CHAPTER 13 



Evolutionary Mechanisms 



I . The Mechanisms of Evolution 



(a) Species Initiation. — From a consideration of the nature of taxo- 

 nomic differences discussed in Chapter 12 and the evidence of the 

 actual course of evolution discussed in the first chapter of this part, we 

 can form some idea of the kinds of mechanism which have to be taken 

 into account in any complete theory of evolution. 



In the first place, taxonomic groups usually differ in large numbers 

 of small factors each with comparatively unimportant effects; these 

 factors may be genes or small inversions, translocations, etc. Examples 

 of groups which differ in this way are the geographical races of animals, 

 but most related species probably also possess the same sort of differ- 

 ences, though if the species will not give fertile hybrids it is not easy to 

 demonstrate this conclusively. 



Secondly, species may differ in a few changes on a coarser scale. 

 Examples are polyploid series, or related species which show a few 

 large translocations, inversions, etc. Groups which differ by such crude 

 chromosome changes are usually only ranked as separate species if they 

 also differ in a large number of genes. Both the large-scale differences 

 and the numerous small-scale ones may occur in a single step if chromo- 

 some rearrangements take place in a hybrid, and this sudden origin of 

 fully fledged new species is clearly very important in plants. It is 

 probably less important in animals, where hybrids usually cannot 

 perpetuate themselves. In considering the origin of an animal species 

 which differs from the old in large-scale chromosome changes, two 

 processes are therefore involved : the origin of the chromosome change 

 and the accumulation of the small-scale changes. 



We see, then, that the accumulation of numerous small differences is 

 important in every type of evolution except in the formation of new 

 species by chromosome changes in a hybrid, and even in this case, the 

 differences between the two parents may have arisen in this way. The 

 process by which the gradual accumulation occurs is usually taken to 

 be by the action of natural selection working on the random gene and 

 chromosome mutations which are continually occurring. The detailed 

 theory of how natural selection acts will be discussed in a later section. 



