ISOLATING MECHANISMS AND SPECIES FORMATION 



mutant could be destroyed by crossing to the original type, the combina- 

 tion would almost certainly be broken up either by independent assort- 

 ment or by crossing over. 



As subspeciation is the ordinary prerequisite to speciation in the neo- 

 Darwinian scheme, the formation of breeding populations which have 

 distinct complexes of genes (subspecies) is of great interest. Completely 

 random breeding within a species would result in even distribution of all 

 of its genetic variability and so no subspecies could be formed, although 

 this would not prevent the mass transformation of a species into a new 

 one if an appropriate selective force were operating, as Wright has pointed 

 out. But complete random breeding probably never occurs, except in very 

 small, endemic species. Typically, local populations breed largely among 

 themselves, with relatively little outbreeding. The result is that different 

 populations of single species can build up genotypes which differ con- 

 sistently in some or many loci. On the basis of the resulting phenotypes, 

 these may be classified as subspecies. Wherever two such subspecies meet, 

 they ordinarily interbreed, and the expected Mendelian recombinations 

 occur, with the result that a single, generally intermediate, and highly 

 variable population is formed. Thus any start toward specific status which 

 the subspecies may have made is lost. If, however, a subspecies is suffi- 

 ciently isolated over a long period of time that interbreeding with its 

 relatives is prevented, it may continue to accumulate differences until a 

 physiological barrier to interbreeding is acquired. The subspecies may 

 now be regarded as a new species. As long as the related groups remain 

 geographically separated from each other they are referred to as allopatric 

 species. There is frequently considerable doubt as to whether two geo- 

 graphically separated groups are in fact allopatric species or simply sub- 

 species. But if they now move into the same territory and fail to inter- 

 breed, with the formation of intermediates, they are regarded as sijmpatric 

 species, and there is much less doubt about their status. 



Hence, the study of the means by which subspecies, and indeed species 

 themselves, may be isolated from one another has played a major role in 

 modern evolutionary studies. Very few would now care to accept the 

 letter of the dictum of Romanes that "without isolation or the prevention 

 of interbreeding, organic evolution is in no case possible." But, with some 

 tempering, this has been the spirit of many recent studies. 



ISOLATING MECHANISMS 



The mechanisms by which subspecies and closely related species can be 

 isolated from one another are classified by Mayr to include three main 

 types of isolating mechanisms. First, there may be a restriction of random 

 dispersal so that potential mates cannot meet. This is largely equivalent 

 to the geographical mechanism of Dobzhansky and others. Second, there 

 may be a restriction of randoin mating so that potential mates do not cross 

 even though they have the opportunity. Finally, there may be a reduction 

 of fertility so that a cross results in few offspring or none. 



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