174 Increase in Number of Species 



analysis of the siibstages of the Lake Michigan lobe of this glacial 

 stage by Frye and Willman (1960) is highly suggestive that (1) 

 the disjunct Ozark populations which are apparently conspecific 

 with their northeastern relatives became separated during the latter 

 part of the Woodfordian substage, about 15,000 years ago, and (2) 

 the disjunct Ozark populations which appear to be distinct species 

 became separated during the latter part of the Altonian substage, 

 about 28,000 years ago (Fig. 75). 



In several animal groups in North America, conditions brought 

 about by the Wisconsin glaciation seem to have caused a division 

 of certain parent species into southeastern and southwestern 

 daughter populations, the actual separation occurring probably 

 from 35,000 to 50,000 years ago. In certain instances these daughter 

 populations have again come into contact, perhaps only a few 

 thousand years ago, and the results indicate that some of these 

 formerly separated populations have developed into species in- 

 dependent in a genetic sense. One of the best known examples is the 

 two species of narrow-mouthed frogs Microhyla olivacea and M. 

 carolinensis discussed on p. 164 (Fig. 69). 



In beetles of the genus Trox, Olson, Hubbell, and Howden ( 1954) 

 found evidence of the evolution of five allopatric species correlated 

 with Pleistocene events in southeastern North America, especially 

 in the general area of Florida. If their suggested correlations are 

 correct, the youngest species are about 30,000 years old and the 

 older and more distinctive species range up to 500,000 years old. 

 In American cave beetles of the genus Pseudanophthalmus oc- 

 curring in southern Indiana and adjacent Kentucky, Krekeler ( 1958 ) 

 found that with few exceptions closely related species occurred 

 along the same stream system. In two of these exceptions the distri- 

 bution of related species coincided with the course of former 

 streams disrupted during the Pleistocene. He concluded that two 

 species, P. bloomi and F. harheri, evolved from daughter popula- 

 tions dating probably from the Teays river system during Nebraskan 

 or Kansan time and that F. jeanneli may have arisen from a coloniza- 

 tion in late Pliocene time. 



The Olympic salamander Rhijacotriton oJympicus of coastal 

 western North America may present a different example. This 

 species (Stebbins and Lowe, 1951) has a northern subspecies oc- 

 curring chiefly in Washington and a distinctive spotted southern 

 subspecies in northern California and southern Oregon with the 

 large intervening area populated solely by intergrading forms 



