Molecular phylogenetic analyses showed that alpine newts split into a western and an eastern group. Each of these again contains two major lineages, which in part correspond to described subspecies (see section Distribution and subspecies above).[1][19] These ancient genetic differences suggest that the alpine newt may be a complex of several distinct species.[1][3][16] Higher temperatures during the Miocene or sea level oscillations may have separated early populations, leading to allopatric speciation, although admixture and introgression between lineages probably took place. Populations from Vlasina Lake in Serbia have mitochondrial DNA that is distinct from and more ancient than that of all other populations; it may have been inherited from a now extinct "ghost" population.[1] The Quaternary glaciation probably led to cycles of retreat into refugia, expansion and range shifts.[1][20]