What is fitness?

The role of evolutionary biology “fitness” is to quantify, using mathematical models, that which natural selection favors. Models assign one form of fitness to genotypes, and use it to derive the longer-term fate of genetic lineages that shape organismal strategies (Smith et al. 2026). While models seem to assign "relative fitness" or "absolute fitness" in either absolute or per-generation time units, implicit within this is the assignation of vital rates of birth, death, and growth. "Invasion fitness" in units of speed can neglect distinct aspects of the probability with which a beneficial allele will establish (Smith et al. 2026). The stochastic fate of lineages can be summarized by the ratio of invasion : counter-invasion in a finite population (Masel 2005, King & Masel 2007), or in the case of balancing selection, the ratio of time-integrals of abundance from appearance to loss (Smith et al. 2026).

Density-dependent selection

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 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

• the birth and dispersion rates of propagules matter more at low density and also affect density.

We analytically solve a novel model of density-dependent selection on these three traits (Bertram & Masel 2019a). It is 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. Propagules compete for territories, a durable resource.

We also modeled competition for consumable resources (Smith & Masel 2025). In contrast to previous R* theory, we found that co-existence is possible between types that are superior at interference competition vs. exploitative competition. We captured this by synthesizing consumer-resource models with the Hawk-Dove game, such that engaging in contests bears an opportunity cost of time that could have been spent searching for and consuming uncontested resources.

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.

Publications :

• Smith DJB, Doulcier G, Bourrat P, Takacs P, Masel, J. (2026) Why there are so many definitions of fitness in models, Genetics: iyag090.

• Gomez K, Aviles NR, Masel J. Improved population fitness in a larger habitat is reduced or even reversed by clonal interference from a zero-sum trait, manuscript in preparation.

• Smith DJB, Masel J, (2025) A Mechanistically Integrated Model of Exploitative and Interference Competition over a Single Resource Produces Widespread Coexistence, The American Naturalist 206(5): 737628.

• Gomez K, Bertram J, Masel J. (2020) Mutation bias can shape adaptation in large asexual populations experiencing clonal interference, Proceedings of the Royal Society B – Biological Sciences 287: 20201503.

• Bertram J., & Masel J. (2019a) Density-dependent selection and the limits of relative fitness, Theoretical Population Biology 129:81–92.

• Bertram J, Masel J. (2019b) Different mechanisms drive the maintenance of polymorphism at loci subject to strong versus weak fluctuating selection, Evolution 73:883–896.

• Gomez K., Bertram J., & Masel J. (2019) Directional selection rather than functional constraints can shape the G matrix in rapidly adapting asexuals, Genetics 211: 715–729.

• Bertram J, Gomez K, Masel J. (2017) Predicting patterns of long-term adaptation and extinction with population genetics, Evolution 71: 204–214.

• Masel J. (2015) Eco-evolutionary "fitness" in 3 dimensions: absolute growth, absolute efficiency, and relative competitiveness, unpublished arXiv preprint.

• King OD, Masel J. (2007). The evolution of bet-hedging adaptations to rare scenarios. Theoretical Population Biology 72: 560-575.

• Masel, J. (2005). Evolutionary capacitance may be favored by natural selection. Genetics, 170(3), 1359-71.