SOME PROBLEMS IN ESTIMATING THE INTENSITY OF SELECTION THROUGH FERTILITY DIFFERENCES IN NATURAL AND EXPERIMENTAL...

We used empirical data from a laboratory population of Heterandria formosa (Pisces, Poeciliidae) and simulations based on those data to examine methods of estimating the intensity of phenotypic selection through fertility differences when generations overlap. The correct intensity was known because we knew the history of every female, as well as the rate at which the population was growing. The net reproductive rate, which is an appropriate measure of fitness when generations do not overlap, underestimated the true intensity of selection. This measure of fitness did not provide a rank order of individuals that agreed significantly with the correct rank order. Simulated schemes of vertical sampling of females, using number of offspring produced during the sampling window as a measure of fitness, provided no consistently useful information about the direction or intensity of selection. Weighting the number of offspring by female age, to give more value to offspring of younger females, produced only a slight improvement in accuracy. Other data indicated that fertility differentials are closely tied to particular environmental conditions. When generations overlap, estimates of fertility differentials will have to be based on horizontal studies of lifetime performance of a cohort coupled with demographic information on the entire population.