THE DYNAMICS OF EVOLUTIONARY CHANGE 



environment, including the other living organisms with which 

 the population is associated. Even if these factors remain con- 

 stant, a gene can change markedly in adaptiveness as it becomes 

 associated with different genes during the continuous process 

 of segregation and recombination which goes on in any cross- 

 fertilizing population. 



These facts have led Dobzhansky (1955) to the "balance 

 hypothesis" of population structure. According to this hy- 

 pothesis, adaptiveness is normally maintained through a com- 

 bination of genes and chromosomal segments kept in the hetero- 

 zygous condition. Individual genes do not usually respond 

 directly to selection as separate units, but rather their adaptive 

 value depends upon the way in which they contribute to the 

 gene pool. Large numbers of genes are thus kept in the germ 

 plasm because their effects are neutral or actually beneficial in 

 heterozygous combinations with other alleles, when these same 

 genes would be strongly disadvantageous or actually lethal if 

 they were present in the homozygous, or "pure," condition. 



The experiments of modern population genetics, therefore, 

 tell us that natural selection acts indirectly upon a spectrum of 

 variability generated largely by genetic recombination which 

 takes place in a very complex fashion. Mutation serves to 

 replenish the store of variability as it becomes depleted through 

 selection. Most of the genes which enable a population to 

 change in response to a new environment are not newcomers 

 which have recently appeared through mutation; they are "old- 

 timers" which presumably originated by mutation many genera- 

 tions ago and have been preserved either as hidden recessives 

 or as contributors to some past adaptive complex. 



This principle does not mean that genes with large effects 

 on the appearance of the individual never play an important 

 role in evolution. On the contrary, some of the most striking 

 demonstrations of the action of natural selection which have 

 been made in recent years have involved such genes. The most 

 striking example is industrial melanism. In the industrial areas 

 of Europe, populations of moths have become completely trans- 

 formed in a few years from light- to dark-colored types through 

 selection of dominant mutations for dark color. The experi- 

 ments of Kettlewell (1955) have shown conclusively that these 



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