Increase in Number of Species 



161 



Originol ronqe 



Moves in direction 

 of orrows 



Successive 

 steps 



Ronqe now 

 divided 



Fig. 67. Diagram of the mechanics of a simple type of range division. 



(3) After becoming separated, the isolated populations diverge 

 genetically because each will start with at least a slightly different 

 genetic composition due to the geographic variation within the pa- 

 rental species and because continuous spontaneous genetic change 

 would accentuate the original differences. In addition the ecological 

 selection pressures would be different for each daughter population. 

 This stems from the fact that no two separated geographic areas 

 are exactly alike ecologically. If a sufficiently detailed examination 

 is made, differences of several kinds can be detected between such 

 areas, perhaps small differences but differences which are nonethe- 

 less perceptible. Points of dissimilarity might be diurnal or seasonal 

 rhythms of precipitation or temperature, soil types, seasonal changes 

 in length of days, among many other factors. As a result, the direc- 

 tion of selection and the consequent adaptive changes would be at 

 least slightly different in each isolated population. 



Opportunities for genetic change through hybridization might be 

 different in each isolated population. In the mosquitoes Aedes aedes 

 and A. albopictus, hybridization apparently does not occur on the 

 Philippine Islands but can occur between populations from Indo- 

 China. Thus the potential flow of genetic material from one to the 

 other is different in various isolated populations of the two species 

 (Mattingly, 1953). 



