J. A. MOORE 327 



This theory starts with the premise that each species, genus and 

 probably each geographical race is an adaptive complex which fits 

 into an ecological niche somewhat distinct from those occupied by 

 other species, genera and races. The adaptive value of such a com- 

 plex is determined not by a single or a tew genes, but is a property 

 of the genotype as a whole. Furthermore, the adaptive complex is 

 attuned to its environment only so long as its historically evolved pat- 

 tern remains, within limits, intact. It is true that interbreeding of dif- 

 ferent adaptive complexes may sometimes result in emergence of new 

 genotypes which fit into unoccupied or sparsely settled ecological 

 niches — hence the evolutionary role of hybridization. Nevertheless, 

 hybridization usually leads to the formation of disharmonious recom- 

 binations. 



Considerations such as these have prompted some writers (Dob- 

 zhansky, 1937a,b; Sturtevant, 1938; ef. Fisher, 1930) to assume that 

 occurrence of hybridization between races and species constitutes a 

 challenge to which they may respond by developing or strengthening 

 isolating mechanisms that would make hybridization difficult or im- 

 possible. Where hybridization jeopardizes the integrity of two or more 

 adaptive complexes, genetic factors which would decrease the fre- 

 quency or prevent the interbreeding would thereby acquire a positive 

 selective value, even though these factors by themselves might be 

 neutral. Race formation is essentially the development of genetic pat- 

 terns which are adapted to a definite environment. Speciation is a 

 process resulting in fixation of these patterns through the development 

 of physiological isolating mechanisms. Clearly, raciation and specia- 

 tion should not be conceived of as entirely independent processes, 

 but the development of physiological isolating mechanisms must nev- 

 ertheless be supposed to intervene only after the divergence of the 

 adaptive complexes had been initiated. If races are to become species, 

 isolating mechanisms must arise when the distinct adaptive complexes 

 are exposed to the risk of disintegration due to interbreeding. 



Dobzhansky has modified his view to some extent in more 

 recent publications. The quotation can be taken, however, as an 

 expression of a theory that is fairly widely subscribed to in 



biology. 



It is my feeling that in the majority of situations it is possible 

 for fully diverged species, adequately equipped with isolating 



