NATURAL SELECTION: I 475 



wastage. We may anticipate, therefore, that species which produced 

 hybrids under these circumstances would be wasting their energies and 

 hence would be at a disadvantage in competition with species that 

 concentrated on production of nonhybrid offspring. 



At this point we come upon a debate of long standing, participated in by 

 Darwin himself in fact (Mayr, 1959). There is abundant observational 

 evidence that isolating mechanisms arise as the more or less incidental 

 accompaniment of adaptive changes mainly concerned with such matters 

 as better adaptation to environment, reduction of competition, and the 

 like. The question is, will natural selection act directly to cause popula- 

 tions to develop isolating mechanisms? Will natural selection favor genetic 

 factors that have as their sole or main effect the production or intensifica- 

 tion of isolating mechanisms (such as hybrid sterility, preferential mating, 

 and the like)? 



We have noted above that one means by which reproductive isolation, 

 and hence failure to hybridize, is achieved is through exercise of "choice" 

 or "preference"' in mating (selective or preferential mating). Koopman 

 (1950) found that he was able to obtain intensification of the tendency 

 to selective mating exhibited by fruit flies of the two sibling species 

 Drosophila pseudoobscura and D. persimilis. These two are so similar 

 that they cannot be told apart by external structure. Yet when hybrids 

 between them are produced, the male hybrids are sterile, and the female 

 hybrids when mated with males of either parent species "lay the usual 

 number of eggs, but the larvae arising from these eggs have such poor 

 viability that in competition with the larvae of the pure species, as in 

 population cages, they never reach the adult state." Obviously, then, such 

 hybrids are worthless to the species, representing true biological wastage. 

 Actually hybrids between pseudoobscura and persimilis have never been 

 found in a state of nature. The species are somewhat isolated from each 

 other ecologically, preferring slightly different environments, but when 

 they do occur together sexual isolation of the selective mating type tends 

 to prevent production of hybrids. Using a modified L'Heritier-Teissier 

 population cage, Koopman demonstrated that in artificial mixed popula- 

 tions of pseudoobscura and persimilis the number of hybrid individuals 

 could be made to decline rapidly by removing the hybrids which were 

 produced. This had the effect of removing from the gene pools of the two 

 species genes contributed by individuals which tended not to mate with 

 members of their own species. Thus the tendency of each species to 

 mate within its own species (homogamic mating) was intensified; con- 

 sequently the number of hybrids produced decreased markedly in the 



