Increase in Number of Species 



165 



5000- 



2200 



1.5 2.0 2.5 



DURATION IN SECONDS 



Fig. 70. Comparison of Microhyla mating calls by polygons constructed from 

 scatter diagrams. A, carolinensis from the overlap zone; B, Texas-Oklahoma 

 carolinensis from east of the overlap zone; C, Texas-Oklahoma olivacea from 

 west of the overlap zone; D, olivacea from the overlap zone; E, carolinensis 

 from Florida; F, presumed hybrids from the overlap zone; G, olivacea from 

 Arizona. (After Blaii-. ) 



mating calls of the two species were presumably only slightly dif- 

 ferent, M. carolinensis as in E (Fig. 70), and M. olivacea as in G. 

 Because of selection against hybrid individuals, the differences be- 

 tween the calls of the two species became intensified and now, in 

 the overlap area, the call of M. carolinensis (A) is lower and shorter 

 than that of M. olivacea (D). 



In a complex of seven Australian species of the frog genus Crinia, 

 Littlejohn (1959) found evidence of a similar history of call dif- 

 ferentiation. Of the seven species, four have become sympatric and 

 in these the calls are sharply different. Only average differences 

 occur between the calls of allopatric species. 



