Variance Models in the Study of Life Histories

We propose a general model that estimates fitness from the joint effects of mean and variance. In this hierarchical model the contribution that each individual trait makes to net fitness proceeds in a stepwise fashion from the individual trait level to the fitness component and cohort fitness levels and, finally, to net fitness. We describe useful mathematical functions that incorporate both mean and variance values at each level. Empirical examples (1) demonstrate that variance, in addition to mean, can be an important determinant of fitness and (2) show how both can be used to estimate fitness, in particular in research addressing the evolution of life history patterns.