Chapter 27 



MUTATION AND SELECTION — 

 NONRANDOM MATING 

 AND HETEROSIS 



Ti 



Ihe composition of the gene 

 pool is dependent upon several 

 factors, including mutation and 

 selection (Chapter 26). Selection acts at the 

 phenotypic level to conserve in the population 

 those genotypes which provide the greatest 

 reproductive potential. Selection takes place 

 at all stages in the life cycle of an individual. 

 If a particular stage has been produced under 

 the action of a haploid genotype, selection 

 will favor the most adaptive phenotypes 

 and thereby conserve the most adaptive hap- 

 loid constitutions. If other stages in the life 

 history involve the action of a diploid con- 

 stitution, selection will favor the most adap- 

 tive phenotypes and therefore the diploid 

 genotypes they contain. Several implications 

 of these statements need to be specified, 

 namely, that selection acts not upon single 

 genes, but upon genotypes, sometimes acting 

 upon the phenotypes produced by single 

 genomes, and at other times, in sexually re- 

 producing organisms, acting upon the com- 

 bined phenotypic effect of two genomes. It 

 should be noted also that what is a relatively 

 adaptive genotype at one stage of the life 

 cycle may be relatively ill-adaptive at another 

 stage, regardless of whether or not these 

 stages have the same or different ploidies. It 

 is, of course, the total adaptiveness of all these 

 separate features which determines the over- 

 all reproductive potential of an individual. 

 Finally, it should be noted that in cross- 

 fertilizing populations selection favors geno- 

 229 



types which produce maximal fitness of the 

 population as a whole. Because selection 

 acts this way, it may be that some portion of 

 the population may receive genotypes which 

 are decidedly disadvantageous to the indi- 

 viduals receiving them. If this is so, the 

 same genetic components would be expected 

 to be advantageous when present in other, 

 more probable combinations. 



Since the human being is primarily of one 

 ploidy, diploidy, it is upon his diploid- 

 produced phenotype that selection principally 

 operates. If we ask, "What is the fate of 

 mutants in the gene pool?" our answer must 

 include knowledge of the frequency with 

 which the mutants arise and of their effects 

 upon reproductive potential in a diploid 

 genotype. The phenotypic effect of a mu- 

 tant gene will depend, not only upon the 

 nature of its allele, but also upon its relation- 

 ship with the nonalleles in the genotype 

 (Chapter 7). Let us consider, in human 

 beings, the fate in the gene pool of mutants 

 whose over-all phenotypic effect is dominant 

 lethal, or dominant detrimental, or recessive 

 lethal, or recessive detrimental, insofar as 

 selection and mutation influence their fates. 



Dominant lethal mutations are lethal when 

 heterozygous, and are eliminated from the 

 gene pool in the same generation in which 

 they arise. Accordingly, the biological fit- 

 ness, adaptive value, or reproductive potential 

 of such mutants is zero. If the normal 

 homozygote {AiAx) is considered to have a 

 selective disadvantage of 0, then the domi- 

 nant lethal has complete selective disadvan- 

 tage, its selection coefficient, s, being there- 

 fore 1 . You can readily see that the mutation 

 frequency or rate, u, to this dominant lethal 

 condition must be equal to one half the fre- 

 quency of affected individuals (AiAo), since 

 each affected individual has one mutant and 

 one normal gene. In the absence of special 

 medical treatment, retinoblastoma, a kind of 

 cancer of the eye, is an example of such a 

 dominant lethal in man. 



