Evidence record

Why is there sex?

Every sentence on the exploration belongs to at least one claim below. Each says what kind of statement it is, what it rests on, what the evidence does not establish, and how far it was checked. Nothing here is a score, and none of it is a verdict on a belief or a theory.

Last checked 24 September 2026.

Source checked: 37
Presented as open: 2
A judgement, not a fact: 8
Partly checked: 1

If something here is wrong, misquoted, out of date or unfair, write to support@unknownoceans.com and give the claim number, which does not change.

What the question takes for granted

A question in evolutionary biology about why a costly way of reproducing is so common. The cost is agreed; which advantage pays for it, and whether any one does, is argued over, with theory and laboratory experiments ahead of evidence from natural populations.

Sex carries a cost that an asexual rival would not pay
Established
All else being equal, a sexual parent passes on half as many of its genes per offspring as an asexual one. The size of the cost in a given species depends on things the simple arithmetic leaves out.
One advantage is the answer
Contested
The question is often put as if a single benefit must pay for sex. The review the page leans on expects some truth in several explanations, varying by organism.
Outcrossing against selfing in a worm stands in for sex against asexuality
Contested
The laboratory tests on the page compare outcrossing with self-fertilisation in a nematode. Selfing shares some of the problems of asexual reproduction, which is why the experiments bear on the question, but it is not the same thing.

How the question is labelled

Claim: The twofold cost

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Thus, unless sexuals produce twice as many offspring per individual, sexuality suffers from a transmission disadvantage, a problem so acute that it has been labeled the cost of sex

What this does not establish

Twofold is the cost with everything else equal. Where sexuals have more offspring, or where the arithmetic differs, the cost is different.

Claim 1 · checked 24 September 2026 · AI-assisted audit

Claim: Sex

Kind: Framing. Signposting. It asserts nothing a source could check.

The question

Claim: An asexual lineage should out-reproduce a sexual one two to one.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: A sexual parent transmits only 50% of its genes to the next generation, compared with 100% for an asexual parent.
Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: Self-fertilizing organisms do not incur the cost of males and therefore should possess at least a two-fold numerical advantage over most outcrossing organisms

What this does not establish

A prediction with all else equal, as the word "should" says. It is the size of the puzzle, not a measured rate in any species.

Claim 3 · checked 24 September 2026 · AI-assisted audit

Claim: Almost everything large reproduces sexually anyway.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: the vast majority of eukaryotic organisms reproduce sexually ... Among named animal species, only ∼0.1% are considered to be exclusively asexual

What this does not establish

"Large" is the page’s word. Otto’s figures are for named animal species and for flowering plants, and many of the sexual ones reproduce sexually only some of the time.

Later work looked for

Europe PMC, 2012 on, "evolution of sex" reviews: Hartfield and Keightley (2012, abstract read) still describe sex as almost universal in higher animals and plants; nothing that changes it.

Claim 4 · checked 24 September 2026 · AI-assisted audit

Claim: Sexual reproduction halves the rate at which genes are passed on and requires finding a partner, so it needs a large compensating advantage.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: In order to outcross, an individual has to find a potential partner, attract it ... A sexual parent transmits only 50% of its genes to the next generation, compared with 100% for an asexual parent.
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction, first paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: Therefore, the selective benefits of outcrossing must more than compensate for this fitness deficit to achieve a high frequency in nature.

Later work looked for

How the cost is reckoned; not a finding later work revises. Hartfield and Keightley (2012, abstract read) state the same cost.

Claim 5 · checked 24 September 2026 · AI-assisted audit

Claim: Several candidates have theoretical and experimental support.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Multiple lines of empirical evidence support the drift hypothesis for the evolution of sex.
Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: outcrossing is favored in populations of C. elegans subject to experimental evolution both under conditions of increased mutation rate and during adaptation to a novel environment.
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: Thus, consistent with the Red Queen hypothesis, coevolving pathogens can select for biparental sex.

Later work looked for

Europe PMC, 2012 on, "Red Queen" with sex or outcrossing and parasites: Hartfield and Keightley (2012, abstract read) report theoretical and experimental evidence for several classes of hypothesis; nothing that changes it.

Claim 6 · checked 24 September 2026 · AI-assisted audit

Claim: Which one does the work, and whether any is sufficient, is unresolved.

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
The Evolutionary Enigma of Sexpp. S10 to S11; Discussion, p. S11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: An open question is whether the drift-based explanation ... is strong enough to account for the evolution of sex in the face of substantial costs of sex.

Later work looked for

Hartfield and Keightley (2012, abstract read) still ask for more data from natural and experimental populations to test the hypotheses against each other.

Claim 7 · checked 24 September 2026 · AI-assisted audit

Claim: An asexual female passes on all of her genes to every offspring.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: A sexual parent transmits only 50% of its genes to the next generation, compared with 100% for an asexual parent.

Later work looked for

How inheritance works; not a finding later work revises.

Claim 8 · checked 24 September 2026 · AI-assisted audit

Claim: A sexual one passes half, and spends effort producing males who do not bear young.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: A sexual parent transmits only 50% of its genes to the next generation
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction, first paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: many outcrossing species produce males that facilitate outcrossing, but are incapable of bearing offspring themselves, resulting in the “cost of males.”

What this does not establish

Morran and colleagues write that many outcrossing species produce males, not that all do.

Later work looked for

How inheritance works; not a finding later work revises.

Claim 9 · checked 24 September 2026 · AI-assisted audit

Claim: That is a twofold disadvantage per generation, which in evolutionary terms is enormous.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: sexuality suffers from a transmission disadvantage, a problem so acute that it has been labeled the cost of sex
Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationintroduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: This halving of the number of offspring-bearing progeny an individual can produce is known as the “two-fold cost of males” and generates a large gap between the mating systems in numerical contribution, and thus fitness, over time

Later work looked for

How the cost is reckoned; not a finding later work revises.

Claim 10 · checked 24 September 2026 · AI-assisted audit

Claim: Something must be paying for it.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own step from claims 4 and 10: a costly habit that is nearly universal is being maintained by some advantage. Morran and colleagues draw the same conclusion in their introduction.

Claim 11 · checked 24 September 2026 · AI-assisted audit

The problem

Claim: The problem

Kind: Framing. Signposting. It asserts nothing a source could check.

The cost is real and the practice is near universal

Claim: The cost is real and the practice is near universal

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: In the face of such legendary costs, we might expect sexual reproduction to be rare. Yet, the vast majority of eukaryotic organisms reproduce sexually

Later work looked for

Europe PMC, 2012 on: Hartfield and Keightley (2012, abstract read) say the same; nothing that changes it.

Claim 13 · checked 24 September 2026 · AI-assisted audit

Claim: Sex is retained by the overwhelming majority of plants and animals.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: the vast majority of eukaryotic organisms reproduce sexually
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction, first paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: Outcrossing (i.e., mating between different individuals) is the most prevalent mode of reproduction among plants and animals.

What this does not establish

Otto adds "at least occasionally": many of these species also reproduce asexually.

Later work looked for

Europe PMC, 2012 on, "evolution of sex" reviews: Hartfield and Keightley (2012, abstract read) call sex almost universal in higher animals and plants; nothing that changes it.

Claim 14 · checked 24 September 2026 · AI-assisted audit

Claim: Around a tenth of one per cent of named animal species are reckoned exclusively asexual, and under one per cent of flowering plants substantially asexual.

Kind: Present-day figure. A count, survey or report finding, as the named source gives it.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: Among named animal species, only ∼0.1% are considered to be exclusively asexual (Vrijenhoek 1998). While the ability to reproduce asexually is more widespread in plants, less than 1% of the approximately 250,000 angiosperm species are thought to be substantially asexual

What this does not establish

Estimates from the surveys Otto cites, the animal figure from 1998. The plant figure is for species that are substantially asexual, a looser test than the animal one.

Later work looked for

Crossref, 2010 on, "proportion of asexual animal species obligate parthenogenesis": no newer census of the proportion found; nothing that changes it.

Corrected

24 September 2026. Previously: “Around a tenth of one per cent of named animal species are reckoned exclusively asexual, and under one per cent of flowering plants.” The sentence carried "exclusively" over to the plants. Otto’s plant figure is for species "thought to be substantially asexual", a different measure.

Claim 15 · checked 24 September 2026 · AI-assisted audit

Claim: Purely asexual lineages exist and are usually young: virtually all of them sit at the tips of the tree of life rather than as deep branches.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: Virtually all of these asexual taxa are at the tips of the tree of life. For example, asexuality in the seed plants is confined to single species or to closely related species complexes

What this does not establish

Otto gives the position on the tree; "usually young" is the page reading that position. She names a possible exception, a small and poorly studied plant genus.

Later work looked for

Europe PMC, 2012 on, "evolution of sex" reviews: nothing found that changes the pattern.

Claim 16 · checked 24 September 2026 · AI-assisted audit

Claim: That pattern suggests asexual lineages arise often and go extinct quickly, which is itself evidence about what sex is for.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own inference from claim 16, and hedged as one. Otto reports the pattern; the reading of it as frequent origin and early extinction is the page’s.

Claim 17 · checked 24 September 2026 · AI-assisted audit

What sex does to a genome

Claim: What sex does to a genome

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Recombination breaks up combinations of genes.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Last, but not least, sexual reproduction breaks apart favorable combinations of genes built by past selection.

Later work looked for

How the mechanism works; not a finding later work revises.

Claim 19 · checked 24 September 2026 · AI-assisted audit

Claim: That lets a beneficial mutation spread without carrying its neighbours along, and lets a harmful one be removed without discarding the good genes beside it.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S9 (the Hill-Robertson effect)
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: when beneficial mutations arise in different individuals, the resulting genetic variation is somewhat hidden from selection by linkage to deleterious mutations
Supports
The Evolutionary Enigma of Sexp. S10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in microbes · Review of the field
The source reads: recombination increased the probability that a specific beneficial mutation fixed and increased its fixation rate, presumably because the mutant could be recombined away from linked deleterious mutations and into fitter lines

What this does not establish

That recombination can do this is theory with laboratory support, much of it in microbes. Whether the benefit is large enough to pay for sex is the open question.

Claim 20 · checked 24 September 2026 · AI-assisted audit

Claim: In an asexual lineage, mutations accumulate in the genome they arose in and cannot be separated from it.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationintroduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: any mutations that arise within separate selfing individuals can not be incorporated into the same lineage or genome
Supports
The Evolutionary Enigma of Sexp. S9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: an asexual population would have to await the right back mutations

What this does not establish

Without recombination the only way back is a new mutation reversing the old one, which is rare.

Claim 21 · checked 24 September 2026 · AI-assisted audit

Claim: H. J. Muller saw what follows, in 1964.

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S9; reference list
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Texts · Review of the field
The source reads: Muller (1964) recognized that the best genotype could be lost by drift

Corrected

24 September 2026. Previously: “Hermann Muller saw what follows.” The cited review gives the author as H. J. Muller and dates the argument to his 1964 paper; the first name is carried by no source on the page. The date is what locates the idea.

Claim 22 · checked 24 September 2026 · AI-assisted audit

Claim: In a finite population, drift can remove the least-damaged class of individuals, and a sexual population can rebuild that class by recombining what is left while an asexual one has to wait for the right back mutation.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Muller (1964) recognized that the best genotype could be lost by drift in finite populations; once lost, this genotype could be rapidly regenerated by recombining the remaining chromosomes in sexual populations, but an asexual population would have to await the right back mutations

What this does not establish

A model of what happens in a finite population. How fast it runs in a real one depends on the mutation rate and the population size.

Corrected

24 September 2026. Previously: “Drift eventually removes the least-damaged class of individuals,” Otto says the best genotype "could be lost by drift in finite populations". The page had made it a certainty and dropped the condition that makes it happen.

Claim 23 · checked 24 September 2026 · AI-assisted audit

Claim: The asexual line’s fitness decays, and the process is called Muller’s ratchet.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: an asexual population would have to await the right back mutations, leading to the decay of fitness in asexuals (Muller’s ratchet). These arguments were originally envisioned as providing a group-level advantage

What this does not establish

A model. Otto notes it was first framed as an advantage of sexual groups over asexual ones, and has since been shown to work within a population too.

Corrected

24 September 2026. Previously: “Fitness decays a notch at a time, which is why it is called Muller’s ratchet.” The account of where the name comes from is carried by no source on the page. The decay and the name are Otto’s.

Claim 24 · checked 24 September 2026 · AI-assisted audit

Claim: Two families of explanation

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own grouping. Otto’s review sets out more conditions than two, among them selection that varies over space and sex that depends on condition; the page compares the two it goes on to discuss.

Corrected

24 September 2026. Previously: “The two families of explanation” The definite article said these were the only two. The review the page cites discusses others.

Claim 25 · checked 24 September 2026 · AI-assisted audit

Claim: Clearing the damage

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Harmful mutations can be separated from good genes and purged

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Sex can then restore variation by breaking down these associations, thus improving the response to directional selection and facilitating the elimination of deleterious alleles.

What this does not establish

What the models propose. Whether it pays a twofold cost in nature is not settled.

Claim 27 · checked 24 September 2026 · AI-assisted audit

Claim: Predicts that lines without it lose fitness

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: leading to the decay of fitness in asexuals (Muller’s ratchet)

What this does not establish

A prediction of the models, tested in the laboratory in claim 29.

Corrected

24 September 2026. Previously: “Explains the short lifespans of asexual lineages” No source on the page says the mutation explanations account for how long asexual lineages last. What the sources carry is the prediction that lines without sex lose fitness, and a laboratory test of it.

Claim 28 · checked 24 September 2026 · AI-assisted audit

Claim: as selfing worms did under a raised mutation rate

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationresults, Figure 1a
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: the latter populations maintained fitness over the course of the experiment in spite of elevated mutation rates, whereas the selfing populations displayed a substantial decline in fitness

What this does not establish

Caenorhabditis elegans in the laboratory, with the mutation rate raised about fourfold by a chemical mutagen, over 50 generations. Selfing is not asexual reproduction, though it shares some of its problems.

Later work looked for

Crossref, 2012 on, "outcrossing Caenorhabditis elegans experimental evolution": later experimental-evolution papers on outcrossing in the worm, none found that overturns this result.

Claim 29 · checked 24 September 2026 · AI-assisted audit

Claim: Needs harmful mutations to interact in a particular way, and measurements of that are mixed

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexpp. S5 to S6
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: where the negative fitness curvature among deleterious mutations was called synergistic epistasis ... the evidence is mixed, with some studies finding negative epistasis but other studies finding no epistasis or positive epistasis, on average.

What this does not establish

This holds for the models of large populations. In Otto’s account the models of finite populations favour sex without that condition.

Corrected

24 September 2026. Previously: “Requires mutation rates and interactions that are hard to measure” Otto’s point is not that the interaction is hard to measure but that it has been measured and the results are mixed. She does not discuss mutation rates here.

Claim 30 · checked 24 September 2026 · AI-assisted audit

Claim: Keeping up with parasites

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Hosts and parasites adapt to each other continuously, so a rare gene combination is valuable

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction, third paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Experiment
The source reads: outcrossing can facilitate rapid adaptation by generating offspring with rare or novel genotypes, which are more likely to escape infection by coevolving pathogens
Qualifies
The Evolutionary Enigma of Sexpp. S6 to S7
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: One limitation of the Red Queen hypothesis is that for the sign of linkage disequilibria to cycle rapidly in haploid models, selection must be quite strong

What this does not establish

The hypothesis as proposed. Otto notes that in models it needs strong selection, and that in diploids it can favour less sex rather than more.

Claim 32 · checked 24 September 2026 · AI-assisted audit

Claim: Predicts sex where parasite pressure is high

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S7
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: This is a promising explanation for the empirical pattern that sexuality is more prevalent than asexuality in locations where parasites are common

What this does not establish

Otto ties the prediction most firmly to one version of the idea, in which offspring meet the parasites their parents and relatives carry.

Claim 33 · checked 24 September 2026 · AI-assisted audit

Claim: which has been seen in natural snail populations and in laboratory nematodes

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Partly checked. Only part of the source could be read. What was not read is listed.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction, fourth paragraph; results
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: The Red Queen hypothesis has been empirically supported in studies of natural snail populations, which show that sexual reproduction is more common where parasites are common and adapted to infect the local host population
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: The results were consistent with the Red Queen hypothesis. In the coevolution treatment, all of the obligately selfing populations became extinct within twenty generations

What this does not establish

The snail studies are reported here as Morran and colleagues describe them; the studies themselves were not read. The nematode result is a laboratory experiment and compares outcrossing with selfing.

Later work looked for

Europe PMC, 2012 on, "Red Queen" with sex and parasites and snails: nothing found that reverses the snail pattern or the nematode result.

Corrected

24 September 2026. Previously: “which has been observed in snails and in nematodes” The two are different kinds of evidence, a pattern in the wild and an experiment, and the page gave them as one.

Claim 34 · checked 24 September 2026 · AI-assisted audit

Claim: Does not obviously explain sex in lineages with little parasite pressure

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own judgement of the limit of the parasite explanation. Otto suggests the Red Queen may also work as a source of ongoing selection inside the drift-based explanations.

Claim 35 · checked 24 September 2026 · AI-assisted audit

The experiment with the parasite

Claim: The experiment with the parasite

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: In 2011 a study took populations of a nematode that can both self-fertilise and outcross, and exposed them to a bacterial pathogen that was itself allowed to evolve against them.

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexintroduction and methods summary
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: The hermaphrodites can reproduce through either self-fertilization or by outcrossing with males ... The coevolution treatment involved repeated exposure (30 host generations) to a potentially coevolving population of S. marcescens, which was under selection for increased infectivity.

What this does not establish

The populations were also exposed to a fixed pathogen and to none, and included lines engineered to only self or only outcross.

Claim 37 · checked 24 September 2026 · AI-assisted audit

Claim: Outcrossing rose from about a fifth to over seventy per cent in the first eight generations whether the pathogen was coevolving or fixed.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults, Fig. 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: Over the first eight generations of the experiment, outcrossing rates increased from 20% to over 70% in both the evolution and coevolution treatments
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults, parasite treatments
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: evolution (repeated exposure to a fixed, non-evolving strain of S. marcescens), and coevolution. The coevolution treatment involved repeated exposure (30 host generations) to a potentially coevolving population of S. marcescens

What this does not establish

Five replicate populations per treatment, of one mutagenised laboratory strain.

Later work looked for

Crossref, 2012 on, "red queen coevolution outcrossing Caenorhabditis elegans": later papers on outcrossing in experimental worm populations, none found that overturns this result.

Corrected

24 September 2026. Previously: “Outcrossing rose from about a fifth of matings to over seventy per cent” The paper reports outcrossing rates, not a share of matings.

Claim 38 · checked 24 September 2026 · AI-assisted audit

Claim: Only the populations facing the coevolving one held it there.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults, Fig. 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: In contrast, populations in the coevolution treatment consistently maintained high levels of outcrossing throughout the experiment, relative to the control treatment

What this does not establish

Thirty host generations in the laboratory. The authors note an earlier worm study in which a coevolving pathogen did not raise outcrossing.

Later work looked for

Crossref, 2012 on, "red queen coevolution outcrossing Caenorhabditis elegans": nothing found that overturns it.

Claim 39 · checked 24 September 2026 · AI-assisted audit

Claim: Against a fixed pathogen the rate slid back to the level of populations facing nothing at all.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults, Fig. 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: the wildtype populations consistently exposed to a fixed population of S. marcescens (evolution treatment) exhibited a steady decline in outcrossing rates after this initial increase, eventually returning to control levels of outcrossing

Later work looked for

Crossref, 2012 on, "red queen coevolution outcrossing Caenorhabditis elegans": nothing found that overturns it.

Claim 40 · checked 24 September 2026 · AI-assisted audit

Claim: Obligately self-fertilising populations exposed to the coevolving parasite were extinct within twenty generations, and none died out under the fixed pathogen or under no pathogen at all.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: In the coevolution treatment, all of the obligately selfing populations became extinct within twenty generations (Fig S1). However, none of the obligately selfing populations went extinct in either the evolution treatment or in the control treatment.

What this does not establish

Five replicate populations of each mating type, in the laboratory.

Later work looked for

Crossref, 2012 on, "red queen coevolution outcrossing Caenorhabditis elegans": nothing found that overturns it.

Claim 41 · checked 24 September 2026 · AI-assisted audit

Claim: That is the Red Queen hypothesis, which Jaenike, Hamilton and Lloyd put forward around 1980,

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S6
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Texts · Review of the field
The source reads: interactions among antagonistic species, such as hosts and parasites (Jaenike 1978; Hamilton 1980; Lloyd 1980; Jokela et al. 2009, in this issue). This mechanism of fluctuating selection is known as the “Red Queen” hypothesis for the evolution of sex (Bell 1982)

What this does not establish

The three papers date from 1978 and 1980. Otto credits the name "Red Queen" for this idea to Bell in 1982.

Claim 42 · checked 24 September 2026 · AI-assisted audit

Claim: tested rather than argued, in a laboratory, with an outcome.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s judgement of claims 38 to 41. The authors describe their results as consistent with the Red Queen hypothesis.

What this does not establish

The test compared outcrossing with selfing in one worm and one pathogen, not sex with asexual reproduction.

Claim 43 · checked 24 September 2026 · AI-assisted audit

The candidate answers

Claim: The candidate answers

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Rare sex can pay its way where frequent sex cannot

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexFigure 5 caption, p. S10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · A simulation · Model
The source reads: Because the cost of sex rises in proportion to the level of sex but the advantage of bringing together fit alleles declines when they are already likely to be together, low levels of sex (left) were more strongly favored than high levels (right).

What this does not establish

A result of Otto’s simulations with two selected loci, not a measurement in any organism.

Corrected

24 September 2026. Previously: “Rare sex takes most of the benefit for a fraction of the price” "Most of the benefit" is a quantity no source on the page gives. Otto’s simulation shows the narrower thing: low levels of sex spread against a twofold cost and high levels did not.

Claim 45 · checked 24 September 2026 · AI-assisted audit

Claim: which makes obligate sex the thing to explain.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own conclusion from claim 45, stated again in claim 57.

Claim 46 · checked 24 September 2026 · AI-assisted audit

Which advantage is decisive

Claim: Which advantage is decisive

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Experiments have tested both families.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationintroduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: allowed us to directly test theories advocating either deleterious mutations or adaptation to ecological conditions as the primary selective forces
Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: We utilized experimental coevolution to test the Red Queen hypothesis

Claim 48 · checked 24 September 2026 · AI-assisted audit

Claim: A coevolving bacterial pathogen kept nematode populations outcrossing.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sexresults
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: Coevolution with S. marcescens, therefore, favored the evolution and long-term maintenance of higher rates of outcrossing.

What this does not establish

One worm, one pathogen, thirty generations in the laboratory.

Later work looked for

Crossref, 2012 on, "red queen coevolution outcrossing Caenorhabditis elegans": nothing found that overturns it.

Claim 49 · checked 24 September 2026 · AI-assisted audit

Claim: Populations of the same worm put under a raised mutation rate also favoured outcrossing over selfing, and the more a population outcrossed, the better it held its fitness.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationresults, Figure 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: The wildtype populations subject to elevated mutation rates exhibit increased levels of outcrossing ... While fitness loss due to selfing is offset to a large extent by the intermediate amounts of outcrossing exhibited in the wildtype populations
Supports
Mutation load and rapid adaptation favour outcrossing over self-fertilizationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Experiment
The source reads: In general, fitness increased with increasing rates of outcrossing.

What this does not establish

Caenorhabditis elegans in the laboratory, the mutation rate raised about fourfold, 50 generations. Obligate outcrossers held their fitness and obligate selfers lost it; nobody measured a twofold cost being paid.

Later work looked for

Crossref, 2012 on, "outcrossing Caenorhabditis elegans experimental evolution": later experimental-evolution papers on the worm, none found that overturns this result.

Corrected

24 September 2026. Previously: “and their fitness rose with the outcrossing rate.” Under the raised mutation rate fitness did not rise: outcrossing populations kept theirs while selfing populations lost it. The paper’s general statement that fitness increased with outcrossing covers all its treatments.

Claim 50 · checked 24 September 2026 · AI-assisted audit

Claim: Both effects are real in the laboratory.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

A summary of claims 49 and 50, and limited to the laboratory as they are.

Claim 51 · checked 24 September 2026 · AI-assisted audit

Claim: Whether either is large enough in a natural population to pay a twofold cost is the open part:

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
The Evolutionary Enigma of Sexpp. S10 to S11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: An open question is whether the drift-based explanation ... is strong enough to account for the evolution of sex in the face of substantial costs of sex.

Claim 52 · checked 24 September 2026 · AI-assisted audit

Claim: Otto’s reading, in a 2009 review, is that there is likely to be some truth in each of the explanations she surveys, depending on the organism, and that too little is known from natural populations to settle it.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of SexDiscussion, p. S11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: There is likely to be some truth in each of these explanations, depending on the organism in question. ... Too little evidence is currently available from natural populations, however, regarding the sign of the recombination and segregation loads

What this does not establish

Otto’s explanations are grouped differently from the page’s two families: she names selection that varies in time and space, drift in finite populations, and sex that depends on condition.

Corrected

24 September 2026. Previously: “Otto’s own reading is that there is likely to be some truth in each explanation, depending on the organism,” Placed after the two families, "each explanation" read as Otto judging those two. Her judgement is about the explanations in her own review, which are grouped differently.

Claim 53 · checked 24 September 2026 · AI-assisted audit

Why it varies so much

Claim: Why it varies so much

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Many organisms reproduce sexually only sometimes: aphids and water fleas turn to sex when their condition is poor, and a large fraction of plants can spread by runners or set asexual seed while still flowering.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S8
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in animals · Review of the field
The source reads: Many organisms, however, are more likely to engage in sex when they are in poor condition (e.g., in yeast, Chlamydomonas, monogonont rotifers, Daphnia, aphids, and other cyclical parthenogens
Supports
The Evolutionary Enigma of SexIntroduction, p. S1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Research in plants · Review of the field
The source reads: Nevertheless, a large fraction of plants are capable of vegetative reproduction (via rhizomes, runners, tubers, bulbils, etc.) ... the production of asexual seed (apomixis).

What this does not establish

Water fleas are Daphnia in Otto’s list. "While still flowering" is the page’s gloss on her point that most of these plants are not substantially asexual.

Later work looked for

Europe PMC, 2012 on, "evolution of sex" reviews: nothing found that changes it.

Claim 55 · checked 24 September 2026 · AI-assisted audit

Claim: In Otto’s models a modifier that raises sex to a low level spreads even against a twofold cost, and one that raises it to a high level does not, because the cost climbs with the amount of sex while the benefit does not.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
The Evolutionary Enigma of Sexp. S11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · A simulation · Model
The source reads: modifier alleles causing the frequency of sex to rise to moderately low levels were able to spread ... With only two selected loci, however, modifiers causing the frequency of sex to rise to higher levels ... did not spread unless costs were much less than twofold.
Supports
The Evolutionary Enigma of SexFigure 5 caption, p. S10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · A simulation · Model
The source reads: Because the cost of sex rises in proportion to the level of sex but the advantage of bringing together fit alleles declines when they are already likely to be together
Qualifies
The Evolutionary Enigma of Sexp. S11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: It remains to be demonstrated but seems plausible that high levels of sex could evolve, given a combination of selective forces acting across the genome, even in the face of substantial costs of sex.

What this does not establish

Two selected loci over 50 generations. Otto thinks it plausible, though not shown, that high levels of sex could evolve with many selective forces acting across the genome.

Claim 56 · checked 24 September 2026 · AI-assisted audit

Claim: That makes the interesting question not why sex exists but why obligate sex exists in animals like us.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own conclusion from claims 55 and 56.

What was read

The Evolutionary Enigma of SexOtto, The American Naturalist · Author’s reprint PDF (Am. Nat. 174 supplement, pp. S1–S14) from the author’s page at zoology.ubc.caRead in full, 24 September 2026 · Copyright; quoted brieflyA review of theory and evidence written for a symposium issue. Read in full; the equations were not checked.
Mutation load and rapid adaptation favour outcrossing over self-fertilizationMorran, Parmenter and Phillips, Nature · Author manuscript, NCBI PMC4183137 via E-utilitiesRead in full, 24 September 2026 · Copyright; author manuscript; quoted brieflyThe supplementary methods and figures were not read.
Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental SexMorran and others, Science · Author manuscript, NCBI PMC3402160 via E-utilitiesRead in full, 24 September 2026 · Copyright; author manuscript; quoted brieflyThe supplementary materials, including Fig. S1 and Table S1, were not read.