Differential Fertility and Human Evolution
Human biological evolution, like evolution in any other species, is ultimately dependent on two phenomena, namely, changes in the gene pool of the species due to genetically based differences in the survival of individuals, and changes due to genetically based differences in reproductive performance. The higher the early mortality rates in a population, and the greater the spread in number of children reared to maturity by those reaching maturity, then the greater the rate at which genetic change can occur, given that there is some genetic basis for either of these phenomena. The truism was more elegantly and more mathematically stated by Fisher (1930) as follows: “The rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time.” This Fundamental Theorem of Natural Selection, as he termed it, measures fitness in terms of reproductive value, that is, as the intrinsic rate of increase associated with a given genotype. This intrinsic rate of increase he has called the Malthusian parameter.
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