CHAPTER 



21 



NATURAL SELECTION: II 



So far in our study of natural selection as an evolu- 

 tionary force we have stressed its operation upon mutations, emphasizing 

 the importance of heterozygote superiority in maintaining a balanced 

 polymorphism contributive to genetic homeostasis. We have inquired into 

 the role played by geographic-environmental isolation in the instigation of 

 genetic change between populations, and have discussed the attainment of 

 reproductive isolation, in its various forms, as an indication that the species 

 level of differentiation has been reached. We have noted that while ab- 

 sence of hybridization between species is the rule, and is favored by natu- 

 ral selection, at times hybridization may form a means of increasing ge- 

 netic variability and thus of providing grist to the mill of natural selection. 

 In the present chapter we shall continue our inquiry into evolution by 

 natural selection, emphasizing the forces operative in species formation 

 (speciation) as well as in production of the adaptive differences by which 

 the higher categories differ from each other. We shall discuss briefly the 

 ancillary theory of sexual selection, and shall then pull the threads to- 

 gether in an outline of the factors and forces operative in producing evolu- 

 tionary change. Brief discussion of directive forces in evolution and of 

 rates of evolutionary change will conclude the chapter. 



SPECIATION 



Evolution may be divided, conveniently but somewhat 

 arbitrarily, into two types or patterns: (1) phyletic evolution, and 

 (2) speciation. Phyletic evolution means evolution in a line or lineage. 

 Species A, living in a certain region, in the course of time undergoes 

 change so that the descendants are sufficiently unlike their distant ances- 



484 



