THE CONCEPT Of EVOLUTION 



705 



environment. Thousands of breeding experiments with plants and ani- 

 mals since the turn of the century have shown that such mutations do 

 occur constantly and that their effects may be of adaptive value. With 

 the development of the gene theory, the term mutation has come to refer 

 to sudden, discontinuous, random changes in the genes and chromo- 

 somes, although it is still used to some extent to refer to the new type 

 of plant or animal. 



In the plants and animals most widely used in breeding experiments 

 —corn and fruit flies— some 400 to 600 mutations, respectively, have been 

 detected. The fruit fly mutations are tremendously varied, and in- 

 clude all shades of body color from yellow through brown and gray to 

 black; red, white, brown or purple eyes; crumj^led, curled, shortened, 

 and peculiarly shaped wings— even the complete absence of wings; oddly 

 shaped legs and bristles; and such extraordinary changes as a pair of 

 legs growing on the forehead in place of the antennae (Fig. 34.2). Mu- 

 tations are found in domestic animals; the six-toed cats of Cape Cod and 

 the short-legged Ancon sheep are two of many examples of the per- 

 sistence of a single mutation. 



Early in the present century there was a heated discussion as to 



FORKED 



DICHAHE 



RUDIMENTARY 



VESTIGIAL 



CURLED 



STUBBLE MINIATURE SCUTE CROSSVEINLESS CUT 



Figure 34.2. Some wing and bristle mutants in the fruit fly, Drosophila melano- 

 gaster. (Drawn by E. M. Wallace; Sturtevant and Beadle: An Introduction to Genetics.) 



