NATURAL SELECTION: I 471 



in environment, and differences in environment involve differences in 

 position in space. The fish in a lake is not occupying the same position in 

 space as is the bird flying overhead. An insect on coniferous trees is not 

 occupying the same position in space as is an insect on deciduous trees. A 

 beetle burrowing in the ground is not occupying the same position in space 

 as is a beetle living on trees. Geographic isolation and environmental isola- 

 tion thus go hand in hand. Sometimes one predominates, sometimes the 

 other. If the main difference between the territories occupied by two species 

 is difference in location, environmental conditions being similar, we say 

 that the species are geographically isolated. In doing so, we merely over- 

 look what environmental differences there are. If, on the other hand, the 

 main differences between the territories occupied by two species are differ- 

 ences in environmental conditions (ground dwelling versus tree living, for 

 example), we say that the species are environmentally isolated. In this case 

 we overlook the fact that the two species are also geographically isolated in 

 the sense of not occupying the same position in space — in the vertical di- 

 mension of space rather than in its horizontal dimension. All sorts of inter- 

 mediate conditions exist. Moore (1949) has well said, "The distinction 

 between geographical and habitat isolation is merely quantitative." 



We have labored this point at some length because of the occurrence of 

 a prolonged but rather footless debate as to which is more important in 

 evolution, geographic isolation or environmental (i.e., ecological) isola- 

 tion. The controversy has centered around the query as to whether 

 environmental isolation can promote evolutionary change in the absence 

 of geographic isolation. If, as we have maintained, the one never occurs 

 without some degree of the other, the point of the query vanishes. It is 

 then apparent that isolation (i.e., nonbiological isolation; see below) 

 always entails some separation in space, this being accompanied by 

 greater or lesser differences in environment. (See Mayr, 1947, 1949, for 

 further discussion of this matter.) 



Under the term reproductive isolation we group a great variety of 

 biological restrictions to gene interchange. The two types of isolation just 

 discussed prevent individuals from coming into contact or reduce the likeli- 

 hood of their doing so. But suppose individuals do come into contact, 

 will interbreeding, with consequent interchange of genes, occur? Many 

 factors may prevent exchange of genes and thus constitute means of 

 biological isolation. 



In some cases interbreeding is not possible. At least two types of factors 

 may be involved: (1) The anatomies of the two populations may be so 

 unlike that copulation between males of one population and females of 



