474 INTRODUCTION TO EVOLUTION 



statement we recall the action of polyploidy, pp. 418-420.) Conversely, 

 when the gene pool does become divided, by geographic-environmental 

 factors, the allopatric populations so produced may become differentiated 

 in ways which give rise to reproductive isolation. Following the develop- 

 ment of reproductive isolation the populations may come into contact 

 (e.g., by migration) and still retain the integrity of their respective gene 

 pools. Two populations living in the same area are said to be sympatric. 

 Sympatric populations must always be reproductively isolated from each 

 other if they are not to merge into a single amalgamated population. 

 Apparently, then, geographic-environmental isolation is primary: the 

 first steps in species formation will not be taken without it (except in the 

 case of polyploidy). The attainment of reproductive isolation is, as pre- 

 viously noted (pp. 314-317), the most conspicuous sign that the species 

 level of evolutionary change has been reached. 



HYBRIDIZATION IN EVOLUTION 



Since the primary role of isolation is the prevention of 

 hybridization, it is now appropriate to inquire further into the evolutionary 

 significance of interbreeding between populations and of the failure of this 

 to occur. Paradoxically, hybridization is important in evolution when it 

 occurs and when it does not. 



Hybridization: Disadvantageous 



We may consider first the importance of the failure of hybridization to 

 take place. As noted, difl'erent species do not usually interbreed or if 

 they do interbreed they do not usually produce viable hybrids. Moreover, 

 when viable hybrids between species are produced they are generally 

 sterile or of such low fertility that the amount of reproduction is negligible. 

 What is the value to a species of failure to produce "successful" hybrids? 



Usually species as we find them under natural conditions are well 

 adapted for particular environmental niches. Suppose that hybrids are 

 produced between two such species, A and B. These hybrids will ordi- 

 narily possess some of the characteristics of species A, some of species B. 

 Consequently, in most cases the hybrids will not be so well adapted for 

 life in the environmental niche occupied by species A as is species A itself. 

 Neither will they be so well adapted for the niche occupied by species B 

 as is species B itself. In other words, the hybrids will usually "fall between 

 two stools." Accordingly, in such cases hybrids would represent biological 



