This article provides a new method for analyzing the evolution of senescence (aging) in populations of organisms.

Allowing genotypic variation in lifespan/fecundity trade-offs generates

predictions conforming to standard theory, including population fecundity trends with age,

population mortality trends with age, Williams’ Hypothesis, the evolution of semelparity and

iteroparity, and differential survival for individuals removed from the influence of an extrinsic

death rate. The Euler-Lotka equation and expressions deriving from it generalize to the genotype-

distinguishing case. In a departure from conventional thinking, the analysis shows that,

even in the presence of genotypes expressing an early-age fecundity advantage, populations

can evolve that favour genotypes with lower fecundity whose intrinsic lifespan is longer. The

analysis also hints that genotype structure is a determinant of equilibrium population size.

A new metric that is laboratory-measurable – mean intrinsic lifespan – follows naturally from

the methodology. This turns out to be also a metric of semelparity/iteroparity.