---
OA_place: publisher
OA_type: hybrid
_id: '20655'
abstract:
- lang: eng
text: 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.
acknowledgement: 'We thank Brian Charlesworth, Deborah Charlesworth, and Sally Otto
for extensive comments and suggestions. We also thank Göran Arnqvist, Adam Eyre-Walker,
Philip Hedrick, Jitka Polechová, and Henrique Teotónio for further helpful comments
on the manuscript. This work was supported by a H2020 Marie Skłodowska-Curie COFUND
Action fellowship (#101034413, to F. R.), the Birgitta Sintring Foundation (#S2024-0007,
to M. K. Z.), the Research Council of Norway (302619, to D. G.), the Alexander von
Humboldt Foundation (to H. K.), the Swiss National Science Foundation (#211549,
to X. L. R.), the Swedish Research Council (#2022-03603, to CO; #2020-03123, to
E. I. S.) and the European Research Council (ERC-2023-STG-#101117517, to C. O.).
We are particularly grateful to the European Society for Evolutionary Biology for
funding a Special Topics Network workshop (to T. C., H. K., E. I. S.), from which
this review began. Open Access funding provided by Institute of Science and Technology
Austria/KEMÖ.'
article_number: 804-825
article_processing_charge: Yes (via OA deal)
article_type: original
author:
- first_name: Filip
full_name: Ruzicka, Filip
id: 347955dd-57b0-11ee-9095-c28bdd368f4b
last_name: Ruzicka
- first_name: Martyna K.
full_name: Zwoinska, Martyna K.
last_name: Zwoinska
- first_name: Debora
full_name: Goedert, Debora
last_name: Goedert
- first_name: Hanna
full_name: Kokko, Hanna
last_name: Kokko
- first_name: Xiang‐Yi
full_name: Li Richter, Xiang‐Yi
last_name: Li Richter
- first_name: Iain R.
full_name: Moodie, Iain R.
last_name: Moodie
- first_name: Sofie
full_name: Nilén, Sofie
last_name: Nilén
- first_name: Colin
full_name: Olito, Colin
last_name: Olito
- first_name: Erik I.
full_name: Svensson, Erik I.
last_name: Svensson
- first_name: Peter
full_name: Czuppon, Peter
last_name: Czuppon
- first_name: Tim
full_name: Connallon, Tim
last_name: Connallon
citation:
ama: Ruzicka F, Zwoinska MK, Goedert D, et al. A century of theories of balancing
selection. Biological Reviews. 2026;101(2). doi:10.1111/brv.70103
apa: Ruzicka, F., Zwoinska, M. K., Goedert, D., Kokko, H., Li Richter, X., Moodie,
I. R., … Connallon, T. (2026). A century of theories of balancing selection. Biological
Reviews. Wiley. https://doi.org/10.1111/brv.70103
chicago: Ruzicka, Filip, Martyna K. Zwoinska, Debora Goedert, Hanna Kokko, Xiang‐Yi
Li Richter, Iain R. Moodie, Sofie Nilén, et al. “A Century of Theories of Balancing
Selection.” Biological Reviews. Wiley, 2026. https://doi.org/10.1111/brv.70103.
ieee: F. Ruzicka et al., “A century of theories of balancing selection,”
Biological Reviews, vol. 101, no. 2. Wiley, 2026.
ista: Ruzicka F, Zwoinska MK, Goedert D, Kokko H, Li Richter X, Moodie IR, Nilén
S, Olito C, Svensson EI, Czuppon P, Connallon T. 2026. A century of theories of
balancing selection. Biological Reviews. 101(2), 804–825.
mla: Ruzicka, Filip, et al. “A Century of Theories of Balancing Selection.” Biological
Reviews, vol. 101, no. 2, 804–825, Wiley, 2026, doi:10.1111/brv.70103.
short: F. Ruzicka, M.K. Zwoinska, D. Goedert, H. Kokko, X. Li Richter, I.R. Moodie,
S. Nilén, C. Olito, E.I. Svensson, P. Czuppon, T. Connallon, Biological Reviews
101 (2026).
corr_author: '1'
das_tickbox: '0'
date_created: 2025-11-19T09:43:50Z
date_published: 2026-04-01T00:00:00Z
date_updated: 2026-07-27T08:08:09Z
day: '01'
ddc:
- '570'
department:
- _id: BeVi
doi: 10.1111/brv.70103
ec_funded: 1
external_id:
isi:
- '001614285900001'
pmid:
- '41235821 '
file:
- access_level: open_access
checksum: 167d95cf0570d6e3653ab349b2a4355c
content_type: application/pdf
creator: dernst
date_created: 2026-07-27T08:07:35Z
date_updated: 2026-07-27T08:07:35Z
file_id: '22409'
file_name: 2026_BiologicalReviews_Ruzicka.pdf
file_size: 1757556
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success: 1
file_date_updated: 2026-07-27T08:07:35Z
has_accepted_license: '1'
intvolume: ' 101'
isi: 1
issue: '2'
keyword:
- evolutionary theory
- population genetics
- balancing selection
- heterozygote advantage
- trade-offs
- negative frequency-dependent selection
- fitness variation
- mathematical modelling
language:
- iso: eng
month: '04'
oa: 1
oa_version: Published Version
pmid: 1
project:
- _id: fc2ed2f7-9c52-11eb-aca3-c01059dda49c
call_identifier: H2020
grant_number: '101034413'
name: 'IST-BRIDGE: International postdoctoral program'
publication: Biological Reviews
publication_identifier:
eissn:
- 1469-185X
issn:
- 1464-7931
publication_status: published
publisher: Wiley
quality_controlled: '1'
researchdata_availability: no
scopus_import: '1'
status: public
supplementarymaterial: yes
title: A century of theories of balancing selection
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short: CC BY-NC (4.0)
type: journal_article
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volume: 101
year: '2026'
...