238 I The Process of Evolution 



through physical isolation, a single evolving unit may become tw^o 

 or more evolving units. As long as such evolutionary units remain 

 isolated, they are free to respond independently to evolutionary 

 forces. However, should circumstances permit such units to regain 

 contact, numerous possibilities are presented. 



Through the course of time there have occurred innumerable 

 environmental changes that might have brought previously isolated 

 populations back together. The effects of ancient cataclysms have 

 been modified or completely erased from the record. Two relatively 

 recent overlapping events have undoubtedly produced effects which 

 are conspicuous today. The first of these events is the Pleistocene 

 glaciations, which changed climate on a vast scale, causing plants 

 and animals to migrate or perish. The glaciers also scoured the 

 earth, mingling soil types, creating lakes, and rearranging drainage 

 systems. 



The advent of man has had similar effects. It is difficult to over- 

 estimate the importance of man as a factor in evolution. First with 

 fire and then with domesticated plants and animals, he has modi- 

 fied the environment. Many of the instances of interbreeding of 

 previously separated forms are the result of man's conscious or un- 

 conscious intervention. By breaking into the vast stored-energy 

 reserves of climax ecological communities, man has diverted energy 

 for his own purposes and grossly modified what we even yet think 

 of as the "natural" ( prehuman ) environment. 



Fusion of Populations. At one extreme, populations may have 

 been isolated for such a short time or subjected to such similar con- 

 ditions that divergence has been minimal. When the populations 

 reunite, individuals mate at random and the offspring from matings 

 between parents of different populations are as successful as those 

 from members of the same isolate. The two previously isolated 

 populations then fuse into a single population. This is seen when- 

 ever Drosophila lines, isolated for a few generations in the labora- 

 tory, are combined, or when guppies kept for a few years in one 

 aquarium are dumped in with those that have been kept in another. 

 The situation is certainly very common in nature also; it has not 

 been given very much attention by zoologists. 



Meeting with No Gene Exchange. At the other end of the spec- 

 trum, populations that have diverged a great deal may come to- 

 gether and all attraction between individuals of the different 

 populations may have been lost, resulting in no interpopulation 

 matings. In most cases this means that gene flow between the evolu- 

 tionary units has ceased, although, as in Drosophila panlistorum, 

 genetic information may still be exchanged by such isolates via a 

 "ring of races." The pattern in Ensotina is of great interest, as it is 



