The Evolutionary Significance of Lifetime Reproductive Success

Abstract

ABsTRAc-r.-Recent studies have reported lifetime reproductive success (LRS) as empirical counts of fledglings, yearlings, or breeders produced by individuals during their lifetimes. In this paper, I show how the mean LRS of the breeding individuals of a population can be calculated from age-specific probabilities of survival and fecundity. An analysis of a simulated population shows that the LRS of males and females from the same population can be different, even though the rates of change in numbers of males and females are identical. Thus, although LRS may be a measure of individual fitness, differences in LRS, even among individuals with different phenotypes and genotypes, cannot be construed to have evolutionary significance. The proper measure of fitness is the Malthusian parameter (mrj). Received 5 March 1991, accepted 30 July 1991. THE ANALYSIS of data from long-term studies of marked birds has resulted in the measurement of lifetime reproductive success (LRS), which refers to the actual production of offspring of specific individuals (Brown 1988, Partridge 1989). Lifetime reproductive success is the product of an individual's reproductive life span (L), fecundity per reproductive year (F), and offspring survival (S; e.g. Brown 1988, Grafen 1988). Usually, investigators calculate the means and variances of LRS and its components (see studies in Clutton-Brock 1988a and Newton 1989a). There is general agreement that LRS is not a measure of Darwinian fitness (e.g. Grafen 1988, Newton 1989b, Partridge 1989). Nevertheless, Newton (1989b:441) suggested that it provides a better basis for estimating biological fitness than any other measure yet available, and Partridge (1989:435) indicated that it is probably a good approximation to [fitness] for many bird populations. In this paper I will show how the mean LRS of a group of breeders can be calculated from life-table probabilities and, having done that, I will discuss the evolutionary significance of lifetime reproductive success.