Reproductive timing and individual fitness

Estimation of individual fitness – i.e. description of the extent to which an individual's genes are represented in future generations – is a feature central to most evolutionary studies. Lifetime reproductive success (LRS) is a commonly used estimate of individual fitness, but because it is rate‐insensitive (i.e. timing of reproductive events is not incorporated), it may give a biased estimate of fitness when reproductive timing is an important component of fitness. A review of all empirical studies which have used a recently derived, rate‐sensitive estimate of individual fitness, λ ind revealed that λ ind ranks the fitness of phenotypes differently from LRS, and that this difference may lead to different conclusions about strength of selection acting on phenotypic traits. However, although λ ind may be a better estimate of individual fitness than LRS in certain situations (e.g. in growing populations), its application is not always unproblematic. For instance, in contrast to rate‐insensitive estimates of individual fitness, the λ ind is sensitive to the age at which offspring are censused and there is little consensus among published studies on when offspring should be counted. Further, rate‐sensitivity does not necessarily improve a fitness estimate in spatio‐temporal variable environments. We suggest that the ultimate test on the applicability of λ ind vs. LRS as practical measures of individual fitness in quantifying selection should come from studies which correlate these estimates with actual number of descendants left more than one generation further in future.