Caucasian cryptic species of rodents as models in research on the problems of species and speciation

Abstract

Problems of species and speciation have been considered with cryptic rodent species of the Caucasus as models. The Caucasian mountain system is characterized by a pronounced altitudinal gradient of the environmental conditions and insular nature of the present-day mountain habitats. These circumstances provide additional possibilities for choosing the concept of species (biological or phylogenetic) and elucidating the speciation pathways (sympatric or allopatric speciation) for model groups of cryptic species, as well as for testing the “refugia” hypothesis. Sibling Sicista species of the “caucasica” group (a group of unstriped birch mice of the Caucasus) and voles belonging to the subgenus Terricola (Microtus, Arvicolinae) were used as model species. Novel data on the karyotype, nucleotide sequences of the mitochondrial gene cytb, multivariate statistical analysis of odontologic features, and biogeography of sibling Sicista species of the “caucasica” group and pine voles belonging to the subgenus Terricola (Microtus, Arvicolinae) were used to reconstruct the evolutionary history of these species in the Caucasus and analyze the concepts of species applicable to the animals studied. An allopatric distribution consistent with the hypothesis of speciation in refugia is characteristic of modern sibling Sicista species of the “caucasica” group. The sympatry of Terricola sibling species in the Caucasus is assumed to be secondary, with the phenotypic similarity of the species assumed to stem from adaptation to similar environmental conditions. The coexistence of Microtus (Terricola) majori and Microtus (Terricola) daghestanicus in the Caucasus and the absence of hybridization between these species confirms the sibling status of these species within the biological concept of species. The existence of Sicista allospecies in the group of unstriped Caucasian birch mice is in best agreement with the phylogenetic concept of species, but considerable chromosomal differences between sibling species, particularly between the extremes of a single evolutionary series within the group (Sicista kazbegica and Sicista kluchorica), do not contradict the biological concept of species.