600 



ECOLOGY AND EVOLUTION 



bach, Robson, and Carr, 1947]) have been 

 shown to induce self-perpetuating (auto- 

 catalytic) gene or genome mutations. 

 Wavelength in various types of radiation 

 has almost no influence, but the frequency 

 of mutations is proportional only to the 

 dosage, whether the dosage is given in a 

 high or low concentration (quickly or 

 slowly), with or without interruptions 

 (Bodenheimer, 1938, p. 148). Mustard oil 

 (allyl isothiocyanate) has a definite though 

 slight eflFect in increasing mutations of 

 Drosophila melanogaster (2.2 per cent 

 lethals detected on the X-chromosomes, as 

 contrasted to 0.4 per cent in the controls), 

 and this substance occurs naturally in a 

 variety of plants such as common mustard, 

 Brassica nigra, and other Cruciferae (Auer- 

 bach and Robson, 1944). 



None of these mutation-inducing agents 

 selects particular gene loci or controls the 

 direction of the effect of the resulting mu- 

 tation. (For specific or adaptive effects, see 

 discussion on page 601.) The effect of the 

 mutations may be either localized accelera- 

 tion or inhibition of growth (Glass, 1944). 



Mutation pressure often has little or no 

 direct ecologic causation (Timofeeff-Res- 

 sovsky, 1940). Spontaneous mutation rates 

 of a given kind of mutation of the order 

 of 10'° to 10'* per gene per generation have 

 been observed in maize, Drosophila, and 

 man. A few genes have higher mutation 

 rates, but probably most have lower rates. 

 These variations are possibly the result of 

 varying chemical stabihty of the protein 

 molecules involved. 



Plough (1941, 1942) indicates that in- 

 creasing temperature is associated with mu- 

 tation frequency in a typical Van't Hoff 

 curve (p. 107) and suggests that it is per- 

 haps this relation of temperature to muta- 

 tion that determines the greater number of 

 species in tropical areas. Wright (1932), 

 after analyzing the effect of increased mu- 

 tation, indicates it would be the spreading 

 of the field occupied by the species and an 

 average lowering of the adaptive level of 

 the species. Reduced selection would have 

 the same result (Herre, 1943). Increased 

 mutation might increase the number of 

 novel favorable mutations with an occa- 

 sional increase in adaptability. By increas- 

 ing the range of variability, more mutations 

 might allow a change in the direction of 

 adaptation. Reduced mutation rate would 



act in a manner similar to increased selec- 

 tion pressure. It would reduce variability, 

 raise the average adaptive level of the 

 species in its environment, and give a 

 smaller chance for novel favorable muta- 

 tions that might change the direction of 

 adaptive evolution. However, differences in 

 rates of mutation are inadequate to explain 

 most of the observed speciation, because 

 other factors almost always exert a stronger 

 influence. 



A high mutation rate is not necessarily 

 an advantage in evolutionary progress (Fig. 

 229). Mutations are usually deleterious to 

 existing adaptations, and the function of the 

 hereditary mechanism is primarily conserv- 

 ative rather than creative. A higher po- 

 tential mutation rate may be counterbal- 

 anced by the selection of a low mutating 

 strain. There is evidence that a moderate 

 rate of mutation is itself an adaptive 

 character (p. 684). In addition, several 

 other factors may obscure the effect of mu- 

 tation rate. Simpson (1944, p. 62) says 

 that "the actual result of mutation rate on 

 evolutionary rate is hkely to be effected not 

 only by the relative number but also by the 

 absolute number of times that a mutation 

 occurs, and this depends upon the abso- 

 lute number of individuals in which it 

 could occur, which is the average size of 

 the population multiplied by the length of 

 time involved and divided by the average 

 length of a generation." Simpson makes it 

 clear that the fossil record shows no close 

 correlation between rate of evolution, var- 

 iability, and length of generations. "Opos- 

 sums have short generations and elephants 

 are near the maximum; but the evolution 

 of the elephants has been many times more 

 rapid than that of opossums" (see also pp 

 654,662,680). 



Tropical areas often contain both more 

 isolating factors and more ecologic niches, 

 and these together constitute a more satis- 

 factory explanation of the greater diversity 

 of the species than increased mutation 

 caused by warmer temperatures. Darling- 

 ton (1943) has shown that, where differ- 

 ences of range of temperature are not great, 

 islands tend to be populated with large 

 numbers of individuals of a few species of 

 carabid beetles, whereas adjacent continen- 

 tal areas contain smaller numbers of in- 

 dividuals of a great many species. Mayr and 

 Serventy (1944) draw the conclusion that 



