On the definition and measure of inclusive fitness and the evolution of menopause

Abstract

In a recent issue of Human Ecology, Mayer (1982) dealt with an apparent paradox in evolutionary theory: the phenomenon of menopause in humans. Briefly, if natural selection designs humans to maximize their reproductive success why do females tend to live well beyond their reproductive years? Mayer noted three different explanations offered by various evolutionary biologists: (a) menopause is an artifact of modern medicine and hygiene allowing females in recent historical times to live beyond their reproductive years and is not designed by natural selection; (b) "postmenopausal survival is an unselected and unfavored by-product of a developmental process which has been favored by selection for other reasons" (Mayer 1982: 478); and (c) menopause developed through the process of kin selection whereby it is more adaptive for women to cease reproduction in old age and instead devote themselves to the enhancement of the reproduction of kin and/or offspring. After an analysis of the genealogies of four families comprising approximately 22,000 individuals spanning a period of 199 years, Mayer suggested that the kin selection hypothesis is correct and states, "As measured by inclusive fitness, adult women who died after age 50 were evolutionarily more successful than adult women who died at earlier ages" (486). Unfortunately, Mayer's demonstration has no bearing on the adaptiveness of menopause. His premature conclusion is a result of a faulty definition of inclusive fitness and an invalid test of the hypothesis. Hamilton (1963, 1964) devised the concept of inclusive fitness to account for certain kinds of behavior (e.g., altruism) that appeared to lower