The stagnation paradox: the ever-improving but (more or less) stationary population fitness

Abstract

Fisher's fundamental theorem states that natural selection improves mean fitness. Fitness, in turn, is often equated with population growth. This leads to an absurd prediction that life evolves to ever-faster growth rates, yet no one seriously claims generally slower population growth rates in the Triassic compared with the present day. I review here, using non-technical language, how fitness can improve yet stay constant (stagnation paradox), and why an unambiguous measure of population fitness does not exist. Subfields use different terminology for aspects of the paradox, referring to stasis, cryptic evolution or the difficulty of choosing an appropriate fitness measure; known resolutions likewise use diverse terms from environmental feedback to density dependence and ‘evolutionary environmental deterioration’. The paradox vanishes when these concepts are understood, and adaptation can lead to declining reproductive output of a population when individuals can improve their fitness by exploiting conspecifics. This is particularly readily observable when males participate in a zero-sum game over paternity and population output depends more strongly on female than male fitness. Even so, the jury is still out regarding the effect of sexual conflict on population fitness. Finally, life-history theory and genetic studies of microevolutionary change could pay more attention to each other.

Evolution of thermal performance curves: a meta-analysis of selection experiments

“…A cautious interpretation of these selection experiments is also warranted because natural systems are obviously more complex than laboratory populations (but see Buckley & Kingsolver, 2021). Specifically, the studies analysed here mainly use fecundity or population growth rate as a measure of fitness (see Table S1) which can be problematic (for a recent discussion, see Kokko, 2021). Fitness is usually multidimensional with different fitness components such as fecundity and viability reacting differently to thermal selection (see e.g., Wadgymar et al ., 2017).…”

Promoting extinction or minimizing growth? The impact of treatment on trait trajectories in evolving populations

“…Adaptation should increase fitness relative to competitors, but what exactly determines fitness in populations that have to adapt to unfavourable conditions? Generally, defining fitness measures is not unambiguous (Doebeli et al, 2017; Kokko, 2021). One possible definition is lifetime-reproductive output, which itself is a composite measure that includes net growth rate, but also the probability that newly founded lineages survive stochastic population size fluctuations.…”

Proceedings B 2021: the year in review

“…The ideas presented by Fortin and colleagues are of considerable value to ecologists and landscape and wildlife managers because they enable predictions to be made about the complex dynamics of species interactions across different spatial scales, trophic levels, and ecosystem functions. This year's annual Darwin review in Proceedings B, 'The stagnation paradox: the ever-improving but (more or less) stationary population fitness' [2] by Hanna Kokko (University of Zurich) is concerned with the fascinating topic of the evolution of population fitness and highlights why an unambiguous measure of population fitness does not appear to exist. Beginning with Fisher's fundamental theorem [3], Kokko shows how fitness can improve over time but still stay constant, a situation she refers to as the 'stagnation paradox'.…”

“…This year's annual Darwin review in Proceedings B , ‘The stagnation paradox: the ever-improving but (more or less) stationary population fitness' [ 2 ] by Hanna Kokko (University of Zurich) is concerned with the fascinating topic of the evolution of population fitness and highlights why an unambiguous measure of population fitness does not appear to exist. Beginning with Fisher's fundamental theorem [ 3 ], Kokko shows how fitness can improve over time but still stay constant, a situation she refers to as the ‘stagnation paradox’.…”