164 



Increase in Number of Species 



An illustration of the process of removing overlapping bands oc- 

 curs in two nearctic frogs belonging to the genus Microhyh ( Blair, 

 1955). The ranges of these two frogs, Microhyla carolinensis and 

 M. olivacea, overlap along a narrow band in Oklahoma and Texas. 

 In this area of overlap the species hybridize in at least some localities 

 (Fig. 69). The chief factor which tends to reduce hybridizing seems 



Fig. 69. Ranges of the two frogs Microhyla olivacea and M. 

 (Adapted from Hecht and Matalas 1946, and Blair 1955.) 



carolinensis. 



to be differences in mating call. Through a study of mating calls 

 Blair found a most interesting phenomenon. In the area of overlap 

 and adjoining areas east and west of it, the calls of the two species 

 differ greatly in the average of both frequency and duration and 

 differ completely when these two factors are considered as a unit 

 (Fig. 70). In distant non-overlapping parts of the two ranges, how- 

 ever, the calls of the two species are almost identical. 



A reconstruction of the past history of these two species indicates 

 that a parent Microhyla species became divided during the Wisconsin 

 glacial advance into a southeastern population which evolved into 

 M. carolinensis and a southwestern population which exolved into 

 M. olivacea. Following glacial retreat these two species spread north- 

 westward and northeastward, respectively, eventually meeting in 

 or near the present zone of overlap. Until the time of meeting the 



