THE EVOLUTIONARY CONCEPT 351 



conjunction with the other isolating mechanisms outlined below. It is the 

 commonest (and some would maintain the invariable) first step in isola- 

 tion. It provides an initial segregation, which may be maintained and 

 strengthened by other mechanisms that continue to operate when popula- 

 tions more or less briefly isolated in space are once more brought into 

 contact. 



Many other kinds of isolating mechanisms have been discovered or have 

 been postulated as a result of the findings of modern genetics and ecology. 

 They may be broadly grouped as physiological isolating mechanisms, for in 

 all of them it is some basically physiological difference that operates to 

 prevent or reduce interbreeding with other groups. There may be some 

 inherent difference in response to environment or to one another that 

 reduces or eliminates opportunity or inclination for breeding. Or there 

 may be structural differences that mechanically prevent mating, or 

 genetic differences that make the offspring infertile or inviable. 



Differences of all the sorts enumerated do serve to prevent effective 

 crossing of closely allied forms that occupy the same geographic range. 

 Whether such forms could have arisen from a common, freely interbreed- 

 ing population without an interval of geographic isolation is a question 

 not yet settled. There is little doubt, however, that all or nearly all of the 

 physiological mechanisms mentioned are effective and important in 

 maintaining the separateness of populations that are unquestionably 

 closely related, although they now occupy the same or overlapping 

 ranges. 



Here also is an illustration of a fact that biologists are realizing more 

 and more — that there are many different evolutionary factors and that the 

 sequence of events that has occurred in any one stock is not necessarily (and 

 probably never exactly) the same as that which has occurred in another. 



Population dynamics 



Another important factor or set of factors in the evolutionary process, 

 which has only recently been appreciated and explored, lies in the mathe- 

 matical combinations and permutations inherent in Mendelian inherit- 

 ance. In the shuffle of heterozygous genes and gene combinations through 

 successive generations, there are so many variables affected by the laws 

 of chance that only the highly abstruse calculations of statistical genetics 

 have revealed the importance of this new approach. 



One of the important findings of these studies has been the phenomenon 

 called drift by Sewall Wright, and referred to as the Sewall Wright effect 

 by others. This is the tendency within a small interbreeding population 

 for each gene that is represented by heterozygous alleles either to be 

 eliminated or to become homozygous, even though neither of the respec- 

 tive alleles confers any adaptive advantage or disadvantage. Thus any 



