EVOLUTIONARY STATUS OF THE ROCKY MOUNTAIN POPULATION OF WOOD FROGS.

Abstract

The Wood frog, Rana sylvatica Le Conte, has an extensive distribution which ranges from Alaska south and eastward through much of Canada and the northeastern United States; a population assigned to R. s. cantabrigensis by Maslin (1947) occurs in the mountains of northcentral Colorado and southeastern Wyoming. Martof and Humphries (1959) studied the geographical variation in morphology of this species and concluded that no subspecific taxon was warranted. These workers noted, however, that the Mountain type, represented in their study by 12 specimens from southeastern Wyoming, 16 from northcentral Colorado, and two questionable ones from Yellowstone County, Montana, was most obviously different from the others. They assumed that Rana sylvatica is also distributed in the intervening areas of the Bitterroots and Rockies and, on their various maps, the Mountain type extends from Montana to northern Colorado and is continuous with the populations in Canada. No locality data were given to support such a distribution, nor are any specimens known from the area between the Medicine Bow Mountains in southeastern Wyoming to the northern tip of Idaho (see Figure 1 for known localities in Colorado and Wyoming). On the basis of habitat distribution alone, the Rocky Mountain population is certainly relict and isolated by a considerable distance from its kind in the north. This southern disjunct population undoubtedly reflects a southward displacement in the Wisconsin and subsequent return northward of these cold-adapted frogs after the close of the Wisconsin. Given sufficient time, speciation as a result of geographical isolation seems inevitable, but scanty data are ava.ilable bearing on ages of species or rates of divergence of sister populations. In a previous study of the Dakota Toad (Porter, 1968), I found that a measureable amount of divergence has occurred between the relict population in Wyoming and its kind in the Dakotas and Manitoba, but breeding experiments indicated the genetic divergence has not been sufficient to affect fertility rates or viability of progeny from interpopulational crosses. Presumably, the southern relict populations of Dakota Toads and Wood Frogs have been separated from their parental populations for about the same length of time. Thus, the purpose of the present study was to use both morphological and nonmorphological approaches to compare the Colorado-Wyoming Wood frogs with those in southern Canada, to determine the stage of speciation of this relict population, and to compare the results with those from the toad study.