NATURAL SELECTION: I 477 



conditions in which hybridization is detrimental. In most instances we 

 know too Httle of the environmental requirements placed upon species 

 to be able to draw valid conclusions on the extent to which hybridization 

 would be detrimental. 



Our tentative conclusion may be that natural selection may under 

 special circumstances favor genetic mechanisms having as their principal 

 function the prevention of hybridization, but that usually such functions 

 arise as a by-product of the genetic changes by which two isolated 

 populations become adapted to their respective environments. (In Rana 

 pipiens, Vermont frogs and Florida frogs will not produce viable hybrids 

 if the experimental attempt is made. Apparently adaptation to develop- 

 ment in cold and warm environments, respectively, has carried with it 

 genetic changes which result in constitutions so unlike that hybridization 

 is not possible. Moore, 1955, 1957.) 



Hybridization between species is less frequent in animals than it is in 

 plants. Nevertheless, instances of natural hybridization between animal 

 species are not lacking (see Stebbins, 1959, for a review of the literature in 

 this field). Among vertebrates some hybridization has been found in all 

 groups but seems perhaps to be most common — at least, it has been most 

 studied — in fresh-water fishes (cf. Hubbs, 1955; Hubbs, Walker, and 

 Johnson, 1943), amphibians (cf. Blair, 1941, and investigations cited 

 above), and birds (e.g., Sibley, 1954). 



Hybridization: Advantageous 



Since, as we have noted, hybrids are sometimes produced we may now 

 ask the question: Under what circumstances might production of hybrids 

 be of advantage in evolution? We have noted that the usual disadvantage 

 faced by hybrids arises from the fact that they are generally not so well 

 adapted for the environmental niche occupied by either parent species as 

 is that parent species itself. In other words, the hybrid is at a disadvantage 

 in competition with its parents and the latter's nonhybrid progeny. Under 

 what circumstances might this situation not be true? It would not be true if 

 there were available to the hybrid other environmental niches than those 

 occupied by the parental species. These other niches would be expected 

 to present somewhat differing hving requirements from those presented 

 by the niches occupied by the parental species. Accordingly, some of the 

 combinations of characteristics possessed by the hybrids might prove to 

 be "just the thing" to enable the hybrids to enter and occupy the new 

 niche. They would afford one means of preadaptation (see pp. 12-13). 



