THE COST OF EVOLUTION VS. THE COST OF NOT EVOLVING

Haldane (1957) under the title "The Cost of Natural Selection" investigated the replacement of a gene by a more favorable allele, in terms of the total number of deaths required to eliminate the less advantageous allele from the population. This he found to be relatively independent of the intensity of selection, with the number of generations required for the change varying inversely with the number of deaths per generation. He estimated the total number of deaths, where selection was moderate (not more than one-third of the adversely affected genotypes being eliminated) to be 10-20 times the number of individuals living at any one time. This figure was based on the assumption that the favored phenotype had a frequency of only 0.0001 at the beginning of the process. He pointed out that this initial frequency is the most important factor influencing the total number of deaths, since a lower frequency serves to protract the initial period during which the viability of the population is at a minimum. On the basis of the apparent burden of excess deaths involved in the process, he hypothecated a limit on the rate at which gene substitutions could take place in a natural species. Kimura (1960) referred to Haldane's work in a discussion of genetic load, which he divided into three types: (1) mutational (the type classically discussed by Muller, 1950), (2) segregational, or due to the loss of homozygotes in a balanced polymorphism (first so called by Crow, 1958), and (3) substitutional or evolutional. In reference to the latter he states "The process of substituting one allele for another through natural selection involves lowering of population fitness and thus creates a genetic load. " The concept was referred to again by Mayr (1963) under the heading "The Cost of Evolution. " In order to clarify these concepts, it is necessary to review some of the conventions customarily used in setting up a mathematical model for selection. It is convenient in such problems to take the viability of the most favored known genotypes as a standard, and measure all others relative to it, so that a conspicuous parameter in the calculations is commonly one of the form (1 x) denoting the viability of a genotype that suffers a net loss of x per generation as compared with an "optimum" genotype. Another convention is to adjust the population size in some manner so that it appears to be constant from generation to generation. These mathematical devices, while necessary to produce a neat model of the selective process, may ignore certain realities. For instance, no account is generally taken of whether one, both, or neither of the viabilities designated by 1 and. (1 x) are adequate to maintain the existence of the population at a constant level. If a population has only one genotype, that genotype will be classed as "optimum" and there can be no "inferior" genotypes. Thus the mathematical model will show no mortality at all, even though, as in the case of some egg-laying organisms, the actual average of individual survival is somewhere in the vicinity of one per million. Even worse, a one-genotype population may appear to be doing very well in the mathematical model, even though in reality it is rapidly becoming extinct because its one genotype is inadequate under current conditions^. On the other hand, a population with a variety of genotypes will always show some selective losses in the model because some of its genotypes have to be rated less than 1, although it may, in fact, be increasing in numbers quite rapidly. The paradox becomes clearer if we imagine a situation in which there occurs within a population a new mutation with favorable effects on viability. In order to incorporate this into the scheme, it is necessary to de-