470 INTRODUCTION TO EVOLUTION 



migration undertaken by individual salmon in returning to spawn in the 

 stream where they began life is another classic example. It is the relative 

 isolation of these local breeding groups which affects gene distribution 

 and hence is important for evolution. 



The local populations just mentioned are frequently not separated from 

 each other by mountain ranges, deserts, and other obvious geographic 

 features. As we noted earlier (p. 445), distance is in itself a barrier — if not 

 a complete one, at least one that in practice is effective (see Wright, 

 1943). Its effectiveness arises in large part from what we may call the 

 homing or territorial tendency of animals. It is the exception rather than 

 the rule for animals to carry on their reproductive activities far from the 

 region in which they themselves first saw the light of day. As mentioned 

 previously (p. 445), individual animals commonly establish more or less 

 clearly delimited home territories. They do not usually stray far from 

 these, at least at the time of breeding. Whatever the psychological con- 

 comitants, "home" evidently has significance in the lives of most animals. 

 Even such able travelers as birds commonly use their wings to return 

 home, if removed from it by some accident such as a storm, rather than 

 to travel to some other locaUty and establish a new place of residence, as 

 Mayr has remarked. 



Environmental isolation is a term which the author feels may be more 

 evident in meaning than is the commonly used term "ecological isolation" 

 or the synonymous "habitat isolation" of Moore (1949). As the term im- 

 plies, populations that are environmentally isolated live under different 

 environmental conditions, at least during the breeding period. A fish and a 

 seed-eating bird are environmentally isolated even though they live in the 

 same locality. Similarly, an insect which inhabits only coniferous trees is 

 environmentally isolated from an insect which inhabits only deciduous 

 trees, even though both live in the same wood lot. Beetles which spend 

 their lives burrowing in the ground are environmentally isolated from 

 beetles which spend their lives on trees. 



We note that, whereas geographic isolation depends upon separation in 

 space, environmental isolation depends upon separation resulting from dif- 

 ferences in the food, habits, and physiological requirements of animals. 

 We may well ask: Do we ever find the one type of isolation without the 

 other? Do animals ever occupy somewhat different positions in space with- 

 out at the same time being faced with somewhat different environmental 

 conditions? Conversely, are animals ever faced with differing environ- 

 mental conditions while occupying the same position in space? Certainly 

 differences in position in space usually, if not always, involve differences 



