The glacial and interglacial cycles of the Quaternary were particularly important in shaping the distribution of marine species, with range expansions and contractions responding to climate oscillations.
The Last Glacial Maximum was one of the most extreme periods of the Quaternary, resulting in severe reductions in the northern distribution of cold-temperate species, due to the advance of massive ice sheets. As climate conditions ameliorated throughout the Holocene, persisting populations (within refugia) or in extended southern ranges, gradually recolonized northward regions.
Climate-driven range shifts can leave distinct footprints on the genetic diversity levels of natural populations. Particularly, regions where populations persisted for the long-term often display high genetic diversity and unique gene pools. In contrast, range shifts can leave small and isolated marginal populations, in which drift and bottlenecks may reduce genetic diversity.
In a recent study we describe and explain the causes of geographical contrasts in genetic diversity and their consequences for the future baseline of the global genetic pool. We compared present geographic distribution of genetic diversity and differentiation with predictive distribution models during past extremes, present-time and future climate scenarios for a brown alga, Fucus vesiculosus.
Machine learning species distribution modelling explained the distribution and structure of present genetic diversity, consisting of differentiated genetic pools with maximal diversity in areas of long-term persistence. Most of the present species range comprises postglacial expansion zones and, in contrast to highly dispersive marine organisms, expansions involved only local fronts, leaving distinct genetic pools at rear edges. Besides unravelling a complex phylogeographical history and showing congruence between genetic diversity and persistent distribution zones, supporting the hypothesis of niche conservatism, range shifts and loss of unique genetic diversity at the southern rear edge of the species were predicted for future climate scenarios, impoverishing the global gene pool.