CHAPTER 19 



POPULATION GENETICS AND 



EVOLUTIONARY CHANGE 



In Chapter 17 we discussed some of the principles of 

 Mendelian inheritance revealed by experiments in which matings were 

 arranged by the experimenter. What happens when members of a popula- 

 tion are free to interbreed as they may, rather than according to some plan 

 of an experimenter? In our discussion of this question we shall refer con- 

 stantly to a unit which we shall call "the population." We shall use this 

 term to mean a Mendelian population, defined by Dobzhansky (1950) as 

 "a reproductive community of sexual and cross-fertilizing individuals 

 which share in a common gene pool." The term deme is sometimes used 

 for such a population, but since the term is also applied to local communi- 

 ties characterized in other ways than that of reproduction within the group 

 we shall not employ it. 



GENETIC EQUILIBRIUM 



It will be convenient to begin our discussion by referring 

 again to the melanistic hamsters (pp. 376-379). As the result of the cross 

 diagramed in Fig. 17.1 (p. 377) an F2 generation was produced consisting 

 of 14 homozygous black {MM), % heterozygous black {Mm), and 14 

 homozygous gray {mm). If these Fj. individuals interbreed at random what 

 types of olTspring will be expected in the next {F.^,) generation, and in what 

 proportions will the various types be expected to occur? For present pur- 

 poses we shall assume that there is nothing about the melanistic condition 

 which affects an individuaKs mating and that no preference is exercised de- 



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