1960 年 35 巻 1 号 p. 7-33
1. Definition of genetic load: Genetic load of a population in CROW'S sense (CROW 1958) refers to the amount by which population fitness is decreased through natural selection acting on genotypic differences. It may either be expressed in terms of selective values or Malthusian parameters depending on whether the continuous or discrete model is used to describe generation time (cf. KIMURA 1958).
2. Kinds of genetic load: (i) Mutational load results from elimination of harmful mutations. (ii) Segregational load arises when heterotic genes (overdominance load) or a meiotic drive mechanism (distortional load) is involved. (iii) Dysmetric load, term proposed by HALDANE (unpublished), refers to a load which may be created when there is a “division of labor” between genotypes. (iv) Internal vrs. external load; The former refers to the decrease of fitness relative to the optimum genotype and thus relates to intragroup selection, while the latter refers to that relative to the optimum condition and therefore relates to intergroup selection. (v) Substitutional (evolutional load) is a cost of natural selection (cf. HALDANE, 1957) which is required in the process of substituting one allele by another through natural selection.
3. Principle of minimum genetic load: This is a hypothesis that in the course of evolution important genetic parameters tend to be adjusted in such direction that the total genetic load will be minimized. More specifically, the spontaneous mutation rate and degree of dominance of mutant genes may be adjusted such that the sum of the mutational and the segregational load
will be minimized. From this principle, the following relations may be derived (KIMURA, 1959):
where ∑μ is the spontaneous mutation rate per gamete per generation, h is the average degree (harmonic mean) of dominance in fitness of deleterious mutant genes, D is the total mutational damage or approximately the rate of inbreeding depression in fitness per unit increase in the inbreeding coefficient and E is the rate of substituion of genes in horotelic evolution (standard rate evolution, cf. SIMPSON 1944).
Taking E=1/300 (cf. HALDANE 1957) and D=2 (cf. MORTON, CROW and MULLER 1956), we obtain from the above equations,
h=0.0196 and ∑μ=0.0585,
which agree fairly well with the corresponding observed values in Drosophila. With these values, mutational and substitutional loads (in terms of Malthusian parameters) become respectively
Lm=0.098 and Le=0.199.
4. Accumulation of genetic information by natural selection: It was pointed out that the rate of accumulation of genetic information (H) by natural selection is directly proportional to the substitutional load Le, namely
With Le=0.199, the value obtained above, this becomes about 0.29 bits;
It was estimated then that the total amount of genetic information accumulated since the beginning of Cambrian epoch (500 million years) may be of the order of 108 bits, if evolution proceeded at the standard rate.
Since the genetic information is transformed into phenotypic information in ontogeny, this figure (108 bits) must represent the amount of information which corresponds to the improved organization of higher animals as compared to their ancestors 500 million years back. It may also be interesting to compare this figure with the one (107_??_109 bits) obtained by ELSASSER (1958) as the probable “information content of the human organism”.