CHAPTER VI 

 CHROMOSOMES AND EVOLUTION 



TWO different concepts are included in the term 

 evolution and are frequently confused with 

 one another — morphological change and the forma- 

 tion of new species. There has recently been a 

 tendency to regard the species as a category which 

 varies from group to group, so that a * species * 

 of beetle is not equivalent to a ' species ' of primate 

 or conifer. From a morphological point of view this 

 is no doubt true — the extent of the taxonomic 

 diiferences between species do vary from group to 

 group. But from a genetical point of view the 

 species (i.e. a group of individuals all of which nor- 

 mally and regularly breed together except in so far 

 as they may be separated by geographical isolation) 

 is still a satisfactory concept which only breaks down 

 in organisms which reproduce without fertilization. 



With the exception of a small number of cytoplas- 

 mic characters in plants (and possibly also in animals, 

 but there is no well-established case as yet) all evolu- 

 tionary changes have clearly arisen in the first place 

 as changes in the chromosomes. It was formerly 

 believed that these changes were of two main types : 

 gene-mutations (which were conceived of as sub- 

 microscopic molecular changes) and structural altera' 

 lions in chromosomes (i.e. microscopically visible 

 rearrangement of whole blocks of genes by transloca- 

 tion, inversion, &c.). It has recently been shown, 

 however, 1^- that this distinction is rather one of 

 degree than of kind ; most (possibly all) gene-muta- 

 tions are minute structural alterations involving 



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