A peculiar case is the Colorado alpine, the nominate subspecies of Erebia callias. This isolated Rocky Mountains population has been lumped with the Siberian brassy ringlets as they are almost alike morphologically. Though one might suspect stronger differentiation and perhaps marked cryptic speciation across the wide range, the Rocky Mountains population is apparently a very recent isolate. Its ancestors apparently crossed over the Bering Strait at the end of the Wisconsinian glaciation, about 15,000 to 10,000 years ago. Thus, brassy ringlets are present on the North American continent quite exactly for the same length of time as a significant human population.