THE ORIGIN OF SPECIES 



sive experiments in bird banding comprise the best evidence for this, as 

 well as the best known example, but similar evidence is accumulating for 

 all groups of vertebrates. 



The homing instinct of many migratory birds is well known, and it is 

 clear that this would also serve to limit random dispersal. The homing 

 instinct has been reported for other vertebrates, and even for some in- 

 vertebrates. The social structure of some species may have a similar effect. 

 Thus, in the geese, family groups do not break up at the end of the nesting 

 season, but rather they migrate together, and separate only after returning 

 to the nesting ground in the following year. The result is a high degree of 

 inbreeding. 



Restriction of Random Mating. The principal factors which limit 

 random mating, in the absence of geographical separation, are ecological 

 differences, behavior differences, and mechanical differences of the copu- 

 latory organs. The first two are well established and will be considered 

 in some detail, but the third has limited validity. 



An important type of ecological difference between subspecies (or 

 species) which may tend to prevent interbreeding is the selection of dif- 

 ferent habitats, so that potential mates from different populations do not 

 meet even though they exist in the same general area. For example. Dice 

 found that the ranges of two subspecies of Peromysciis maniculatus over- 

 lap in northern Michigan, but there is no evidence of their interbreeding 

 in nature, although they will interbreed in the laboratory. One of these 

 races is found principally in forests, the other on sandy beaches. A similar 

 case in the same species has been studied by Murie in Glacier National 

 Park. Here, one of the races is confined to forests, the other to open 

 prairies. The common water snake, Natrix sipedon, presents a comparable 

 situation in Florida, where fresh-water and salt-water races may come 

 very close together, but they are kept separate by their habitat prefer- 

 ences. Pictet has described a curious case in Swiss moths. Nemeophila 

 plantaginis occurs in different altitudinal races, with a race above 2700 

 meters and one below 1700 meters differing in a single gene. At 2200 

 meters, there is a hybrid population, in which all of the moths are hetero- 

 zygous. When the two pure races are crossed in the laboratory, the off- 

 spring include all three types in the Fo so it appears that the homozygous 

 types must be subject to a very severe selective elimination at the 2200 

 meter level, if these data are correct. Data similar to the above are being 

 gathered for many different groups, both plant and animal, and it seems 

 clear that such differences in habitat requirements are of general impor- 

 tance as barriers to random mating. 



Another ecological factor is seasonal isolation, that is, breeding seasons 

 are sufficiently different to make interbreeding improbable. Thus, in Cali- 

 fornia there are five described species of cypress trees (Ciipressus). These 

 are subdivided into ten distinct entities which could be called subspecies, 

 all of which have quite hmited distribution, some being represented only 

 by a single grove. These may grow side by side with only a very few 

 hybrid trees formed, or none at all. The factor which prevents interbreed- 

 ing appears to be simply this, that the several races pollinate at somewhat 



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