360 INTRODUCTION TO EVOLUTION 



What we may call the varying effects of the mutations submitted to the 

 action of natural selection constitutes another variable factor. Although 

 most mutations are recessive, some of them produce detectable effects in 

 individuals heterozygous for them, as noted previously (p. 347). Thus an 

 individual heterozygous for a recessive mutation (Aa in constitution) may 

 have lowered, or increased, viability as compared to an individual homo- 

 zygous for the dominant characteristic (AA). Dobzhansky (1947), and 

 Reed and Reed (1948) have investigated some configurations of chromo- 

 somes in Drosophila which seem to be disadvantageous to flies possessing 

 them in double dose (i.e., homozygous for them). These disadvantageous 

 chromosomal arrangements are prevented from complete elimination, 

 however, because individuals heterozygous for the arrangements are bet- 

 ter adapted to survive under certain environmental conditions than are 

 homozygotes for the advantageous arrangement. It is as though Aa indi- 

 viduals were more viable than either A A or aa individuals. (See further 

 discussion of these experiments on pp. 459-464, and of "The role of het- 

 erozygotes," pp. 457-469.) 



At times, also, a mutation which produces a visible characteristic of nei- 

 ther advantage nor disadvantage to its possessor may be acted upon by 

 natural selection because of associated effects of the same gene. Thus a 

 structural change harmless in itself may be associated with lowered viabil- 

 ity. Natural selection will reduce the proportion of the population having 

 the lowered viability and in doing so will necessarily reduce pari passu the 

 proportion of the population having the harmless structural feature. (See 

 also p. 350.) 



Another variable of importance is the role of chance in the form of ge- 

 netic drift (pp. 349-351). We recall that the effect of drift varies with 

 varying population size, being particularly effective when populations are 

 small, as in isolated animal communities, or following a severe winter 

 which has killed large numbers of the population. 



Mutation pressure, selection pressure, varying effects of mutations, ge- 

 netic drift, and many other factors combine in varying proportions to pro- 

 duce the type and degree of evolutionary change observable in any one 

 animal group at any given time in its history. Further discussion of these 

 factors will be found in the following chapters. 



Mutations with Large Effects 



From the foregoing discussion the reader will have gained the impres- 

 sion that a single mutation always produces a small effect — a small in- 



