The Evolution of Sex

This has long been an interest of mine. It dates back to the old talk.origins days, prompted by a Creationist taunt with familiar tone – “I’d like to see someone explain the evolution of sex …” (with the implicit “hurr, hurr”). I articulated some thoughts, then was rounded on by the ‘mainstream’ community. I got a flavour of the world through Creationist eyes – an equally familiar tone: some very sharply expressed contempt and an invitation to f*** off back to high school and learn meiosis.

But, the more I thought about it, the more convinced I became that the ‘twofold cost’ picture traditionally presented, one that demanded an offsetting benefit of similar weight, was simply incorrect, however lofty the figures proposing it. The problem as expressed for a dioecious population (one with separate males and females: the ‘twofold cost of males’) or from the perspective of a locus on a genome (the ‘twofold cost of meiosis’) leads to the same result: an apparent halving of reproductive output for a female, or of survival odds for an allele. Yet since dioecy is quite rare, and ‘selfish alleles’ only exist when recombinant sex does, these cannot be taken as relevant to the origin of sex, and not all that relevant beyond it. The sensible perspective, it seems to me, is that of the haploid genome. From such a perspective, we are binary organisms – temporary unions of haploid genomes. It cannot be inherently more costly to unite then separate than simply to do nothing. The view that sex (as syngamy: gamete fusion) should have existed for only a moment before being erased by permanent diploidy seems wrong. Meiosis is the brief return of the native organism – the haploid. Permanent diploidy is cancer: a trap.

It’s more complicated than that …

I have written the above essay summarising my views. It is a bit of a dry and technical read – few pictures, even fewer jokes! This venue is not particularly conducive to a sensible discussion of the subject. It may be of interest to no-one: too amateurish for the professionals, too technical for the interested layman. Almost none of it is really original thought, being more a synthesis than anything groundbreaking. Beyond the choice of perspective, there are only a couple of ideas I have not seen elsewhere (I might remain coy about which those are!).

I’d prefer comments to be held off until the paper has been read (a big ask; it’s 50 pages!) but I hope it doesn’t descend into yet another discussion about semantics, metaphysics, the evidence for common descent, or any other of the conversations we have all had on a few dozen other threads.

My own time defending this is limited – I’m off to the Pyrenees for 5 weeks on June 24th. But, if anyone is interested …

237 thoughts on “The Evolution of Sex

  1. Alan,

    keiths will just tell you that it must be useful, because the evolutionary costs are so high! See, that’s all the evidence you need, it proves it is useful evolutionarily!

  2. Well, fuck that.

    Is there a way to read this paper that does not require that I sign up for membership at Academia.edu or other site, and that does not insist on invading privacy?

  3. Allan Miller:
    phoodoo,

    For God’s sake phoodoo. 50 fucking pages! Arguing that the costs are not high.

    Huh??

    Is it cold where you are? The brandy takes away some of the nip?

  4. Allan Miller:
    phoodoo,

    Did you read below the fold?

    Did I read your 50 pages? You are kidding right?

    keiths was arguing that it costs so much to have a big brain, that is must be evidence for its useful evolution. So he would make the same argument for sex.

    But then if you argue that the costs are NOT high (its your argument, but in evolution ALL arguments seem to hold the same validity), so that is evidence for the reasonable evolution of sex, well, one can only smile at the irony, right.

  5. phoodoo,

    keiths was arguing that it costs so much to have a big brain, that is must be evidence for its useful evolution. So he would make the same argument for sex.

    But then if you argue that the costs are NOT high (its your argument, but in evolution ALL arguments seem to hold the same validity), so that is evidence for the reasonable evolution of sex, well, one can only smile at the irony, right.

    I can only smile at the irony of you having a guess what my argument might be.

  6. I’m off to the Pyrenees for 5 weeks on June 24th.

    Which bit, or are you walking from the Atlantic to the Med? Maybe we can meet up!

    I’ll own up to finding the paper a bit of a slog on first reading but I’ll have another go.

    @ Neil
    We can host it here and make it available as a download, I’m sure.

  7. Alan Fox,

    Which bit, or are you walking from the Atlantic to the Med? Maybe we can meet up!

    Yep – Haute Route from Hendaye to Banyuls-sur-Mer. My itinerary is obviously a bit unpredictable, mind!

  8. Allan, stop arguing this shit with phoodoo. He wouldn’t get it if he got paid to. Not getting it is his life’s foundation. He’s just here to mock and troll.
    Wait for people who at least have an interest in trying to understand the arguments to have read your assay.

  9. OK, I have amended the link to be a PDF hosted here. Didn’t realise you could.

    Can’t do much about the dry content though!

  10. The evolution of sex was on my list of The Mysteries of Evolution, which I’m going to do anyways since your speculations are confusing and unfounded as all speculations are…

  11. J-Mac,

    The evolution of sex was on my list of The Mysteries of Evolution, which I’m going to do anyways since your speculations are confusing and unfounded as all speculations are…

    That was a quick skim!

    I accept that my presentation may well be confusing. I am happy to try and clarify.

    I look forward to your alternative view. dot-dot-dot.

  12. Neil Rickert,

    Yeah, sorry if that route was unpalatable. I’ve now made it a PDF hosted here – I thought ‘media’ was for images etc.

  13. I have not had a chance to read through Allan’s manuscript yet, but I do notice that a major issue is the evolution of dioecy (asymmetry of the sexes). The classic paper on this is Baker and Parker in Journal of Theoretical Biology in 1972 which can be downloaded as a free PDF here: https://www.researchgate.net/publication/18089267_The_Origin_and_Evolution_of_Gamete_Dimorphism_and_the_Male-Female_Phenomenon
    (just ignore the box that pops up and asks you to join ResearchGate).

    I notice that this is not referenced in the article. When Allan gets back from the Pyrenees perhaps he can put his argument in the context of Baker and Parker’s.

  14. Joe Felsenstein,

    Thanks – as a general point, I recognise that the essay is light on references, which I am happy to remedy!

    I’ll read up, but my main point on dioecy is that it seems unjustifiably central to arguments on sex in toto. The cost of males is what many people mean when they say ‘twofold cost’, and yet dioecy is something of a minority case. If a complex sexual clade and ecology arises first, with nary a whiff of dioecy, then the simple models showing what happens if you compete a dioecious sexual with an asexual ‘female’ population in isolation seem insufficient to inform our entire attitude to sex.

  15. Allan Miller:
    phoodoo,

    I can only smile at the irony of you having a guess what my argument might be.

    No, no, you got it wrong, I wasn’t stating your argument. I said whether one wants to say the cost of something in evolution is high (like keiths said), or if they want to say they are very low (like you said), evolutionists will claim BOTH of these explanations are evidence for something having evolved.

  16. keiths:
    phoodoo,

    Go play with Mung.The grownups are talking.

    This is one of your best posts.

    Guests that are new here might think I am joking.

  17. phoodoo,

    This is one of your best posts.

    Guests that are new here might think I am joking.

    While old-timers have long since learned to disregard your assessments.

  18. phoodoo,

    No, no, you got it wrong, I wasn’t stating your argument. I said whether one wants to say the cost of something in evolution is high (like keiths said), or if they want to say they are very low (like you said), evolutionists will claim BOTH of these explanations are evidence for something having evolved.

    To the irritation of some, I don’t mind discussing points with anyone, up to a point … in this instance, what you say is untrue. It’s not that both high cost and low cost are evidence for evolution, but that in an evolutionary scenario, we’d like to know what the benefit is, if a cost really is high (it would be no different for Design, to be honest, if there were the same intellectual curiosity).

    This is at the heart of a long debate in evolution. There is a school of thought that everything must be beneficial, and a cottage industry in coming up with possible benefits, often admittedly with little actual evidence. Don’t quote mine me on that … It’s a fun intellectual exercise, and a challenge, but many people rightly caution against over-promoting a plausible explanation without further evidence.

    In the matter of sex evolution, there seem to be massive costs, and most work has been trying to find benefits to offset them. There is no shortage of ideas, but not much consensus. The view I’m pursuing here is that, despite the apparently compelling nature of those costs, they are not (as stated) real.

  19. Joe Felsenstein,

    I’ve had a look, and it’s certainly interesting that stable gamete asymmetry can arise under a range of assumptions – some of them quite unrealistic! My chapters on gender are a bit light – I was flagging by then, and aware it was already long!

    A couple of areas of omission for me:

    1) The role of mitochondria
    2) The relation with hermaphrodity and pre-existing outcrossing mechanisms.

    1. Equal cytoplasmic compartments will contain equal numbers of mitochondria. To the extent that contests between such mitochondria on zygote formation can be damaging, this may be a source of additional disruptive selection favouring incremental dimorphism.

    2. The models all seem to assume isogamy in a population in which every gamete can initially fuse with every other. The drive towards dioecy is considered simultaneous with the drive towards anisogamy. More likely, I think, is that mating types pre-existed, gamete dimorphism arose in hermaphrodites, and dioecy evolved from hermaphrodity via mixed hermaphrodite/monoecious individuals.

    It is not certain that the assumed isogamous stage of a multicellular lineage would initially lack a mechanism to avoid selfing. Given this, if there is already a binary state in the population, likely predating multicellularity itself, such a state would be more susceptible still to disruptive selection on gamete size.

    But, for each individual, neither strategy is definitively best at first, and so organs specialising in both types would emerge on the same individual, either simultaneously or sequentially. This is separate to a ‘sex determining’ system. In fully dioecious and anisogamous individuals, a mating type distinction would cease to be necessary.

    For this reason incidentally, I prefer the term ‘cost of males’ to the frequently used ‘cost of anisogamy’, because they aren’t necessarily the same.

  20. Allan Miller,

    Allan,

    Do you understand, even a little, that keiths made the argument for the big brain being valuable evolutionary, AS EVIDENCED BY THE FACT THAT IT IS COSTLY?

    Do you get that part? Now, feel free to disagree with him, but that’s his argument. THAT is what I am commenting on here.

    So if you are saying the reason sex could evolve, is because it wasn’t very costly to do so, well, then I am just pointing out the two feuding materialists.

  21. phoodoo,

    Do you understand, even a little, that keiths made the argument for the big brain being valuable evolutionary, AS EVIDENCED BY THE FACT THAT IT IS COSTLY?

    Do you get that part? Now, feel free to disagree with him, but that’s his argument. THAT is what I am commenting on here.

    So if you are saying the reason sex could evolve, is because it wasn’t very costly to do so, well, then I am just pointing out the two feuding materialists.

    There is no feud. Sex and large brains are two completely different things. You seem to think that, if evolution were true, everything in it would subject to identical constraints.

    Nonetheless, as a general principle, if a feature is costly, there should indeed be an offsetting benefit, otherwise one would reasonably expect its loss. I don’t know why you think Keith and I are saying the opposite to each other on that. And indeed, as I said, Design enthusiasts ought (I’d have thought) to be equally interested in what benefit a genuinely costly feature provides.

    I’ve said the same thing twice now.

  22. Allan Miller: Sex and large brains are two completely different things.

    Now there’s a special argument. They are different things!

    Then why even argue that sex has a lower cost benefit than some presume. What does it matter the cost benefit, if evolution can handle anything thrown at it?

    What does the cost benefit even matter, if we can just say everything is different?

  23. phoodoo,

    Now there’s a special argument. They are different things!

    It is a reasonable counter to the idea that everything should be the same.

    Then why even argue that sex has a lower cost benefit than some presume.

    Because I think that is a more accurate position (lower cost).

    What does it matter the cost benefit, if evolution can handle anything thrown at it?

    Precisely because evolution is not expected to handle everything thrown at it. Which is one reason why lineages go extinct.

    What you see is people exploring ideas. You don’t seem to notice that they don’t just swallow the first idea that comes along, as if anyone saying what they feel like saying at that moment makes it true, ‘cos it’s evolution. There has to be some rational basis for the argument.

    Do you think a separate thread might be in order? I realise I’ve been as guilty as anyone of honouring my second-last paragraph in the breach, but I’d like this one to be about sex.

  24. I’d prefer comments to be held off until the paper has been read (a big ask; it’s 50 pages!) but I hope it doesn’t descend into yet another discussion about semantics, metaphysics, the evidence for common descent, or any other of the conversations we have all had on a few dozen other threads.

    I promise to not tell you what’s in the paper until I’ve read it.

  25. I beg your pardon for not having read the paper. (50 pages: daunting.) But are you careful to distinguish between the evolution of sex and its maintenance? Sex might have evolved in haploids, and in fact appears to have been completely separate from reproduction, and still is in many protists. But most discussions of the cost involve the maintenance of sex in multicellular diploids, whose reproduction is what entails the main cost, not gametogenesis, and is closely tied to meiosis.

    And of course there are other costs even in haploids, notably the extra steps of meiosis and the effort/risk in finding a partner.

    I’m curious if you discuss any of this.

  26. John Harshman,

    (50 pages: daunting.)

    Granted!

    But are you careful to distinguish between the evolution of sex and its maintenance?

    One of my key points is that this distinction is somewhat artificial. It is as if sex can be explained for a few days, but not forever. It seems an odd view that sex could get going relatively unimpeded by costs (isogamy) but then as soon as mitotically competent diploids arise as a result of it, it should be wiped out by them. That is, sex should only exist as a stepping stone from haploid to diploid asexuality, a life-cycle fossil. I don’t see why.

    Sex might have evolved in haploids, and in fact appears to have been completely separate from reproduction, and still is in many protists. But most discussions of the cost involve the maintenance of sex in multicellular diploids, whose reproduction is what entails the main cost, not gametogenesis, and is closely tied to meiosis.

    Sure, when people talk of ‘the mystery of sex’, they really mean ‘the mystery of multicellular sex’ – ‘what about us?’

    What I think is often missed is that, by the time multicellular diploids come on the scene, sex has had plenty of time to establish a significant clade during the “Age Of Isogamy”, with nearly all the features of modern sexual systems. That ecology is a challenge to any novel species, including asexual diploids. This is just blanked out in many models.

    As to multicellularity itself, given my central thesis that sex is not ‘for’ diploids at all, but for their constituent haploids, the whole ‘point’ of multicellular bodies is to increase and protect those haploids, not to make more multicellular bodies. It’s a bit of a Dawkinsian flip of perspective, but not down to the individual allele. Multicellularity involves the addition of diploid and/or haploid mitoses without dispersion. These allow production of a soma, or other non-reproductive set of cell lines. Increase in the germ line or equivalent benefits both haploid partners. But because of the gametic exit, there is stable co-operation between soma and germ line. Because every cell is mitotically competent, without gametes any cell could reproduce on its own account. But because release of haploids is itself a tissue specialisation, somatic cells work on behalf of the gene copies in the gamete line, specialising for their own, non-reproductive, role.

    Therefore, multicellular bodies are built for haploid increase, albeit frequently paired in mitosing diploid cells. You don’t get multicellular bodies at all without sex (I would argue). Sex is responsible both for the diploidy of its diploid components and for the co-ordination of the whole. But when a perpetual asexual diploid line arises, it is not doing that which was the whole point of the structure in the first place. It can carry on for many generations, as some cancer lines can. But its challenges are many, for reasons I go into.

    And of course there are other costs even in haploids, notably the extra steps of meiosis and the effort/risk in finding a partner.

    Yep, cover that.

  27. Joe Felsenstein,

    Just to mention that the version I originally uploaded had corrupted a couple of my diagrams. The link is now a PDF, closer to the original than the Academia paper I originally linked you to.

  28. Allan Miller,

    I’m a little disturbed by the teleological language. The whole “point”, built for haploid increase, the whole point of the structure? Different environments select for different things. Just because x was advantageous in the past, and so arose by selection, doesn’t mean it’s advantageous now, and so is maintained by selection. What was “the point” once isn’t necessarily “the point” now. And anyway, bdelloid rotifers.

  29. John Harshman,

    I’m a little disturbed by the teleological language.

    Don’t be. Hard to avoid sometimes, particularly in a relatively informal blog comment. See also: “The function of a heart is…”, “Females attempt to maximise their …”. I thought the scare quotes might be enough, but will express it any other way you like if it retains the essence of my point. I am merely saying that multicellularity is a mechanistic cause of paired haploid genomes becoming increased in number (through germline mitosis of both genomes) and protected (through somatic mitosis of both genomes). These complementary roles are stabilised by sex, and the entire body is tuned (“tuned”?) in light of the generation of those haploid outputs – the native organism from the beginning, in my view. To revert from this state to asexuality is to provide a functionally similar organism without the actual end product. Multicellular asexuals are built by sex yet, since they don’t do it, they are possessed of redundant function and have lost a cause of cohesion – the gametic exit. They don’t just fall apart, of course.

    And anyway, bdelloid rotifers.

    Yes, the poster bugs of explanations of sex.

    I don’t find them central myself. They offer a valid counterexample to the general trend by which homologous chromosomes become increasingly homozygous in long term diploid lines and suffer other degradative, evolutionary and ecological constraints.

    There appear to be mechanistic reasons why they haven’t gone extinct with the comparative rapidity that other asexual diploid lines appear to, and may indeed be equipped to survive indefinitely. I don’t know why everything isn’t rotifer-like in its genetic system, but nor can I see why everything should be. No sex, no rotifers – their ancestors were sexual. Should it be a life-cycle fossil in the general case? Why?

    The exception that proves the rule, I would say, rather than the exception that fatally undermines it.

  30. John Harshman: I’m a little disturbed by the teleological language.

    I’m not. But I’m glad that you brought it up.

    Teleology runs through natural language. It is almost impossible to avoid.

    In any case, talk of the cost of sex already involves teleology. The standard view seems to view replication as the central purpose. Allan has a different evaluation of the cost, because he assumes a different purpose.

    I’ve never found analyses of the cost of sex to be persuasive. That’s perhaps because I don’t subscribe to the standard teleology — which is not to say that I don’t have my own assumptions about purpose.

    By the way, I have not finished reading the paper.

  31. Allan Miller: To revert from this state to asexuality is to provide a functionally similar organism without the actual end product.

    That’s the problem with teleological language: it has a tendency to be reified. What it shows is that “end product” is a problematic notion. It implies a raison d’etre where there is none. Dispensing with a haploid stage is just a thing that can happen to a species, but you seem to be implying that it’s a betrayal of the purpose of life.

    I don’t know why everything isn’t rotifer-like in its genetic system, but nor can I see why everything should be. No sex, no rotifers – their ancestors were sexual. Should it be a life-cycle fossil in the general case? Why?

    The exception that proves the rule, I would say, rather than the exception that fatally undermines it.

    I always hated “the exception that proves the rule”, because of course exceptions don’t prove rules. Why would the ancestors of bdelloid rotifers being sexual make any sort of point for you?

  32. John Harshman:

    I always hated “the exception that proves the rule”, because of course exceptions don’t prove rules. Why would the ancestors of bdelloid rotifers being sexual make any sort of point for you?

    Just a note on this particular concept. “The exception that proves the rule” does not mean that some given exception to some standard either proves some rule about the standard or proves some rule about the exception. Rather, the phrase actually means, “the exception that implies a rule exists”. Basically, the concept works like this: a sign on a store that reads, “closed on Monday” implies that the store is open the other six days of the week.

  33. Neil Rickert,

    In any case, talk of the cost of sex already involves teleology.

    Yep, that’s pretty insightful. When one wonders “what’s it for?”, one has an entity in mind, a perspective ‘for whose benefit’ a feature exists. One can restate that down in non-teleological language, but it isn’t especially necessary unless people are out to misunderstand.

    The traditional entities are diploids and individual genetic loci. From their perspective, sex makes little sense, because for both, it causes a ‘halving’. So the stance I urge – even though one might initially wonder ‘what’s the point?’ – is that of the haploid genome. From their perspective, sex makes sense. To me, at least.

  34. John Harshman,

    […] you seem to be implying that it’s a betrayal of the purpose of life.

    God no! The line I take is that, logically, sex must have begun with haploid syngamy and reduction. The haploid is the ‘native organism’, and the diploid a temporary union between two of them. When the diploid returns haploids, that’s one turn of the cycle. Now of course a diploid might not return haploids. But while it does, it is a sexual species, and from the perspective of the haploid, diploidy is a temporary state whose scare-quoted ‘purpose’ is to again-scare-quoted ‘improve the lot’ of both haploid genomes. As far as multicellularity is concerned, the ultimate role is to provide increase and protection to those genomes.

    Of course perpetual diploidy can occur, and haploids never be released. If it proves more advantageous to two haploid genomes to remain in harness, they will prevail over the reducing population. Nonetheless, while reduction remains in place, the ‘end product’ of the cycle is indeed haploid genomes. Enhancements to the diploid phase benefit both haploid genomes, and the eventual result is more haploid genomes.

    I always hated “the exception that proves the rule”, because of course exceptions don’t prove rules.

    I agree it’s an annoying phrase, because people tend to interpret it as you do. I meant, as Robin pointed out, that it is the exception that shows there is more generally a rule, to which this is an exception.

    Why would the ancestors of bdelloid rotifers being sexual make any sort of point for you?

    The point against which you raised them related to my suggestion that multicellularity depends on sex. I should have been clearer: the evolution of multicellularity from unicellularity depends on sex. Without sex, doesn’t happen. I know of no exception, and there are good theoretical grounds relating to the gametic exit.

    Now this is not at all at odds with the evolution of parthenogenetic lineages subsequently. Having obtained (via sex) a multicellular body, you can abandon sex, though only rarely does this strategy work. If it does, it then looks as if ‘the point’ all along was to make more diploids …

    It’s something of a paradox. The ‘mystery of sex’ in these terms is not a mystery of sex at all, but the general failure of secondary asexual lines. But why should we expect them to prevail? I’ve no reason to suppose that this kind of multicellular asexuality – one that can’t even happen without having sexual ancestors – should extinguish the other descendants of those very ancestors, leaving diploidy a frozen mystery, in the general case.

  35. Allan Miller: One can restate that down in non-teleological language

    I would like to see some evolutionists try that sometime. One wonders why it is almost never done. Well, actually, I don’t wonder, I now why it is not done, because you then have to talk about evolution being random, and evolutionists try to hide from that as best as they can.

  36. phoodoo,

    I would like to see some evolutionists try that sometime. One wonders why it is almost never done.

    Rubbish. In their scientific works, that’s pretty much all they do. But it can result in stodgy prose, particularly in text aimed at the general public. Yet even then, they do explain why they take the stance they do. Unless you’re Mary Midgeley or a Creationist, you can follow the reasoning without being overly bamboozled by the teleological manner in which things are expressed.

    Dawkins does not stick to the ‘selfish gene’ stance throughout his book without ever explaining what he really means by it. Frequently, he drops the stance explicitly in order to do so.

    And there ain’t much teleology in my essay. It was an expression I used talking casually to John, knowing full well (or thinking I did) that he wouldn’t take me literally.

    Well, actually, I don’t wonder, I now why it is not done, because you then have to talk about evolution being random, and evolutionists try to hide from that as best as they can.

    ‘Random’ (in whatever sense you mean the word) has absolutely nothing to do with the teleological stance. It’s not that genes are depicted as ‘selfish’ when they’re really ‘random’, whatever that would mean.

    Anyway, sex, phoodoo. Focus!

  37. Just to break down the actual issue of haploid vs diploid vs gene as an appropriate stance in sex:

    If we had a simple system in which two haploids adhered their cells for a moment then separated, we would see quite clearly that the haploids were the units in this transaction. While joined, there is a higher-level unit which is itself subject to selective forces, but there is no gene in the doublet which is anything other than one in one of the haploids.

    We could make the association more intimate, by fusing the cytoplasm and the nucleus before separation. This makes no difference. We would not scratch our heads and wonder why the diploid separates into two ‘halves’, when we can clearly see that it’s just haploids going back to being haploids.

    The longer the doubled state persists, the more central it seems to appear. But there is not a clear point to me at which the diploid suddenly takes over as the fundamental unit of the transaction. Even if haploidy reduces to a few hours and diploidy extends to 70 years, nothing has changed in terms of the relation between the entities – it’s just become more asymmetric.

    If, conversely, a mutation prevents the constituent haploids from ever being released, the diploid really does become a unit with evolutionary interests. But only then. The diploid is not such a unit until the lineage has abandoned sex. While returning haploids, it makes no sense to wonder why the diploid (the temporary coupling) does not stop doing it. Why should it? There is no gene asserting the rights of the diploid. Genetically, it is nothing more than its two components until it abandons sex.

  38. Prompted by discussion, I’ve added this section – unfortunately taking the page count up to an even more unwieldy 54!

    The Rotifer

  39. Allan Miller,

    OK, that’s one thing I don’t get. Why must one part of the cycle be the central one and the other part “not a unit”? Are they not both units of selection if selection takes place there? Now, in animals, most selection takes place in the diploid body, and in fact most genes are inactive in the haploid phase. I’d say a diploid individual is definitely quite a bit more than its two haploid genomes, even if the haploid phase appears regularly. And the reason sex isn’t abandoned more often is that its benefits generally outweigh its costs. That asexuality has benefits is shown by the frequent appearance of asexual species, and that it has costs is shown by the frequent extinction of most of them. Bdelloid rotifers do nothing to demonstrate that any rule exists. If anything, it’s the transient asexual lineages that do that.

  40. John Harshman,

    OK, that’s one thing I don’t get. Why must one part of the cycle be the central one and the other part “not a unit”? Are they not both units of selection if selection takes place there?

    It’s almost exactly the same ‘levels of selection’ issue one sees expounded by Dawkins. I’m guessing you’re not a fan.

    Dawkins portrays ‘individuals’ – the diploid, in this case – as being temporary vehicles. Selection indeed happens mostly in that state, while they persist, but what is being increased or decreased in the population are genes. The vehicles themselves don’t persist.

    But it’s not really ‘genes’, it’s the evolutionary allele in Williams’s sense – that which recombines with appreciable frequency. Haploid genomes are evolutionary alleles while reduction remains. They increase or decrease in the population, albeit mostly existing in the paired state.

    As soon as the paired state becomes permanent, that diploid combination becomes the evolutionary allele.

    Now, cost-based analyses usually apply to a recombining locus within a population. It’s an adaptive explanation – an allele increases in frequency, with an attached selection coefficient somewhat independent of all the selection coefficients of its linked loci. The moment sex is abandoned, that locus-based view loses traction. No locus can recombine into the population. The entire genome becomes an evolutionary allele with a composite selection coefficient dependent on all manner of interactions both in that genome and with the wider ecology, and with an uncertain future trajectory to the relative parameters in the sexual and asexual subpopulations.

    But no – we are told categorically that asexuality is an adaptation to be treated exactly as if it were a locus within a recombining population. But once the population is left, it cannot be returned to. The asexual is a separate species, and the change is irreversible (I am unaware of any counterexamples). It’s not duelling alleles inside a population, increasing and decreasing under the ebb and flow of selection, it’s ecological competition between species. Do Red Squirrels in Europe suffer a Cost Of Not Being Greys?

    Sex is the only allele for which we are told what its selection coefficient is, rather than it being empirically discovered. That selection coefficient, we are assured, is precisely half that of the asexual ‘allele’, but there’s probably something going on we don’t know about to bring it back up to equality. But that’s only necessary in multicellular dioecious populations.

    That seems completely wrong.

    Ironically in view of your discussions with Charlie M, you are insisting that an us-based view is the way to go – sex is for diploids. But if you take that stance, sex has a notional twofold cost, if you take the haploid stance, it doesn’t. Rather than insisting that the cost is definitely twofold, despite being an artefact of one’s choice of stance, perhaps it’s worth exploring the stance that does not incur it?

  41. John Harshman,

    Bdelloid rotifers do nothing to demonstrate that any rule exists.

    Yes they do – if there weren’t a rule, that persistent asexual species are rare, no-one would mention ’em!

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