VARIABLE GENERATION TIMES AND DARWINIAN FITNESS MEASURES

Reproductive value (RV) and net reproductive output (R(o)) are frequently used fitness measures. We argue that they are only appropriate when intervals between reproductive events are fixed, as they are dimensionless generation-to-generation scalings with units offspring per parent. A fitness measure should account for two different effects of a decrease in generation time: (1) increased survival due to shorter exposure to mortality agents and (2) increased frequency of reproduction. R(o) and RV deal with the first of these two effects, while a measure with a physical dimension per time [T-1] is needed to account for the second. The Malthusian growth parameter, r, meets this requirement and in situations where time to reproduction is variable, we propose rho, the instantaneous rate of spread of descendants (from an individual) be used instead of R(o). As an alternative to RV, we suggest using the instantaneous difference PHI = rho - r, where r is the population rate of increase. While RV and R(o) are dimensionless ratios, PHI, and rho are per time rates which are appropriate in accounting for alterations in generation time.