A century of theories of balancing selection

Biol. Rev.

published article

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Authors

Affiliations

Filip Ruzicka

Institute of Science and Technology Austria

Martyna K. Zwoinska

Uppsala University

Débora Goedert

Norwegian University of Science and Technology

Hanna Kokko

University of Mainz

Xiang‐Yi Li Richter

University of Bern

Iain R. Moodie

Lund University

Sofie Nilén

Lund University

Colin Olito

Lund University

Erik I. Svensson

Lund University

Peter Czuppon

University of Münster

Tim Connallon

Monash University

Published

November 14, 2025

Abstract

Traits that affect organismal fitness are often highly genetically variable. This genetic variation is vital for populations to adapt to their environments, but it is also surprising given that nature – after all – ‘selects’ the best genotypes at the expense of those that fall short. Explaining the extensive genetic variation of fitness-related traits is thus a longstanding puzzle in evolutionary biology, with cascading implications for ecology, conservation, and human health. Balancing selection – an umbrella term for scenarios in which natural selection maintains genetic variation – is a century-old explanation to resolve this puzzle that has gained recent momentum from genome-scale methods for detecting it. Yet evaluating whether balancing selection can, in fact, resolve the puzzle is challenging, given the logistical constraints of distinguishing balancing selection from alternative hypotheses and the daunting collection of theoretical models that formally underpin this debate. Here, we track the development of balancing selection theory over the last century and provide an accessible review of this rich collection of models. We first outline the range of biological scenarios that can generate balancing selection. We then examine how fundamental features of genetic systems – non-random mating between individuals, ploidy levels, genetic drift, linkage, and genetic architectures of traits – have been progressively incorporated into the theory. We end by linking these theoretical predictions to ongoing empirical efforts to understand the evolutionary processes that explain genetic variation.

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Citation

BibTeX citation:

@article{ruzicka2025,
  author = {Ruzicka, Filip and Zwoinska, Martyna K. and Goedert, Débora
    and Kokko, Hanna and Li Richter, Xiang‐Yi and Moodie, Iain R. and
    Nilén, Sofie and Olito, Colin and Svensson, Erik I. and Czuppon,
    Peter and Connallon, Tim},
  publisher = {Cambridge Philosophical Society},
  title = {A Century of Theories of Balancing Selection},
  journal = {Biological Reviews},
  date = {2025-11-14},
  url = {https://doi.org/10.1111/brv.70103},
  doi = {10.1111/brv.70103},
  langid = {en},
  abstract = {Traits that affect organismal fitness are often highly
    genetically variable. This genetic variation is vital for
    populations to adapt to their environments, but it is also
    surprising given that nature – after all – “selects” the best
    genotypes at the expense of those that fall short. Explaining the
    extensive genetic variation of fitness-related traits is thus a
    longstanding puzzle in evolutionary biology, with cascading
    implications for ecology, conservation, and human health. Balancing
    selection – an umbrella term for scenarios in which natural
    selection maintains genetic variation – is a century-old explanation
    to resolve this puzzle that has gained recent momentum from
    genome-scale methods for detecting it. Yet evaluating whether
    balancing selection can, in fact, resolve the puzzle is challenging,
    given the logistical constraints of distinguishing balancing
    selection from alternative hypotheses and the daunting collection of
    theoretical models that formally underpin this debate. Here, we
    track the development of balancing selection theory over the last
    century and provide an accessible review of this rich collection of
    models. We first outline the range of biological scenarios that can
    generate balancing selection. We then examine how fundamental
    features of genetic systems – non-random mating between individuals,
    ploidy levels, genetic drift, linkage, and genetic architectures of
    traits – have been progressively incorporated into the theory. We
    end by linking these theoretical predictions to ongoing empirical
    efforts to understand the evolutionary processes that explain
    genetic variation.}
}

For attribution, please cite this work as:

Ruzicka, F., M. K. Zwoinska, D. Goedert, H. Kokko, X. Li Richter, I. R. Moodie, S. Nilén, et al. 2025. A century of theories of balancing selection. Biological Reviews.