Increase in Number of Species 



163 



these behavioral characters possessed a slightly different mean or 

 mode in each species. The selection would therefore remove the 

 overlapping bands of gene frequencies, producing in each species a 

 new distinctive mode for these characters. Figure 68 shows diagram- 



Origlnal species 

 in isolotion 



bcdef obcde 



Species A Species B 



Mode of genes controlling moting behovior 



Steps in sympotry : 

 Changes in gene 

 frequencies when 

 ranges overlap 



\ Initiol 

 frequencies 



Subsequent 

 frequencies 



50% crossmoting 

 in this range 



50% crossmoting 

 in this range 



Result: Nearly 50% progeny loss 

 from b to e through gametic 

 wostoge. .-. selection for Sp A 

 towards o, for Sp. B towords f. 



Result: 50% progeny loss from c 

 to d, .■- continual selection for Sp 

 A towords 0, for Sp B toward f. 



3 Final 

 frequencies 



Result: Behavior characteristics 

 sufficiently for opart to ovoid 

 cross-moting. Modes hove moved 

 from c to b for Sp A, from d to 

 e for Sp. B 



Fig. 68. Diagram of hypothetical development of sexual isolation through 

 shifts in behavioral characteristics following range overlap between species 

 whose hybrids are infertile. Hypothetical units of behavior are indicated by the 

 letters a through f. Note that originally Species A had its mode at c and 

 Species B at d. 



matically the probable steps in this process. The speed would vary 

 with the extent of cross-mating, the percentage viability of the 

 hybrids, and the differences in behavior pattern at different stages 

 of the process. 



