Collapsing landmasses and secondary contacts: introgression and the evolution of adaptive diversity in Caucasian rock lizards.

Abstract

Caucasian rock lizards (Darevskia) are known for high species diversity, adaptation to a broad range of habitats, and widespread hybridization patterns and gene introgression between the species. We explored the speciation history within a highly diverse "caucasica" clade of this genus by analyzing phylogenies based on the mitochondrial cytochrome b gene in representative samples of 16 described species, and the genome-wide SNP data genotyped using ddRAD-seq in six ingroup taxa. We also inferred geographic expansion pathways, which led to secondary contacts among the evolutionary lineages after periods of isolation. The analyses showed multiple introgression patterns in the secondary contact areas, evident from D-statistics and TreeMix analyses and the discordance between the mitochondrial and nuclear phylogenies. These processes may have shaped ecological niches and phenotypes in the incipient species, particularly in the rock-dwelling, ground-dwelling, and intermediate adaptive phenotypes. The role of introgression during a period between establishing secondary contacts among the diverging lineages and the development of effective prezygotic isolation in the speciation process is discussed.