THE CONCEPT OF EVOLUTION 707 



may bring about some spontaneous mutations without the intervention 

 of external agents. 



Both spontaneous and artificially induced mutations occur at ran- 

 dom; the appearance of a mutation bears no relationship to the kind of 

 inducing agent or to the particular need of the organism at that time. 

 There is no way of producing to order a particular kind of mutation— 

 a particular kind of biochemical mutant in Neurospora, for example. 

 An investigator who wants to use some particular mutant has no choice 

 but to irradiate many organisms, produce hundreds or even thousands 

 of mutations, and then select the one he particularly wants. 



Whatever the causes of mutations may be, their central role in 

 evolution as the raw material for natural selection is now generally ac- 

 cepted. Some evolutionists have in the past objected that the spontaneous 

 or induced mutations observed in the laboratory could not be the basis 

 for evolution for almost all of them are deleterious, and because the 

 differences between species are usually slight variations, affecting many 

 different parts of the organism and inherited by means of multiple 

 factors, whereas the mutations observed in the laboratory are usually 

 large variations, involving a single organ and inherited by single gene 

 differences. Studies in the genetics of wild populations have shown that 

 mutations that occur in the wild, like the ones observed in the labora- 

 tory, are usually for detrimental traits. We must keep clearly in mind 

 that the animals and plants living today are the result of a long and 

 rigorous process of natural selection. In the course of their evolution, 

 most of the possible mutations have occurred, and the beneficial ones 

 have been selected and preserved. The organisms are well adapted to 

 their surroundings and further mutations are much more likely to be 

 harmful than helpful. However, a few of the mutations seen in the 

 laboratory and in wild populations are beneficial and have survival 

 value. Mutations may produce traits which are deleterious in one en- 

 vironment but advantageous in another. Sickle cell anemia, for example, 

 is generally disadvantageous but its resistance to malaria is advantageous 

 in regions such as Central Africa where malaria is very widespread. 



Closer study of populations has shown that the sort of variations 

 which differentiate a species do appear in stocks bred in thfe laboratory. 

 However, being somewhat more difficult to detect and study, they were 

 missed in some of the earlier work. More recent experiments indicate 

 that such mutations occur at an even greater rate than the larger, more 

 obvious ones. 



303. Straight-Line Evolution 



Many of the earlier paleontologists and other students of evolution 

 were led to the conclusion that there are trends in evolution, that 

 evolution tends to progress in a straight hne. The term orfhogenesis 

 was coined to refer to straight-line evolution; some investigators had the 

 somewhat mystical belief that organisms have an inherent tendency to 

 evolve in a predetermined direction. More recent, fuller exanunations of 

 the accumulating fossil data, however, have shown that many of the 



