352 THE CHANGING GENERATIONS 



small interbreeding population tends to become homozygous without 

 benefit of selection; pure chance eliminates one allele and establishes the 

 other. 



If unopposed by a source of new genes (which may be either mutation 

 or some degree of crossing with other stocks) or by sufficiently strong 

 selection for adaptive value, genetic drift would result in the population 

 becoming stable and unchangeable. Such a stock, stabilized without 

 benefit of selection, would almost certainly show many poorly adapted or 

 deleterious features and would be especially liable to extinction. It is 

 probable that in most cases the conditions within a small population are 

 such that drift is opposed by varying degrees of selection, mutation, and 

 outbreeding. In this event drift will be effective only in establishing alleles 

 of relatively small adaptive significance. 



The real importance of the Sewall Wright effect comes from the fact 

 that in nature most large populations are wholly or partly broken up into 

 several or many subpopulations. The latter are often of sufficiently small 

 size at some time or times in their existence to be affected by drift. Since 

 drift is wholly random, it should result in producing a variety of dif- 

 ferentiated subpopulations. The first phenotypic manifestations of many 

 recessive alleles are likely to occur where these alleles have become numer- 

 ous through drift. If isolation between the subpopulations is only partial, 

 or if there are episodes of increase and spreading of the more successful 

 ones, the existence of differentiated subpopulations sets the stage for 

 modification of the whole species from centers where favorable modifica- 

 tions have become established, through inter group competition. Sewall 

 Wright has spoken of such centers as population sources and of the sub- 

 populations which disappear as a result of intergroup competition as 

 population sinks. 



This resume of the status of evolutionary concepts nearly a century 

 after the publication of the Origin of Species has necessarily been partial 

 and incomplete. Many of the most promising lines of investigation involve 

 technical details beyond the scope of this book. Space prohibits any 

 account of the interplay between mutation, isolation, selection, drift, 

 and the changes in environment that may operate simultaneously or in 

 sequence to produce diversity and change in populations of organisms. 



Evolutionary theory is still essentially Darwinian or neo-Darwinian, 

 with variation, inheritance, and selection playing vital roles. After this 

 lapse of time the biologist knows far more than did Darwin about the 

 source and limits (if not about the cause) of variation. With respect to 

 inheritance, he has- learned to distinguish between phenotype and geno- 

 type, and he knows now that inheritance is particulate. He has discovered 

 the intricate and precise laws of combination and permutation that 



