Reconstructing hotspots of genetic diversity from glacial refugia and subsequent dispersal in Italian common toads (Bufo bufo)

Genetic diversity feeds the evolutionary process and allows populations to
adapt to environmental changes. However, we still lack a thorough
understanding of why hotspots of genetic diversity are so 'hot'.
Here, we analysed the relative contribution of bioclimatic stability and
genetic admixture between divergent lineages in shaping spatial patterns
of genetic diversity in the common toad Bufo bufo along the Italian
peninsula. We combined population genetic, phylogeographic and species
distribution modelling (SDM) approaches to map ancestral areas, glacial
refugia, and secondary contact zones. We consistently identified three
phylogeographic lineages, distributed in northern, central and southern
Italy. These lineages expanded from their ancestral areas and established
secondary contact zones, before the last interglacial. SDM identified
widespread glacial refugia in peninsular Italy, sometimes located under
the present-day sea-level. Generalized linear models indicated genetic
admixture as the only significant predictor of the levels of population
genetic diversity. Our results show that glacial refugia contributed to
preserving both levels and patterns of genetic diversity across
glacial-interglacial cycles, but not to their formation, and highlight a
general principle emerging in Mediterranean species: higher levels of
genetic diversity mark populations with substantial contributions from
multiple genetic lineages, irrespective of the location of glacial
refugia.