654 



ORGANIC EVOLUTION 



later, have no effect on ttie genes, fience are 

 of no significance in evolution. Mutations 

 may involve only a single gene (micromu- 

 tations), which is the most usual, or they 

 may include a group of genes (macro- 

 mutations ) , which account for sudden large 

 changes such as short legs in sheep or extra 

 toes in cats. Most biologists believe that 



Fig. 26-3. Three mutations that commonly occur in ani- 

 mals. Curled wings in Drosophila is recessive to nor- 

 mal wings. Hornless (polled) condition in cattle is 

 dominant to horned. Fused or webbed toes (syndac- 

 tyly) and extra toes (Polydactyly) in man are dominant 

 to normal toes. 



species have originated from the accumula- 

 tion of a great many micromutations rather 

 than by one or more macromutations. Ac- 

 cordingly, it is doubtful that a full-blown 

 species ever appears in one generation, it is 

 more likely that many tiny, almost imper- 

 ceptible, mutations occur and by the proc- 

 ess of natiual selection accumulate until a 

 new species appears. 



The first careful records made of muta- 

 tions were tliose of the Dutch botanist, 

 Hugo DeVries, who described them in the 

 evening primrose. In fact, DeVries named 

 these sudden and spontaneously occurring 

 changes mutations, a term which today ap- 



plies to any alteration in the genie compo- 

 sition of an organism. DeVries thought that 

 evolution occurred as a result of mutations 

 and not by natural selection. Now we know 

 that mutations provide the basis for the 

 operation of natru'al selection. In other 

 words, both mutations and natural selec- 

 tion are essential for evolution. 



Mutations have been noted in a large 

 number of plants and animals under experi- 

 mental conditions and they are undoubtedly 

 occurring continuously among organisms in 

 their natural habitats (Fig. 26-3). Over a 

 thousand mutations have been recorded for 

 Drosophila alone. A great many gene 

 changes probably go unnoticed which, in 

 nature, might have considerable survival 

 value. For example, a mutation might oc- 

 cur in an animal living in a region where 

 some element in the diet was becoming 

 deficient, which would so affect the enzyme 

 systems of the animal that it could get 

 along on a minimum amount of the ele- 

 ment. Such a mutation might show no visi- 

 ble difference in the animal itself and yet 

 be most important in survival. Animals with 

 the favorable gene would be more apt to 

 live to produce progeny than those without 

 it, hence selection would be toward the 

 favorable gene. While we have a great deal 

 of information concerning anatomical mu- 

 tations we know almost nothing about bio- 

 chemical mutations (see p. 586). Neverthe- 

 less, such physiological changes must have 

 been and must continue to be very impor- 

 tant in their evolution. 



Isolation 



Darwin realized, as have most biologists 

 since, that isolation played a very important 

 part in the origin of new species, but it was 

 not until inheritance was thoroughly under- 

 stood that the full meaning of isolation be- 

 came apparent. It is known now tliat in 

 addition to geographic isolation, genetic 

 isolation is also essential for the origin of 

 new varieties and eventually new species. 

 Genetic isolation may even be the more 



