The Gene Pool; Equilibrium Factors 



203 



causes changes in gene frequencies and shifts 

 in the genotypic frequencies found at equi- 

 librium. 



The Martian population was also pre- 

 sumed to be large. Suppose, however, that 

 the Martian population (whose gene pool is 

 2B and .8b) ran short of food, and only 

 one couple, determined by chance, could 

 have children. The chance that this hus- 

 band and wife would be blue-eyed is 

 .64 X .64, or about .41. Accordingly, there 

 is a 41% chance that the gene pool will 

 drift at random in this particular manner, 

 producing the new gene frequencies of 1 .0 

 for b and for B. This random genetic drift 

 can also be illustrated in a less extreme 

 situation: If a population is very large, and 

 a certain family happens to produce a rela- 

 tively large number of children for several 

 generations, then the proportion of all indi- 

 viduals in the population with this family 

 name is still very small. But if the popula- 

 tion decreases while this family's reproduc- 

 tive rate is unchanged, the proportion of the 

 population with this surname increases. Ac- 

 cordingly, when populations are very large, 

 oscillations in the number of children pro- 

 duced by different genotypes occurring by 

 chance are unimportant, for they do not 

 change the gene pool. In small populations, 

 however, such chance oscillations can change 

 gene frequencies via random genetic drift. 



In our Martian model, the possibility that 

 the colony would have emigrants or immi- 

 grants was not considered. If the emigrants' 

 gene frequencies are different from those re- 

 maining in the population gene pool, then 

 the gene frequencies in the remaining popu- 

 lation will be changed. If the immigrants' 

 gene frequency is different from the natives', 

 and they interbreed, the gene pool will again 

 be changed. In this way migration can shift 

 the genetic equilibrium. 



We see then that a cross-fertilizing pop- 

 ulation remains static — in genetic equilib- 



rium — in the absence of mutation, selection, 

 random genetic drift, and differential migra- 

 tion. The occurrence of one or another or 

 all of these factors changes the frequencies 

 of genes in the gene pool and thereby shifts 

 the frequencies of genotypes at equilibrium. 

 Different species possess different gene pools, 

 and it is natural to presume that they are 

 different species because of their different 

 gene pools. Accordingly, the factors which 

 change gene frequencies are considered to 

 be the main causes of species formation. 

 Insofar as the formation of higher taxonomic 

 categories is, like speciation, based upon 

 change in gene pools, the principal causes 

 of biological evolution are: 



1. Mutation (which supplies the raw ma- 

 terials) 



2. Selection (which shapes these raw ma- 

 terials into the biologically fit geno- 

 types of races and species) 



3. Random genetic drift (which can pro- 

 duce rapid changes in gene frequency 

 in small populations) 



4. Differential migration (which can shift 

 gene frequencies via interchange of in- 

 dividuals between populations). 



* Selection of Genotypes 

 The disequilibrating effect of selection upon 

 the gene pool has already been noted. Selec- 

 tion acts at the phenotypic level to conserve 

 in the population those genotypes which pro- 

 vide the greatest reproductive potential. Se- 

 lection takes place at all stages in the life 

 cycle of an individual. Since it acts to pre- 

 serve whole phenotypes and not single traits, 

 selection conserves genotypes and not sing'e 

 genes. Sometimes selection acts upon the 

 phenotypes produced by single genomes in 

 haploid species or stages; at other times — 

 in sexually reproducing organisms — it acts 

 upon the combined phenotypic effect of two 

 genomes. It should be noted that what is 

 a relatively adaptive genotype at one stage of 



