhibit low genetic diversity and greater genetic differentiation as a consequence of smaller effective population size and greater geographical isolation relative to geographically central populations. We reviewed 134 studies representing 115 species that tested for declines in within-population genetic diversity and/or increases in among-population differentiation towards range margins using nuclear molecular genetic markers. On average, 64.2% of studies detected the expected decline in diversity, 70.2% of those that tested for it showed increased differentiation and there was a positive association between these trends. In most cases, however, the difference in genetic diversity between central and peripheral population was not large. Although these results were consistent across plants and animals, strong taxonomic and biogeographical biases in the available studies call for a cautious generalization of these results. Despite the large number of studies testing these simple predictions, very few attempted to test possible mechanisms causing reduced peripheral diversity or increased differentiation. Almost no study incorporated a phylogeographical framework to evaluate historical influences on contemporary genetic patterns. Finally, there has been little effort to test whether these geographical trends in putatively neutral variation at marker loci are reflected by quantitative genetic trait variation, which is likely to influence the adaptive potential of populations across the geographical range.