NATURAL SELECTION: I 473 



of the mating. Sperm cells may fail to reach the eggs, or if they reach 

 them may not enter and fertilize them. In plants, pollen tube growth may 

 be arrested before the ovule is reached. (2) Fertilized eggs may be 

 formed, but the hybrid individuals may prove to be inviable. This in- 

 viability may express itself at any time: as soon as the fertilized eggs are 

 formed, early in embryonic development, later in embryonic life, or after 

 birth but before sexual maturity is reached. (3) Hybrids may live to sexual 

 maturity but may be sterile so that they cannot pass on the genes they 

 have received. Mules, hybrids between horse and donkey, form well- 

 known examples of this type of isolation (although rare fertile individuals 

 occur). 



All of the isolating mechanisms we have enumerated, geographic, en- 

 vironmental, and reproductive, may result in isolation which is complete 

 or partial in its effectiveness. And obviously several of them may be 

 operative simultaneously in any given situation to prevent or reduce gene 

 interchange between populations. 



Action of Isolation 



With regard to their function in evolution we may group the three types 

 of isolation into two categories: (1) geographic-environmental; (2) re- 

 productive. The two play different roles in evolution (Mayr, 1959). 



Geographic-environmental isolation causes two populations or sub- 

 populations to be separated so that each goes its own way in acquiring 

 mutations, and in being acted upon by such forces as genetic drift and 

 natural selection. Two geographically separated populations are said to be 

 allopatric. We should note that isolation between two allopatric popula- 

 tions may not always be complete. If it is not, neighboring populations 

 may interbreed in regions in which they come into contact (frequently 

 the case between neighboring subspecies, pp. 320-321). Or individuals 

 may migrate from one population to another. If interbreeding occurs in 

 either of these ways the gene pools of the two populations will not remain 

 as completely separate as they would under conditions of complete isola- 

 tion. The effect of this mingling of genes may be small or large, depending 

 upon the extent to which isolation is incomplete. (See Wright, 1931; also 

 discussion of effects of hybridization, pp. 474-481.) 



We conclude that at least some degree of geographic-environmental 

 isolation is necessary as a first step in the development of genetic diversity 

 between populations. So long as the gene pool remains undivided, two 

 differing populations cannot arise from it. (As an exception to this 



