ecolution

Does it make sense to separate ecology and evolution? Conceptually yes, but practically no. This week let me draw your attention to this recent paper in Ecology Letters which has been covered by the BBC. You have to love ANOVAs (and related methods) to make friends with this paper – but so you should because understanding variance and covariance is at the heart of understanding evolution. For an experimental evolutionist this paper also makes for a refreshing read because it uses soil mites as a model system.

By growing lineages of mites in conditions of limited resource and under different harvesting regimes, the authors conclude that life history parameters that influence ecology undergo significant responses to selection during the course of the experiment. In particular limited food leads to prolonged development which, in turn, leads to greater fecundity and evolutionary rescue (R0, the basic reproductive ratio, goes from < 1 to > 1). Harvesting modulates this by shortening development (when juveniles are harvested) or lengthening it (when adults are harvested), but with a smaller effect size than the main effect attributed to diminished food availability. These manipulations affect ecological responses to fluctuations in food supply throughout the experiment (with adult harvesting potentiating shifts in population size).

This obviously has practical implications for how we understand conservation, but I’d like also to alert you to the conceptual distinction between ecology and evolution or what Andrew McColl of Nottingham University, UK calls the ecological causes of evolution. The key point here is that ecology plays a mediating role between phenotype and fitness. It does this for all traits because the fitness of (to use one of McColl’s examples) a finch’s beak depends on the availability of seeds of difference hardnesses, which in turn depends on abiotic factors such as the climate. (McColl’s point is that we ought to pay attention to this more often and more rigorously as have the Galapagos finch folk).

Ecological dependence gets serious with phenomena such as frequency- or density-dependent selection for which a model must account for fitness relative to other members of the population. The old idea of soft selection (introduced to deal with the problem of mutation load) captures this concept well I think (contrasted with hard selection on viability), but so too does the idea of relative fitness (which differs from mutation load in considering fitness relative to the mean fitness rather than to some ideal). I recently re-read this short paper by Allen Orr describing how simply moving from absolute to relative fitness influences our expectations about how selection behaves. I’ll write more on bet hedging soon, but the conclusion is that, when the fitness of others matters, selection becomes conservative and acts to decrease the variance in (absolute) fitness as well as to increase the mean.

So a lot of what we think of as evolutionary theory entails ecology, but being precise about the distinction (and about concepts like fitness and selection) is necessary to avoid confusion.

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