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CHAPTER 26 



the mutation rates to and from it are different. 

 In either or both events, the genetic equihb- 

 rium will be shifted until a new one is attained. 

 Thereafter, the new equilibrium will be main- 

 tained until some new factor acts on mutation 

 rate in a directional way. 



We have also presumed in our model that 

 the reproductive potential is the same regard- 

 less of what the genotype is with respect to eye 

 color. But it is possible, under certain con- 

 ditions, that persons with blue (or those with 

 brown) eyes are preferred as mates, in which 

 case the reproductive potential of an indi- 

 vidual would not be independent of the alleles 

 under consideration. Accordingly, if indi- 

 viduals with a certain genetic endowment 

 produce more surviving offspring than do 

 those of a different genetic endowment, the 

 genes which confer this higher biological fit- 

 ness will tend to increase their frequency in 

 the population, while those genes with lower 

 fitness will tend to decrease. In this way, 

 selection, operating on genotypes of different 

 biological fitness, causes changes in gene fre- 

 quencies, and shifts in the genotypic frequen- 

 cies found at equilibrium. 



We have also presumed that the Martian 

 population was a large one. When popu- 

 lations are very large, oscillations that occur 

 by chance in the number of children pro- 

 duced by different genotypes do not matter 

 for they do not change the gene pool. In 

 small populations, however, such chance 

 oscillations can change gene frequencies. 

 Suppose, for example, the Martian popula- 

 tion whose gene pool is M and 2B runs 

 short of food, so that only one couple, de- 

 termined by chance, can have children. The 

 chance that this husband and wife will be 

 blue-eyed is .64 X .64, or about .41, or 41%, 

 which is the chance that the gene pool will 

 drift at random in this particular manner, to 

 produce the new gene frequencies of 1.0 for 

 b and for B. This random genetic drift can 

 be illustrated also in a less extreme situation. 

 If the Martian population is very large, and 



a certain family chances to produce a rela- 

 tively large number of children for several 

 generations, then the proportion of all indi- 

 viduals having this family name will still be 

 very small. But if the Martian population 

 decreases while the reproductive rate of this 

 family is unchanged, the proportion of all 

 people with this surname will be increased. 



Finally, we have not mentioned the possi- 

 bility that the Martian colony will have emi- 

 grants or immigrants. If the emigrants have 

 gene frequencies that differ from those in the 

 population gene pool, the gene frequencies 

 in the remaining population will be changed. 

 If the immigrants have a different gene fre- 

 quency from the natives and interbreed with 

 them, this also will change the gene pool. 

 In this way migration can shift gene equi- 

 librium. 



We see then that a cross-fertilizing popula- 

 tion will remain static, in genetic equilibrium, 

 in the absence of mutation, selection, ran- 

 dom genetic drift, and differential migration. 

 The occurrence of one, another, or all of these 

 will change the frequencies of genes in the 

 gene pool and thereby shift 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. 

 In accordance with this view, since these four 

 factors serve to change gene frequencies, they 

 may be considered to be the principal causes 

 of species formation. Insofar as the forma- 

 tion of higher taxonomic categories is, like 

 speciation, based upon change in gene pools, 

 mutation (which supplies the raw materials), 

 selection (which shapes these raw materials 

 into the biologically fit genotypes of races 

 and species), random genetic drift (which 

 can produce rapid changes in gene frequency 

 in small populations), and differential migra- 

 tion (which can shift gene frequencies via 

 interchange of individuals between popula- 

 tions) are the principal causes of biological 

 evolution. 



