What is fitness?

Density-dependent selection

Evolutionary biology “fitness” can be defined either in relative terms, normalized to the competition, or against an external standard. Many binary distinctions have been made between different kinds of fitness: not only absolute vs. relative fitness, but also r- vs. K-selection, natural vs. sexual selection, selection on individuals vs. groups, and hard vs. soft selection.

Another way of looking at this is that some traits (e.g. contest ability) are more important at high densities while others (e.g. propagule production rate) are more important at low densities. A separate distinction is that some traits (e.g. absolute birth and death rates) affect density, while other traits (e.g. success probability in a zero-sum contest) do not. We have used the different ways that fitness-associated traits and density interact to produce a model of three traits. Adult death rate is density-independent but affects density, juvenile contests to become adults matter more at high density but do not affect density, and the birth and dispersion rates of propagules matter more at low density and also affect density. We have used these distinctions to devise and analytically solve a novel model of density-dependent selection, as a generalization of the lottery model of ecology, which is itself a generalization of the Wright-Fisher model to overlapping populations, to now incorporate variable density.

Adaptation can then be modeled as a travelling wave in up to three fitness dimensions, which we are currently developing. We are also using this approach to define the conditions for escape from extinction in a deteriorating environment, beyond the short-term scenarios handled by existing evolutionary rescue models.

I also wrote a book, Bypass Wall Street: A Biologist's Guide to the Rat Race, applying ideas of relative vs. absolute competition to economics, in particular the difference between money as a relative points system and wealth as an absolute store of value.

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