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. MungMung

    Rumraket thinks there is but one single canonical tree from morphological features which matches up precisely with the one single canonical tree from genetics.

    Either that, or when he argues for “the twin nested hierarchy” what he really means is that given a variety of data sets some can be made to match up. Sort of.

  2. RumraketRumraket

    Mung: Rumraket thinks there is but one single canonical tree from morphological features which matches up precisely with the one single canonical tree from genetics.

    I was not aware that I believed this.

    Either that, or when he argues for “the twin nested hierarchy” what he really means is that given a variety of data sets some can be made to match up. Sort of.

    So those are the only two possiblities? Either I believe the above, or this second tailor-made-to-sound-stupid option?

    I see that you have nothing more to contribute to this discussion. I accept your concession.

  3. Alan FoxAlan Fox

    For those interested in Allan’s progress on his 500 mile hike in the Pyrenees along Haute Route from Hendaye to Banyuls, he spent last night in Gavarnie, roughly a third of the way along.

  4. MungMung

    Alan Fox: For those interested in Allan’s progress on his 500 mile hike in the Pyrenees along Haute Route from Hendaye to Banyuls…

    Can we Un Feature his OP given that he’s not responding?

  5. Alan FoxAlan Fox

    Mung,
    Yes, Allan is mostly out of signal except near population centres. The weather in the high Pyrenees is thunderstorms at the moment which will hamper communications, too. At least the EU has just abolished roaming charges.

    I’m sure he’ll return refreshed and recharged and wanting to pick up points now hanging. We can always re-stick this OP in that case.

  6. Neil Rickert

    phoodoo: You see, the point is, in evolution, we start off with this assumption that everything adapted, without a plan, to end up perfectly suited to its environment. Everything is perfectly suited to its environment at any given moment. We never seem to describe a living organism as not well suited for its environment.

    I missed this (skimmed past it) when it first appeared. I’ll comment now.

    Most evolutionists will admit that suboptimal can be good enough. So the charge that perfect adaptation is assumed seems to be off.

    From my perspective, an organism is always adapted to its environment. It may be suboptimally adapted, but that can be good enough. As long as the population can stay roughly stable, that’s good enough. If the population is in decline, that’s the pathway to extinction. If the population is expanding too rapidly, that cannot continue without destroying the environment.

    According to the Darwinian picture, the explanation of biodiversity is to be found in natural selection optimally adapting a population to its environment. But if that worked, it would lead to exponential growth and destruction.

    I prefer to look at it the other way. Natural selection isn’t the solution; it’s the problem. Natural selection leads to extinction.

    If the population is to survive, it needs to stay sufficiently well adapted to have a reasonably stable population. Darwinists tend to think of a fixed environment and varying organisms, with natural selection as fitting the population to the environment.

    But environments can change too. Instead of fitting the population to the environment, there’s the possibility of fitting the environment to the population. That’s roughly what farmers and horticulturalists do.

    So why not a combination of both? That’s how I see evolution. I see a population experimenting with change, and using that change to explore its ability to expand into other niches. And I see the environment changing for reasons beyond the control of the population. So the population is exploring change, and that gives it the possibility that it can track environmental change. And, of course, sexual reproduction is an important part of what allows the population to explore change.

  7. Alan FoxAlan Fox

    I see folks are breathless for news of Allan Miller and his marathon march of five hundred miles across the Pyrenees from the Atlantic at Hendaye to the Mediterranean at Banyuls-sur-Mer.

    I can report that Allan is on schedule to arrive in Banyuls tomorrow and then should be back communicado within a couple of days.

  8. Allan Miller Post author

    Hi gang – back at the screen now, complete with a coffee and a bit more free time.

    I will endeavour to address comments made in my absence shortly – just wondering the best format for doing that, continuation or new post ((s), as there seem to be several themes).

  9. Alan FoxAlan Fox

    The simple thing would be to re-feature the post. If there’s no interest, it can be unfeatured just as simply.

  10. Allan Miller Post author

    Alan Fox,

    Sure, there’s no technical problem, it’s just a threading issue, when multiple thought streams are being pursued simultaneously.

  11. Allan Miller Post author

    Mung,

    How can the naturalistic origin of sexual reproduction be distinguished from a miracle, that’s what I’d like to know.

    Using your tried and trusted methodology for distinguishing other things from a miracle, I’d guess.

    Mung,

    I’ve downloaded your paper. Where do you explain the evolution of sex?

    Within its pages.

    Mung,

    Q: Which scientific model for the evolution is sex is best supported?

    A: Who cares. It exists, therefore it must have evolved. All else is irrelevant detail.

    Mine.

  12. Allan Miller Post author

    Rumraket,

    This discussion made me wonder if it is possible that there can be some initial selective pressure that will bring a population towards greater reproductive success compared to it’s ancestor, and if this process repeats long enough, something eventually evolves that happens, if you were to reintroduce the population to the original environment, to be deleterious compared to the “starting” ancestor.

    I think there is, and this is in a sense what you are suggesting happened with sex, Allan?

    Not really! I think many things would struggle to survive in historic environments.

    My argument has sex commencing, initially as fusion (syngamy), in a population of mitotically competent ‘haploids’ (scare quoted; haploidy is a relative term enforced by the existence of diploidy). It introduced a diploid phase to their life cycles, where previously there was none. Whether or not modern organisms with a periodic diploid phase to their life cycle would survive in that milieu is not an issue. This is related to the common separation of sex into ‘origin’ and ‘maintenance’, a distinction I think artificial and misleading.

    I think it the case that those haploids indulging such a diploid phase have outcompeted those haploid lineages that did not. There are no strictly mitotic haploid lineages today. Either there never were (unlikely), or they are extinct, and the sexual clade is a prime candidate for the agency of that extinction. We have a similar issue with hypothetical RNA organisms (rendered extinct by protein coders), with endosymbiosis (primitively amitochondriate eukaryotes rendered extinct by endosymbiotic ones) and indeed with every other step on the road to eukaryogenesis from pre-LECA precursors (sex itself, I consider a fundamental component of eukaryogenesis).

    Each step, we may legitimately suppose, was so successful that ancestral populations and their relatives were eliminated in toto.

    The common (albeit often implicit) supposition with sex is that it should have been eliminated by secondary asexuality – by diploids, whose own origin is unlikely without sex. That’s another issue I’ll deal with separately. But as to primary asexuals – such as the mitosing haploid population in which I argue fusion arose – any benefits of diploidy would inevitably reduce the population of ‘primitive’ haploids. To the point, with prolonged tuning and cladogenesis, of extinction of that entire class of organism.

  13. phoodoo

    Alan Fox,

    Its good (for him) that you will still be around to interfere in discussions and protect your friends like the hack moderator you have turned into Alan.

  14. Allan Miller Post author

    phoodoo,

    Its good (for him) that you will still be around to interfere in discussions and protect your friends like the hack moderator you have turned into Alan.

    Huh? Have I been protected from some unpleasantness on the internet? Thank goodness! I don’t think I could cope.

  15. Allan Miller Post author

    Rumraket,

    I’m wondering about whether some hypothetical gradual change of selective pressures can explain the origin of the sort of sexual reproduction we see, from an initially asexuallly reproducing species, and if so, is there something from real biology we know that would in effect predict the evolution of sex under the right conditions?

    I’m inclined to think not. Sex is like endosymbiosis, a bit of a one-off, dependent on local circumstances at a singular origin rather than overarching evolutionary trajectory. It just happens to have had massive consequences.

    In fact I think viewing sex as an adaptation at all is one of the causes of the ongoing confusion over it, which is why I take my extremist ‘anti-cost’ stance. Those costs are viewed from the wrong perspective, as if sex were a feature of diploids, rather than the other way around.

    There is an adaptive element to it, of course, but it’s very important to be clear about what one means by adaptation – for the benefit of what, precisely, are we saying that the adaptation exists?

    Typically, adaptation is the increase of genetic loci at the expense of others within a collection of interest, when there is a reward provided by the environment for one type against another. The genomic extent of such a locus is dependent upon the genetic system and (if that is sexual) on the extent of recombination.

    In a cyclic haploid-diploid without crossover, the smallest such units are entire chromosomes, which forms a useful model system uncluttered by recombination. If we suppose a haploid with just a single chromosome, adaptation is provided by increase or decrease of such entire linked units. Chromosomes with the capacity to invoke a diploid phase will (given a benefit occurring in that phase) increase in frequency in the overall collection of all haploids. But also, within these cyclic diploids, there will be a competition between different versions of the chromosome. The wider collection adapts by promoting a diploid phase. The narrower collection (the cyclic diploids) also adapts, by selectively promoting the best individual chromosomes through their effects in that phase.

    Now it’s perfectly OK to see sex as an adaptation in these terms. But it’s an adaptation for haploids. They increase in the population-of-all-haploids. What would be inappropriate – yet it’s what everyone does! – is to see it as an adaptation for the diploid, and hence to wonder why it happens at all without clear benefits from reduction of said diploid. As the diploid phase becomes longer and longer, people start to wonder why it splits. That is entirely where the mystery of sex comes from, IMO: what I call diplocentricity. Most adaptations within a sexual population can be viewed from the diploid stance with complete validity. That’s where they generally find phenotypic expression; most adaptations are adaptations of diploids. But not sex itself.

  16. phoodoo

    Alan Fox:
    phoodoo,

    Reminder to phoodoo.
    Complaints about moderation should be made in the “Moderation Issues” thread.

    It wasn’t a complaint about moderation, I don’t believe this site has moderation.

    It was a complaint about you.

    Don’t try to tell me what I can and can’t post Alan.

  17. MungMung

    Chapter 8 of Macroevolution: Pattern and Process by Steven M. Stanley is Why Sex Prevails. In case you want to check it out.

  18. Alan FoxAlan Fox

    @ Allan Miller

    Something triggered me to revisit the late John Davison’s paper (PDF) entitled Semi-meiosis as an Evolutionary Mechanism.

  19. Allan Miller Post author

    Alan Fox,

    Davison – I approach with caution!

    First line of the abstract – sex has evolved independently in the animal kingdom. That’s contradicted by modern evidence – Spo11, for instance, which initiates Double Strand Breaks, is highly conserved, derived from an archaeal topoisomerase. Same goes for most of the meiotic machinery. Indeed meiosis as a process seems too complex to have been subject to repeat origin (the complexities arose incrementally, in one lineage, in my view, prior to the Last Eukaryote Common Ancestor).

    The evidence is that sex arose once only – or at least, no other origins have left descendants.

  20. Allan Miller Post author

    Mung,

    Making the same fundamental error – the expectation that sex ‘should have’ been obliterated by diploid asexuality and gamete asymmetry (neither of which can even arise without sex) is what I am critiquing.

  21. Alan FoxAlan Fox

    Allan Miller: Indeed meiosis as a process seems too complex to have been subject to repeat origin

    Too complex! Too complex to have happened more than once! Are you an ID proponent!!!

    Meiosis does indeed seem overly complex just to have sex. It was just I recall John Davison making the same point in his paper. I think he may have been wrong for the right reasons, or perhaps right for the wrong ones.

  22. Allan Miller Post author

    Alan Fox,

    Too complex! Too complex to have happened more than once! Are you an ID proponent!!!

    Heh! Its complexities were serially acquired – rerunning the tape, it probably wouldn’t happen exactly the same way twice. Just conventional parsimony!

    Meiosis does indeed seem overly complex just to have sex.

    Well, it’s a vital component, but did not need to be complex at the start – IF you start in the right place! Sex is periodic diploidy and haploidy, by my preferred definition. Meiosis is just the haploid-generation part – reduction. In principle, not that complex, and easily derivable, I think, from mitosis. Mitosis in a haploid involves generation of a transient diploid – chromosome pairs – and then their separation. If you have a fusion-derived diploid, I think that the mitotic separation process would very likely be triggered by that state. The cell doesn’t know where the diploid came from; it just ‘knows’ what to do with it. The initial problem might be in deferring this reduction, not invoking it.

    That is, Meiosis Mkl was originally just the back end of mitosis. The rest is layered upon this, adding a meiosis-specific replication step (2-step meiosis), crossover, etc, for reasons not demanded on day 1.

  23. llanitedavellanitedave

    Mung:
    In my experience, sex is highly improbable.

    That’s possibly the saddest comment I’ve ever read on this site.

  24. MungMung

    llanitedave: That’s possibly the saddest comment I’ve ever read on this site.

    That’s an award I’ve long coveted here at TSZ. As keiths will happily tell you, I specialize in making sad comments. But to have finally made the saddest?

    woot!

  25. J-MacJ-Mac

    If evolution of sex is such a advantage to biological organisms, why quite a few species supposedly lost this ability?

    I can’t see how that would be an advantage to Joe Felekstain if he were to lose his leaky hose…Unless his wife can self reproduce without a yogurt shotgun I find this story to be whoopie-pie-flapdoodle like…

  26. J-MacJ-Mac

    Allan Miller,

    Too complex! Too complex to have happened more than once! Are you an ID proponent!!!

    Heh! Its complexities were serially acquired – rerunning the tape, it probably wouldn’t happen exactly the same way twice. Just conventional parsimony!

    Nothing is too complex if you want to believe it…All you have to do is speculate… I call it a speculative fluff … no proof required…You call it science?

  27. Allan Miller Post author

    J-Mac,

    Didn’t Darwin’s theory predict that biological organisms should reproduce asexually?

    If you can find a citation, then maybe. Though I don’t think we are bound by ancient texts.

  28. Allan Miller Post author

    J-Mac,

    If evolution of sex is such a advantage to biological organisms, why quite a few species supposedly lost this ability?

    How are the two in any way contradictory? Advantage is not a universal, nor is advantage the only reason things happen.

    I can’t see how that would be an advantage to Joe Felekstain if he were to lose his leaky hose…Unless his wife can self reproduce without a yogurt shotgun I find this story to be whoopie-pie-flapdoodle like…

    You what?

  29. Allan Miller Post author

    J-Mac,

    Nothing is too complex if you want to believe it…All you have to do is speculate… I call it a speculative fluff … no proof required…You call it science?

    The elliptical expression of many a Creationist (or possibly all the same one) … yadda yadda … yadda yadda … not really saying anything … yadda … trail off …

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