Analysis of control region mtDNA haplotype data shows two major lineages. The Arctic and the subarctic migratory population includes the lesser sandhill cranes. The other lineages can be divided into a migratory and some indistinct clusters which can be matched to the resident subspecies. The lesser and greater sandhill cranes are quite distinct, their divergence dating to roughly 2.3–1.2 million years ago , some time during the Late Pliocene or Early Pleistocene. Glaciation seemingly fragmented off a founder population of lesser sandhill cranes, because during each major ice age, its present breeding range was frozen year-round. Still, sandhill cranes are amply documented from fossil and subfossil remains right to the modern era. Conceivably, they might be considered distinct species already, a monotypic "G. canadensis" and the greater sandhill crane, "G. pratensis", which would include the other populations.