THE EVOLUTIONARY CONCEPT 349 



ous genes first known by their association with some visible phenotypic 

 effect have been found to affect the fertility or the general viability of the 

 organism and thus to be definitely vulnerable to the effects of selection. 

 Another (and largely modern) concept of the role of selection has to 

 do with intergroup competition, as contrasted with the intragroup com- 

 petition of classical Darwinian theory. A very large number of potentially 

 interbreeding natural populations are from time to time segregated into 

 more or less disjunct subpopulations, as will be briefly discussed in connec- 

 tion with isolation. After these subpopulations have been isolated long 

 enough to develop some ecological or genetic deterrent to free inter- 

 breeding, they may again be brought into direct competition with each 

 other. Here selection is particularly effective. It may result in establishing 

 the better adapted stock throughout the common range, with disappear- 

 ance of the less well adapted; or it may cause restriction and further 

 fragmentation of the less favored population, which survives in those 

 parts of a varied range where its particular characteristics are less 

 disadvantageous . 



Isolation 



One of the great difficulties in the way of accepting segregation of 

 inherited variations as an evolutionary process was to show how it could 

 operate in nature. No mechanisms were known that would prevent the 

 free spread of accumulating diversity throughout the whole of an inter- 

 breeding population. Accumulating mutations might well produce 

 gradual change in a stock or increase its variability; but speciation re- 

 quires segregation of variability into restricted portions of a population. 

 What is needed is some type of isolating mechanism that is inherent in 

 the conditions normally encountered by populations in nature. 



This problem was recognized by Darwin and was emphasized by many 

 of his immediate followers. They lacked, however, the data of genetics 

 and ecology, and the detailed knowledge of geographical and ecological 

 relationships of closely allied forms, that have enabled modern workers 

 to deal with this difficulty. 



Actually a wide variety of real and potential isolating mechanisms 

 exists in nature. The most obvious and most fully studied and documented 

 cause of isolation is spatial. Partial or complete separation in space 

 (geographic isolation) has been a factor in the origin of many, probably of 

 most subspecies and species. A once continuous range occupied by a 

 continuous interbreeding population has often been divided by physio- 

 graphic or climatic changes into two or more isolated areas, thus effec- 

 tively restricting the spread of subsequently acquired genetic changes. 

 Not infrequently a physiographic, climatic, or even a man-made change 



