The Evolution of Sex
Since 2017 I've been working on a number of questions related to the evolution of sexual reproduction. This research can be broken down into a number of subthemes:
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The evolution of mating types ("ancestral versions of the sexes")
The evolution of stress-induced sexual reproduction
The “paradox of sex” lies in explaining how the benefits of sexual reproduction can outweigh its costs. The benefits of sexual reproduction arise primarily from genetic recombination, which generates individuals with advantageous gene combinations that would not have arisen under clonal reproduction alone. Meanwhile the costs of sex vary between species. Species with males can pay the famous "two-fold cost of sex". However even in species without sexes, sexual reproduction can take eight times longer than asexual reproduction, resulting in a staggering 256-fold (2^8) cost of sex!
To make things more complicated many species produce both sexually and asexually (facultative sexual reproduction). In this context, the observation that many undergo sexual reproduction in response to stress is an old one. To explain this empirical pattern, most research has focussed on how the benefits of recombination might be maximised in stressful conditions. This approach makes intuitive sense as we might expect the benefits of the gene shuffling in recombination to be maximized when an organism is poorly adapted to its environment. However this all assumes that the "cost of sex" is generally constant.
In reality, the cost of sex changes based on ecological context. The "opportunity cost" of reproducing sexually is high when there is plenty of opportunity for asexual reproduction; conversely it is low when opportunities for asexual reproduction are limited by nutrient starvation.
In a recent paper, we showed how the evolution of stress-induced sex in many microorganisms could be explained by the reduced cost of sexual reproduction under resource limitation:
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Stress, Sex, Cysts and Spores: Selection for Syngamy, Meiosis, and Dormancy Under Resource Limitation in Eukaryotic Microbes - Integrative and Comparative Biology (2026).
This built on work that showed how syngamy (the fusion of haploid cells that precedes genetic recombination) could be selected for under stressful conditions in unicellular organisms:
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Cell size and selection for stress-induced cell fusion in unicellular eukaryotes - PLoS Computational Biology (2025)
The evolution of sexes (anisogamy)
The nature of the sexes is complicated*. However, one definition often used in biology relies on classification based on the size of gametes (sex cells); females produce large well-provisioned eggs and males produce numerous small sperm. However, many species (including algae, fungi
and amoebae) feature no such dimorphism in gamete sizes; instead, individuals often belong to complementary but self-incompatible mating types that can be viewed as "proto-sexes".
Explaining how anisogamy (e.g. sperm-egg systems) arose from isogamy (morphologically similar gametes) is one of the great success stories of evolutionary theory. It is now widely understood that this transition has its origins in what is essentially a quality–quantity trade-off. Large zygotes are assumed to survive better than small zygotes, but from any given energy budget, a parent can produce a smaller number of large than small gametes; thus, quality trades off with quantity. Egg-producing females enhance their fitness by producing larger gametes that form large zygotes with higher survival probabilities, while sperm-producing males enhance their fitness by producing more numerous gametes that have higher fertilization success.
Key differences between sexes and mating types.
We extended this classic theory to show the transition from isogamy to anisogamy might be altered when organisms had the ability to develop parthenogenetically (i.e. in the absence of fertilization):
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Parthenogenesis and the Evolution of Anisogamy - Cells (2021)
We found that such facultative sex had the ability to stabilize isogamy, consistent with empirical observations. Essentially if "going it alone" is an option, then the fitness of "proto-males" might be decreased by producing smaller gametes with poor survival outcomes in the absence of a mate.
We extended this work to also explore what happens to the fertilization rate when parthenogenetic reproduction is an option:
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Parthenogenesis, sexual conflict, and selection on fertilization rates in switching environments - Theoretical Population Biology (2025)
Interestingly when costs to sex are high we found that females (whose gametes have large survival probabilities even in the absence of males) can evolve to avoid fertilization. While this might sound surprising, an empirical example of such behaviour has been seen in the brown alga Scytosiphon lomentaria, where so-called "amazon" populations in the northern seas around Japan completely suppress pheromone production and consist entirely of females reproducing parthenogenetically.
*Prof Letitia Meynell | ”Are There Two Sexes? Yes and No, But Mostly No (and Gender is Something Else Anyway – More or Less).” - Philosophy, Theory, and Practice in Biology (2026).
The evolution of mating types
Schizophyllum commune
Expected number of mating types, M, as a function of effective population size, N, and per-generation mutation rate, m. As the rate of asexual reproduction is increased (increasing c), the number of expected mating types decreases.
Expected extinction time, T, for an mating type allele in a population of size N with M resident mating types. As the rate of asexual reproduction increases, the extinction time decreases but remains large. In grey shaded regions T exceeds 10^11 generations, the evolutionary age of fungi.
In many species, male and female sexes do no exist; instead of producing sperm and egg gametes (anisogamy) these species produce mating type gametes of equal size and behaviour (isogamy), as illustrated in the figure above. Sex is (as with the sexes) restricted to occurring between gametes of distinct types. However, while anisogamous species have just two sexes, isogamous species can potentially have many thousand mating types. This is exemplified by the fungus Schizophyllum commune, which has over 23,000 mating types.
An unresolved question has been what governs the number of mating types in isogamous species. Naively, we might expect that many thousands of mating types is the norm; any new mating type that evolves will initially reproduce more quickly than its ancestors as it has more opportunities for reproduction with potential non-self partners. However most isogamous species, such as the green alga Chlamydomonas reinhardtii and the yeast Saccharomyces cerevisiae
have just two.
Hanna Kokko and I proposed that the number of mating types might simply be a result of a balance between mutations and extinctions. We constructed a mathematical model that accounted for the fact that many isogamous species reproduce asexually as well as sexually. Our model predicted that when sexual reproduction was rare, fewer mating types could be maintained. On evaluating the empirical literature, we found that this insight was well supported:
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The rate of facultative sex governs the number of expected mating types in isogamous species - Nature Ecology and Evolution (2018)
For more information about this paper, there's a wonderfully written commentary by Sujal Phadke:
Sex begets sexes - Nature News and Views.
And a blog-post by myself on how the paper came to be:
Why do most species have so few mating types, yet some have so many? - Behind the Paper, Nature Community.
Follow up work with Peter Czuppon has demonstrated that although the number of mating types observed in natural populations may result from a mutation-extinction balance, this does not imply a high turnover rate of mating type alleles: in fact, low numbers of mating types can be very stable over long, trans-specific, evolutionary periods:
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Invasion and extinction dynamics of mating types under facultative sexual reproduction - Genetics (2019)
Meanwhile work with Yvonne Krumbeck and Tim Rogers has shown that small fitness differences between mating type alleles may also play a key role in limiting mating type diversity:
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Fitness differences suppress the number of mating types in evolving isogamous species - Royal Society Open Science (2020)
Transitions in sex determination systems
In species with sex determination at the diploid level, sex is often determined genetically by a sex-specific chromosome: the Y chromosome in species with male heterogamety (XX females, XY males) an the W chromosome in species with female heterogamety (ZW females, ZZ males). However transitions between these states occur frequently in fish (e.g. Xiphophorus maculatus) amphibians (e.g. Rana rugosa) reptiles and invertebrates (e.g. Musca domestica). That these transitions are common is surprising given that the sex-specific chromosome should degrade over time, leading theorists to speculate that direct selective forces such as sex-specific selection may be responsible.
Working with Carl Veller, Pavitra Muralidhar and Martin Nowak, we were able to show in fact that such transitions could be observed in a simple null model that accounted for genetic drift:
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Drift-induced selection between male and female heterogamety - Genetics (2017)
Here we proved mathematically that there is a drift-induced bias favoring dominant mutations (see Figure, left). In an extremely minimal way our model predicts that dominant sex determining mutations are more likely than recessive ones, qualitatively recapitulating the empirical observation that that sex determining cascades were built from the bottom up by a series of dominant mutations.
You can read more about the paper, and its context in the literature, in a review by Prof. Deborah Charlesworth:
