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  • Review Article
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Fitness and its role in evolutionary genetics

Nature Reviews Genetics volume 10, pages 531–539 (2009)Cite this article

Key Points

  • This article starts by reviewing the differences between various measures of fitness, for example, individual fitness, absolute fitness, relative fitness and geometric mean fitness.

  • Differences in fitness (when measured appropriately) can be used to derive selection equations, which show how natural selection changes the genetic composition of a population through time.

  • Quantitative geneticists derived the secondary theorem of natural selection, which shows how selection on fitness will change other, genetically correlated traits.

  • Fitness landscape models, whether continuous or discrete, can be used to analyse how natural selection will drive a population to the top of a fitness peak.

  • Evolutionary geneticists are currently pursuing several empirical approaches to the study of fitness, including direct fitness assays, microbial experimental evolution and the use of DNA sequence data to infer a history of positive natural selection.

  • The concluding section sketches several major unresolved problems in the experimental study of fitness.

Abstract

Although the operation of natural selection requires that genotypes differ in fitness, some geneticists may find it easier to understand natural selection than fitness. Partly this reflects the fact that the word 'fitness' has been used to mean subtly different things. In this Review I distinguish among these meanings (for example, individual fitness, absolute fitness and relative fitness) and explain how evolutionary geneticists use fitness to predict changes in the genetic composition of populations through time. I also review the empirical study of fitness, emphasizing approaches that take advantage of recent genetic and genomic data, and I highlight important unresolved problems in understanding fitness.

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Figure 1: Allele frequency versus time.
Figure 2: A three-dimensional Wrightian fitness landscape.

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Acknowledgements

This work was suppported by a grant from the US National Institutes of Health.

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  1. Department of Biology, University of Rochester, River Campus Box 270211, Rochester, 14627-0211, New York, USA

    H. Allen Orr

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Nature Reviews Genetics Series on Modelling

Glossary

Instar

A developmental stage of insect larvae.

Random variable

A quantity that might take any of a range of values (discrete or continuous) that cannot be predicted with certainty but only described probabilistically.

Covariance

A measure of association between two variables that characterizes the tendency for the two variables to covary around their means in a systematic way.

Genetic drift

Random fluctuation in allele frequency owing to the sampling of gametes in a finite population.

Interdemic selection

Selection wherein local populations (demes) compete with each other. Wright believed that fitter populations would also produce more migrants.

NK model

A class of fitness landscape model that considers evolution through a discrete sequence space. The model allows the ruggedness of a landscape to be varied from very rugged (many local optima) to very smooth (one optimum) by changing one parameter.

Hardy–Weinberg equilibrium

A state in which the frequency of each diploid genotype at a locus equals that expected from the random union of alleles (genotypes AA, Aa and aa will be at frequencies p2, 2pq and q2, respectively). These expectations are based on a stable population undergoing random mating in the absence of selection, new mutations and migration.

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Orr, H. Fitness and its role in evolutionary genetics. Nat Rev Genet 10, 531–539 (2009). https://doi.org/10.1038/nrg2603

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