284 AN INTRODUCTION TO MODERN GENETICS 



carried them. If, however, a population is already split into two groups 

 by some extraneous agency, such as a geographical barrier, it is easy 

 to imagine that each group may accumulate factors which are not 

 deleterious within the group but which lead to infertility or lack of 

 breeding between the groups. It is probable, then, that most species 

 are initiated in this way^ by a preliminary geographical isolation which 

 allows the different groups to diverge in evolution before they have a 

 chance to mingle. We shall find (p. 295) that at least some degree of 

 geographical isolation also provides the conditions under which the 

 modification of the genotype proceeds most rapidly under the influence 

 of natural selection. Geographical isolation therefore appears to play a 

 very important part, both in the original separation of a population 

 into two parts, which we may call species initiation, and in the subse- 

 quent divergence of these two parts into two different species. 



Some types of chromosome change, such as polyploidy and trans- 

 location, may immediately lead to sterility of the new type with the 

 old. It is clear that in Nature the occurrence of polyploidy takes place 

 and may initiate new species in plants. The position as regards trans- 

 locations is more obscure; it is known that many species differ by 

 translocations (e.g. in Drosophila) but no translocations have yet been 

 found in natural populations, except for the segmental interchanges 

 discovered in plants (p. 1 10), and in fact it is difficult to see how a 

 translocation could ever be perpetuated. 



(b) Species Divergence. — We have seen that evolution includes both 

 the changing of the genetic constitution of a population and often its 

 spHtting up into separate groups which become distinct species. 

 Although Darwin named his great work The Origin of Species, he 

 actually dealt mainly with the first of these processes, and the theory 

 of species initiation has ever since lagged far behind that of species 

 divergence. 



The greater part of evolutionary theory deals with species divergence, 

 or the alteration of the genetic constitution of populations, and this, 

 since it is the part dealing with the origin of new characters, is the more 

 important part of the theory. The main theory which requires con- 

 sideration is that of natural selection, though we shall also have to 

 discuss the hypothesis of the inheritance of acquired characters. The 

 theory of natural selection is as follows. If a species contains two 

 hereditary varieties, and if one of the varieties produces proportionately 

 more offspring than the other, then the rapidly breeding variety will 

 become relatively more frequent and will eventually entirely replace the 



^ Sturtevant 1938. 



