J. A. MOORE 333 



and aquatic organisms. The chance dispersal of organisms from 

 a continent to a remote island, the breaking of connections be- 

 tween continents or bodies of water, the extermination by bio- 

 logical or physical causes of the individuals in portions of a 

 species' range are some situations that can divide an originally 

 continuous population. In many instances the isolated groups 

 will evolve with no further relation to each other. 



The situations just described exclude the possibility of hybrid- 

 ization, and we will now consider those instances where once 

 separated populations become continuous. 



When two once continuous and then separate populations re- 

 establish contact, a variety of consequences can ensue. If the 

 period of separation has been short, or if genetic divergence 

 has been slight, the two groups would be expected to become 

 parts of the same breeding population in the zone of overlap. 

 If the period of separation has been long and if genetic diver- 

 gence has been sufficient for effective isolating mechanisms to 

 develop, the two overlapping populations could maintain their 

 discreteness. 



The two situations just considered are extremes and there is 

 little doubt about what will occur. It is with the intermediate 

 situation that we shall now give our attention, for it is here 

 that there is doubt as to the course of events. The situation will 

 be this: Two portions of a population become geographically 

 isolated and during their period of isolation an appreciable 

 amount of genetic divergence occurs. The amount of divergence 

 will be defined as sufficient to result in a hybrid less well adapted 

 than either parent population. It is in this situation that Dob- 

 zhansky has postulated that natural selection will promote the 

 development of isolating mechanisms that will decrease the wast- 

 age of gametes due to their combining in poorly adapted hybrids. 



When the two populations reestablish contact, under the con- 

 ditions stipulated, they come together as differently adapted 

 groups. In their period of separation each evolved along paths 

 leading to better adaptation to their separate environments. The 

 very fact that the genomes are no longer able to collaborate 

 normally may be taken as indicative of considerable genetic di- 



