54 I The Process of Evolution 



to reduce the possible deleterious effects of minor events such as a 

 single base-pair substitution. 



Polygenic systems that express relatively little variability may 

 store tremendous potential variability simply because they have the 

 ability to respond to selection by producing genotypes which, in the 

 absence of selection, would never be produced. Let us suppose that 

 a character is controlled by 40 loci, at each of which there are -|- and 



— alleles, and that the effects of the genes are additive (e.g., the 

 most extreme phenotypes have all loci homozygous -\ — h or homo- 

 zygous ) . If the gene frequency at each locus were +.50 and 



— .50, then, in the absence of selection, the probability of a single 

 diploid individual having the extreme + phenotype (being homo- 

 zygous -| — h at each locus ) would be ( V2 ) ^", a number infinitesimally 

 smaller than one divided by the number of electrons in the universe 

 —for all practical purposes, zero. However, this potential could be 

 realized in perhaps 8 or 10 generations by selection favoring indi- 

 viduals with a maximum of -j- alleles. Multiple-factor systems of 

 inheritance provide, then, an important mechanism for maintaining 

 balance between fitness for the immediate environmental situation 

 and flexibility for response to long-range change in the environment. 



SUMMARY 



In the majority of organisms, genetic material, DNA, is associated 

 with long protein strands forming chromosomes. The chromosomes 

 are linearly differentiated into functional units called genes, exist- 

 ing in numerous allelic states, which control the formation of spe- 

 cific enzymes. Mutation of genes to diflFerent allelic states occurs 

 spontaneously with a frequency of from 10~''' to 10^*^ per generation. 

 Meiosis and crossing-over result in recombinational units, usually 

 equivalent to the functional genes. Except for chromosome linkage, 

 genes segregate and recombine independently in the zygotes. Intra- 

 allelic interaction or dominance and interallelic interaction or epis- 

 tasis occur. Some characters are aflFected by genes with conspicuous 

 major effect, although modifying factors also may be found. Most 

 characters are controlled by a very large number of nonhomologous 

 genes, each with relatively small effect. Study of the resulting quan- 

 titative variation is complicated by the diflBculty of separating the 

 various fractions of the genetic component of variation from each 

 other and from the environmental component. The basic source of 

 variation is gene mutation. In populations of sexual higher organ- 

 isms, recombination is more important as a source of immediate 

 variability in the short-term analysis. 



