Presumable incipient hybrid speciation of door snails in previously glaciated areas in the Caucasus

•Micropontica populations from Lagonaki plateau in the Caucasus are homoploid hybrids.•They form a genetic cluster distinct from the parental species and are characterized by a mtDNA haplotype clade.•These haplotypes are presumably better adapted to high altitude habitats.•They replaced the haplotypes of the parental species in a selective sweep.•The fixation of an adaptive haplotype clade is an important step towards speciation.

Homoploid hybrid speciation, speciation by hybridization without a change in chromosome number, may be the result of an encounter of closely related species in a habitat that is different from that usually occupied by these species. In the northwestern Caucasus the land snail species Micropontica caucasica and M. circassica form two distinct entities with little admixture at low and intermediate altitudes. However, at higher altitudes in the Lagonaki plateau, which were repeatedly glaciated, Micropontica populations with intermediate characters occur. Admixture analyses based on AFLP data demonstrated that the populations from the Lagonaki plateau are homoploid hybrids that now form a cluster separate from the parental species. The Lagonaki populations are characterized by a mtDNA haplotype clade that has been found in the parental species only once. The fixation of this haplotype clade in most hybrid populations suggests that these haplotypes are better adapted to the cooler conditions in high altitude habitats and have replaced the haplotypes of the parental species in a selective sweep. The fixation of a presumably adaptive mitochondrial haplotype clade in the Lagonaki populations is an important step towards speciation under the differential fitness species concept.

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