706 GENETICS AND EVOLUTION 



whether evolution was the result of natural selection or of mutation. 

 As more was learned about heredity, it became clear that natural selec- 

 tion can operate only when there is something to be selected, that is, 

 when mutations present alternate ways of coping with the environment. 

 The evolution of new species, then, involves both mutation and natural 



selection. 



One of the current controversies in evolutionary theory concerns 

 the possible role of small and large mutations in the origin of new 

 species. The Neo-Darwinists argue that new species (and all the higher 

 categories) evolve by the gradual accumulation of many small muta- 

 tions; thus there should exist many forms intermediate between the 

 original species and the new one. Other biologists believe that new 

 species and genera arise in a single step by a macromutation, a major 

 change in the genetic system which produces a major change in the pat- 

 tern of development. This results in an adult which is morphologically 

 and physiologically quite different from its parents. The macromuta- 

 tionists would hold that one should not expect to find forms which are 

 intermediate between the original species and the new one. Many 

 macromutations result only in "monsters" which would be unable to 

 survive. (The term monster simply means any form which is markedly 

 different from the usual type of the species, and does not necessarily 

 imply that it is ugly.) Other macromutations may give rise to what 

 Richard Goldschmidt of the University of California calls "hopeful 

 monsters," organisms which are enabled by their mutation to occupy 

 some new environment. The evolution of the extinct ancestral bird, 

 Archaeopteiyx, into modern birds, he believes, may have occurred by a 

 macromutation. Archaeopteryx (Fig. 24.7) had a long, reptile-like tail 

 covered with feathers; a macromutation which altered development so 

 that the tail was gieatly shortened would result in a "hopeful monster" 

 with the fan-shaped arrangement of tail feathers seen in modern birds. 

 This new shape of the tail, which is better suited for flying than the 

 long tail of Archaeopteryx, gave its possessors an advantage in the 

 struggle for existence. There is, of course, no proof that modern birds 

 evolved in this way, but there is ample evidence that similar marked 

 skeletal changes may result from a single mutation. The stubby tail of 

 the Manx cat is the result of a mutation which causes the tail vertebrae 

 to shorten and fuse. Professor Goldschmidt does not deny that small 

 mutations may occur and accumulate, but holds that they can lead only 

 to varieties or geographic races, and not to species, genera and the 

 higher taxonomic divisions. 



The causes of natural or spontaneous mutations are unknown. Both 

 gene and chromosome mutations can be produced artificially by a vari- 

 ety of agents: x-rays, alpha, beta and gamma rays emitted by radio- 

 active elements, neutrons, ultraviolet rays, chemicals such as the war 

 gas known as nitrogen mustard, even heat and cold are slightly effec- 

 tive. Cosmic rays and other particles bombarding the earth may account 

 for some of the spontaneous mutations, but since genes are exceedingly 

 complex molecules it is quite likely that metabolic processes in the cell 



