338 INTRODUCTION TO EVOLUTION 



them valuable "raw materials" for the construction of evolutionary change. 



Of primary importance is the fact that mutations are inheritable changes. 

 As noted above, they are inheritable because they are ( 1 ) changes in the 

 germ plasm (genes) followed by (2) changes in the bodies of offspring 

 arising from the altered germ plasm. 



What is the nature of this change in the germ plasm? Two main types of 

 change may be distinguished. The most common type, and the one of 

 greatest significance in the evolution of animals, is called gene mutation. It 

 is the type mentioned in the first paragraph of this section — a chemical 

 change in a single gene. As a result of its changed nature the gene then pro- 

 duces something other than it produced previously — black body color in- 

 stead of gray in an insect, for example. By far the most common gene muta- 

 tions involve the change of a dominant gene to a recessive one, although 

 the reverse sometimes occurs. 



Aside from changes in individual genes, changes in numbers and arrange- 

 ments of genes sometimes occur through what are called chromosomal muta- 

 tions or, better, chromosomal aberrations. Chromosomes are bodies in the 

 nuclei of cells; unlike the genes within them, they are visible with the ordi- 

 nary compound microscope. As noted previously (p. 336), each chromo- 

 some contains many genes — a thousand or so, frequently. Chromosomal 

 aberrations arise in various ways: Chromosomes break into fragments which 

 later join together but in arrangements diff'erent from the original ones. 

 Small chromosomes fuse together to form large ones. Chromosomes in- 

 crease in number, by addition of one or two extra chromosomes or, on the 

 other hand, by addition of multiples of the number originally present. Dis- 

 cussion of the processes and of their effects upon inheritance will be found 

 in books on genetics. Chromosomal aberrations seem to be of considerable 

 importance in plant evolution (see pp. 418-420). The "mutations" of the 

 evening primrose discovered by De Vries were later found to be mainly of 

 this type. For further discussion of the mutation process see Chapter 17. 



Mutations vary greatly in magnitude of effect produced. The change in 

 length of legs in the Ancon ram and the changes observed by De Vries in 

 the evening primrose were large changes. Subsequent research has shown, 

 however, that by far the most abundant mutations produce only small 

 changes. 



What Causes Mutations? 



We have spoken of mutations as being "spontaneous"; they arise unex- 

 pectedly for reasons which we do not know. The darkness of our ignorance 



