The more recent research program of evolutionary ecology focuses on the evolution of behavioral and demographic parameters in ecologically less-simplified situations. Yet, in textbooks on the subject, simple viability selection models figure heavily in any chapters devoted to genetics, although on a verbal level fitness tends to be identified unrestrictedly with expected lifetime offspring production (= average number of offspring contributed to the next generation). In addition, the Malthusian parameter, also called intrinsic rate of
(natural) increase, is introduced as an appropriate measure of fitness. Usually it is left somewhat open when, and more importantly why, one or the other measure is relevant, although one finds statements to the effect that for arriving at evolutionary predictions one should maximize lifetime offspring production in the case of stable environments, and the intrinsic rate of increase in environments that are constant except for occasional decimating catastrophes. Although steps in the right direction, neither statement fully captures the intricacies of the issue.
Below it will be argued that for a fair collection of ecological scenarios and given certain minimal requirements, there exists a unique fitness concept, to be referred to as invasion fitness, that may serve as universal reference. This invasion fitness is in the first place a theoretical entity that earns its keep as tool in powerful deductive arguments and from being calculable from models fed with more concrete data. Measuring it directly is very difficult, except in the simplest possible ecologies. To make things worse, single measurements are of little use, as the interest of invasion fitness derives from what its values are for potential mutants and how these values change with changes in the environment. For general ecoevolutionary models, it is only possible to calculate invasion fitnesses by numerical means. Only under special circumstances and for purposes that are more restricted, invasion fitness can be expressed in a simple manner or replaced by simple proxies like lifetime offspring number. In a still smaller subset of cases it may even be possible to directly measure some component of fitness (say, survival probability or offspring number) that by invoking a ceteris paribus assumption can be argued to be monotone-related to fitness. This proxy then may be used to rank types in order of increasing fitness. The section 'Uses' discusses the theoretical restrictions on the application of such simplified procedures.
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