The evolution of post-reproductive life: adaptationist scenarios

Abstract

Introduction In adaptationist scenarios, traits are selected for if they are positively correlated with reproductive success (Fisher, 1930; Price, 1970). It seems counter-intuitive, then, that natural selection would select for a dampening, or complete cessation, of reproductive ability before the end of the somatic lifespan. If the production of offspring is a means by which favorable traits can be passed to future generations, then the continued production of offspring – even the costliest of offspring – would seem to be beneficial. Why was there a phylogenetic shift in female reproductive strategy away from continued egg production in fish, amphibians, and reptiles, to a finite number of eggs that slowly dwindles across the lifespan to the point, in some mammalian species, of follicular exhaustion and low levels of ovarian hormones prior to death? In other words, why is there a menopause? This chapter reviews the evidence for two categories of adaptationist scenarios. In the first, menopause and post-reproductive life are the direct products of natural selection. In the second, menopause and post-reproductive life are the indirect by-products of natural selection for other traits. A similar differentiation between menopause as either adaptation or epiphenomenon has already been made (Peccei, 2001), but the argument presented here takes a slightly different tack by emphasizing the byproduct scenario as also adaptationist. In the byproduct scenarios presented here, mammalian patterns of early oogenesis (egg production) and lifelong atresia (ovarian follicle loss) are the reproductive strategies that underwent positive selection.