Distribution and variation of mice group Sylvaemus microps & uralensis in Eastern Europe: fragmentation and clines

The species names Sylvaemus microps and Sylvaemus uralensis are markers of the two key stages in the recognition of the taxonomic heterogeneity of Sylvaemus sylvaticus (s. str.), to which they have long been included. The first was described in 1952 from Central Europe, while the second 140 years earlier (1818) from the Southern Urals. Both taxa have undergone a complicated taxonomic history and are now considered conspecific. However, there are some gaps between them, in particular geographic ones. They are considered here as two groups of populations: a western (microps) and an eastern (uralensis). The taxonomic history of wood mice of the group Apodemus microps & Sylvaemus uralensis and the history of increase in knowledge on their distribution are considered. An analysis of geographic variability of key diagnostic characters of the species was carried out, which revealed a quite well-expressed morphological homogeneity. Within the Western Palearctic populations, there is a clear clinal variation from relatively large southern and eastern to small northern and western forms. Geographic range analysis shows the presence of several relatively isolated fragments of the range, including a clear gap between the western forms of the microps group (Central Europe, the Balkans, and the Western Carpathians region) and the eastern forms of the uralensis group (from the Dnipro region and Baltic states to the Urals, including the Caucasus and Asia Minor). Proposals to recognise the specific level of differentiation between the northern and southern forms seem far too hypothetical, as well as the idea to assign the Caucasian form (ciscaucasicus) to the Central European microps and the Crimean form (baessleri) to the Upper Volga mosquensis. In general, the available data indicate differences between the western and eastern forms of Sylvaemus uralensis. Detailed information on marginal records of the two groups of populations is given, including the easternmost (essentially north-eastern) findings of the microps group and the westernmost findings of the uralensis group. There is a significant geographic gap between these two forms covering the entire area of Volyn, Podillia, Western and Central Polissia, but narrowing to the south, towards the Black Sea. Connection between the two range fragments may exist in the south, but the available and verified data essentially show a gap across the interfluve between the Dnister and Tyligul rivers