Chapters 5 and 6 deal with the questions which arise when the strategies brought forth by evolution, which we define in terms of non-invasibility, are searched by means of maximizing simple optimization criteria. In other words, when is the fittest (in the environment set by itself) also the best (in a more general sense)?
In chapter 5, I concentrate on monomorphic populations in demographic steady state, and present three different conditions under which the evolutionarily stable life-history strategy can be characterized as the life-history strategy at which a relatively simple function is maximal. Depending on the way density dependence shapes the environmental feedback loop, this function can be the individual lifetime reproductive success (R0), the intrinsic rate of natural increase (r), or another quantity from a large range of possible optimization criteria. I illustrate this by examining the optimal age at maturity for a hypothetical example organism, showing that the details of the population dynamical embedding may influence our evolutionary predictions to an unexpected extent. This analysis is extended in chapter 6, where I derive necessary and sufficient conditions for the equivalence between non-invasible strategies and maxima of optimization functions.
These results show that optimization functions should be derived, using all information available about the precise way in which density dependence limits population growth, instead of assumed, as is common practice in evolutionary ecology and life-history theory. If we want to assess the validity of prevalent optimization functions, more empirical research on mechanisms of density dependence is of crucial importance.
Taken together, the six chapters demonstrate that it pays to incorporate detailed information concerning individual life-history or population dynamics in an early stage of evolutionary model building. By taking biologically validated approximations, one can still end up with simple models, but with qualitatively different results than from models arrived at in more phenomenological ways.