VARIABLE GENERATION TIMES AND DARWINIAN FITNESS MEASURES

Reproductive value (RV) and net reproductive output (R(o)) are frequen tly used fitness measures. We argue that they are only appropriate whe n intervals between reproductive events are fixed, as they are dimensi onless generation-to-generation scalings with units offspring per pare nt. A fitness measure should account for two different effects of a de crease in generation time: (1) increased survival due to shorter expos ure to mortality agents and (2) increased frequency of reproduction. R (o) and RV deal with the first of these two effects, while a measure w ith a physical dimension per time [T-1] is needed to account for the s econd. 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) b e 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 alteration s in generation time.