Increase in Number of Species 



179 



differentiation would be proportional to such selective differences. 

 Relative size of populations would exert some effect on genetic 

 divergence, especially if one population were large and the other 

 small, as in the case of small remnant populations existing in local 

 pockets (Fig. 74). 



Intrinsic genetic mechanisms affecting the rate of becoming 

 genetically incompatible have been observed in several groups. 

 Phylogenetic hues in which genetic change tends to be accompanied 

 by chromosomal change appear to acquire genetic incompatibility 

 between isolated populations faster than those in which chromo- 

 some number and morphology remain constant. Thus, those Dro- 

 sophila species having large numbers of distinctive inversions appear 

 to have a lower interbreeding potential than species of about the 

 same relationship without such differences in chromosome struc- 

 ture (Patterson and Stone, 1952). Keck (1935) and Clausen (1951) 

 have observed examples of this effect in various plant genera. In 



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Fig. 78. Chromosome morphology of four species of the plant genus 

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 Evolution of Plant Species, courtesy of Cornell University Press.) 



