LECTURES IN BIOLOGICAL SCIENCES 



ing barriers are evident from a great wealth of observational and 

 experimental evidence, which is summarized in part by Steb- 

 bins (1950), Dobzhansky (1951), and Mayr (1957). If all of the 

 barriers separating two related species are studied, they are 

 found to be of various sorts, and any two species are usually 

 isolated by many different kinds of barriers. Sometimes the 

 species can be easily hybridized artificially, and fertile offspring 

 can be raised under human supervision, but they nevertheless 

 fail to hybridize in nature because of differences in their breed- 

 ing seasons, mating instincts, or similar factors. The distinct- 

 ness of various species of pines in California is of this nature 

 (Stebbins, 1950), as is also the separation of the mallard from 

 the pintail duck. In other instances, as in the wild rye (Elymus) 

 genus of grasses and its relatives (Stebbins, 1959a), natural hy- 

 brids are common, but they are so sterile that they rarely, or 

 never, reproduce. More common, however, are examples in 

 which two related species are separated from each other by a 

 variety of barriers. A classic example is Drosophila pseudo- 

 obscura and D. persimilis, which are separated by different tem- 

 perature requirements for maximum sexual activity, an instinc- 

 tive tendency for females to mate with males of their own 

 species, sterility of Fj hybrids (which is complete in the male 

 sex), and weakness or sterility of backcross progeny from the 

 partly fertile F x females (Dobzhansky, 1951). 



We can also see how diverse and complex the reproductive 

 isolating barriers between two species can be by crossing two 

 related species many different times, using as parents different 

 races of the same pair of species. Such experiments have been 

 performed in Drosophila (Dobzhansky, 1951) and various groups 

 of higher plants, such as the tarweeds, Madiae (Clausen, 1951); 

 the phlox family, Polemoniaceae (Grant, 1957); and the grasses 

 (Stebbins and Vaarama, 1954). In each example, the same kind 

 of result has been obtained. Both the ease of crossing and the 

 fertility of the F x hybrid differ greatly depending upon the par- 

 ticular parental strains used. Any widespread species contains 

 a great store of genetic variability, including not only genes 

 which affect the adaptation of the population to its environ- 

 ment, but also those which help or hinder the ability of a species 

 to cross and exchange genes with individuals of another species. 



52 



