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In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).
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How is variation in fitness maintained?

Brian Charlesworth1

  • 1Institute of Ecology and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.

Evolution; International Journal of Organic Evolution
|March 6, 2026
PubMed
Summary
This summary is machine-generated.

Evolutionary genetics faces a puzzle: how can substantial additive genetic variance in fitness persist? This study explores how genetic drift and fluctuating fitness can maintain this variance at loci under balancing selection.

Keywords:
fitnessgenetic driftgenetic variancemutationsselection

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Area of Science:

  • Evolutionary genetics
  • Population genetics
  • Quantitative genetics

Background:

  • The persistence of additive genetic variance in fitness is a fundamental problem in evolutionary biology.
  • Balancing selection is known to maintain genetic diversity but typically reduces additive genetic variance.

Purpose of the Study:

  • To investigate the impact of random genetic drift and temporal fluctuations in fitness on loci under balancing selection.
  • To explore mechanisms by which substantial additive genetic variance can be maintained despite balancing selection.

Main Methods:

  • Theoretical modeling of genetic drift and fluctuating selection.
  • Analysis of loci maintained by balancing selection.

Main Results:

  • Random genetic drift can cause loci under balancing selection to deviate from their selective equilibria.
  • Temporal fluctuations in fitness, in conjunction with drift, can lead to the maintenance of substantial additive genetic variance.

Conclusions:

  • Departures from selective equilibria, driven by drift and fitness fluctuations, offer a potential resolution to the paradox of persistent additive genetic variance in fitness.
  • This theoretical framework provides insights into the maintenance of quantitative trait variation in natural populations.