Climate changes produces distributional shifts and shapes genetic diversity levels of species world-wide. In regions where climate conditions allowed populations to persist (i.e., refugia), high and unique genetic diversity is expected. Where ranges contracted or unfavourable conditions prevailed, diversity is generally lower due to bottlenecks, genetic drift and/or reduced gene flow.

Early studies placed strong emphasis on glacial refugia. But inferring regions of long-term persistence also requires considering the warmer extremes of climate change producing genetic erosion at lower latitudes. Moreover, for marine species able to colonize deeper colder waters, depth shifts might be important for local persistence, preventing some latitudinal shifts during unfavourable sea surface changes.

In a recent study published on Journal of Biogeography we found that the southernmost warmer ranges of a marine forest species are the richest in genetic diversity, indicating long-term persistence of large populations. Analyses performed with machine learning distribution models suggested that this could only have been possible due to stable refugia in deeper reefs; the deep rear edge populations of marine forests play a fundamental role during periods of extreme warmer climate, allowing persistence and retaining some of the largest genetic diversity pools of the species’ distribution.