Genetic Loads and Their Population Effects 



221 



majority of point mutants are detrimental, 

 and that, perhaps, only one point mutant in 

 a thousand minutely increases the reproduc- 

 tive potential of its carrier. Yet, provided 

 the mutation rate is not too large and there 

 is sufficient genetic recombination, these rare 

 beneficial mutants offer the population the 

 opportunity to become better adapted. 

 Moreover, mutants which lower biological 

 fitness under one set of environmental con- 

 ditions may be more advantageous than the 

 normal genes under different environmental 

 circumstances. 7 For example, a mutant 

 producing vestigial wings in Drosophila is 

 clearly inferior to its normal genetic alterna- 

 tive in an environment where flight ability 

 is advantageous; but this mutant might be 

 advantageous for Drosophila living on a 

 small island where flight is not only un- 

 necessary but harmful because insects that 

 fly can be blown out to sea and lost. Con- 

 sider a second example of this type. Several 

 decades ago the environment was DDT-free. 

 and mutants which confer immunity to DDT 

 were undoubtedly less adaptive than the 

 normal genetic alternatives present. But 

 once DDT was introduced into the insect 

 environment, such mutants — even if detri- 

 mental in other respects — provided such a 

 tremendous reproductive advantage over 

 their alternatives that they became estab- 

 lished in the population as the new wild- 

 type genes. Still other examples can be 

 cited involving antibiotic-resistant mutants 

 in microorganisms, which in an antibiotic- 

 free environment are less adaptive than the 

 genes normally present. 



It becomes clear, then, that mutation pro- 

 vides the opportunity for a population to 

 become better adapted to its existing en- 

 vironment. It also provides the raw mate- 

 rials needed to extend the population's range 

 to different environments, either those al- 

 ready existing elsewhere or those that will 



•See Th. Dobzhansky (1964). 



arise through changes. A population that 

 is already very well adapted to its present 

 environment is appreciably harmed by the 

 occurrence of mutation. But environments 

 differ, and any given environment will even- 

 tually change, so that a nonmutating popu- 

 lation though successful at one time will, 

 in the normal course of events, eventually 

 face extinction. Mutation, therefore, is the 

 price paid by a population for future adap- 

 tiveness to the same or different environ- 

 ments. We can now appreciate that muta- 

 tion and selection, together with genetic 

 drift and migration, are primarily responsible 

 for the origin of more adaptive genotypes. 

 We can also better appreciate the advantage 

 of genetic recombination in speeding up the 

 production of adaptive genotypes and the 

 importance of the genetic mechanisms which 

 regulate mutation frequency. 



Somatic Mutations 



In view of the preceding discussion, it is not 

 at all difficult to predict the consequences of 

 increasing the mutation frequency in human 

 beings, an increase that doubtlessly is oc- 

 curring as a result of our exposure to man- 

 made penetrating radiations and certain re- 

 active chemical substances. Man-made as 

 well as spontaneous mutations can occur in 

 either the somatic line or the germ line. 

 Somatic mutants are, of course, restricted 

 to the person in which they occur. The 

 earlier the mutation occurs in a person's 

 life, the larger will be the sector of somatic 

 tissue to which the mutant cell gives rise. 



When an adult is exposed to an agent 

 which causes a mutation to occur in a cer- 

 tain percentage of all cells, the cells carrying 

 induced mutants will usually be surrounded 

 by nonmutant ones of the same tissue whose 

 overall action produces a near-normal phe- 

 notypic effect. When an embryo is exposed, 

 a proportionally smaller number of its cells 

 will mutate. Mutant embryonic cells can, 

 however, give rise later to whole tissues or 



