Another unanswered question about evolution for BBC Focus

Here’s the second “unanswered question of evolutionary biology” that I discussed in my BBC Focus piece (#1 is here).  This one appears only in print, so I’ll put my answer online.

Why is there sex? It’s possible to reproduce without sex: many species do it, including microorganisms, lizards, fish, and plants. In fact, it turns out that reproducing asexually has a big evolutionary advantage.  Imagine that each human couple can leave two offspring: on average one male and one female. If a female had a mutation that allowed her to reproduce asexually, she would have a tremendous evolutionary advantage. In the next generation, such asexual females would produce two females instead of one, meaning that our asexual females would effectively have double the reproductive rate of their sexual neighbors. Sexual females (and their male offspring) would therefore eventually be crowded out. This disadvantage of sexuality is often called the “twofold cost of sex,” but it’s really the cost of producing males who can’t bear children.

Despite this cost, the vast majority of species on Earth reproduce sexually. Why?  There must be some big evolutionary advantage of sex that offsets the cost of producing those useless males.  Evolutionists have offered many suggestions, but it’s not clear that any of them provides a sufficient advantage to overcome that two-fold cost.

One promising explanation involves parasites and pathogens.  Every plant and animal is afflicted with these burdens, and natural selection favors the evolution of resistance. Yet pathogens themselves evolve to defeat these defenses, yielding a never-ending evolutionary “arms race” between hosts and parasites.  Asexual hosts can’t respond quickly because their offspring are genetically identical to the parents, lacking the variation that fuels evolution. Sexuality gives you a leg up, however, because you can produce genetically variable offspring by mixing and shuffling your genes with someone else’s. Indeed, there’s some evidence that sex helps adapt to disease: lab experiments with microscopic worms, for example, show that sexual strains will outcompete asexual ones, but only in the presence of infectious bacteria.

Is this a general explanation? Once again we’re stymied by evolution’s historical nature.  Even if the presence of parasites can account for sex in contemporary populations, it does not necessarily follow that this was the reason why sex first arose.  Sex is, in fact, a surprisingly tricky problem.

Actually there are several possible answers to the conundrum of sex, but I had space to highlight just one.

70 Comments

  1. Rick
    Posted July 28, 2012 at 10:22 am | Permalink

    Interesting evolutionary conundrum.

    • lisa
      Posted July 29, 2012 at 12:47 am | Permalink

      Please understand that I am fully cognoscente of the fact that I am far behind most all the people (or cats; I don’t recall how many get concerned with such humans trivialities or bother posting at all) who post to this site. My formal education was interrupted for many reason that are not important. But having been forced to hang about the place mostly alone, I have tried, through books and documentaries, etc, to at least follow what’s going on with the world of science. So I know this is probably just my naiveté, but I have considered the duo-sexual nature of humans (not having much else to really observe) was one of the reason homo sapiens developed and survived, even beyond all of our ’cousins’ that didn’t make the cut. I do not know how the brain of Neanderthal and the gang developed, nor if they had gender specific differences not related to reproduction. No matter how PC or otherwise, studies have shown that, especially after puberty, women and men become very different, well beyond the outside appearance or mating ritual or anything of the kind. In general (please put that in bold italics and underline it; I more than know women’s superior nature, so please do not take any of this as minimizing female capabilities) men’s and woman’s brains, in fact serveral physical aspects, are structured differently. Men are generally better at things such as spacial recognition, where woman are much better (again generally) at such things as multitasking; men are all about ‘the big picture’ where woman are detail oriented. I would think that any evolving species would have an enormous advantage if , far and above the simply reproductive, about half of a population was best capable of perfecting half of the necessary survival skills while the other half perfected the rest. Obviously when situations compelled, either half could still handle the totality of these skills, but for optimal survival, if a species could use this splitting of skills, when paired, they could be quite a contender. Does this sound logical, or have I just not educated myself enough yet?

      • lisa
        Posted July 29, 2012 at 12:52 am | Permalink

        I didn’t spell check that last posting, and with my keyboarding skills, i know there are several corrections that should have been made. And please don’t tell Sister Rose Electra; she would haunt me ’til Judgement day!

  2. Posted July 28, 2012 at 10:26 am | Permalink

    Let me make the question harder. Why separate sexes, rather than non-self-fertile hermaphrodism, like sea squirts, which gives you the shuffling benefits without making half the population incapable of childbearing?

    • andreschuiteman
      Posted July 28, 2012 at 11:02 am | Permalink

      Most flowering plants are in fact hermaprodites, only a minority (so-called dioecious plants) have separate sexes.

      As to Jerry’s unanswered question, it seems plausible that non-sexually reproducing organisms are less capable to respond to external changes (including climate change) because of lower genetic variation. As a result they may simply be weeded out relatively quickly.

      • andreschuiteman
        Posted July 28, 2012 at 11:05 am | Permalink

        ‘hermaphrodites’

        I may add that not all hermaphroditic plants are self-fertile.

    • matunos
      Posted July 28, 2012 at 11:17 am | Permalink

      If I had to guess, it would be that different evolutionary strategies evolve for each side of the equation over time, especially once gametes separate (which may be driven by the necessities of cell structure?) and a species reaches a stage where care for the child is necessary.

    • RF
      Posted July 28, 2012 at 1:45 pm | Permalink

      Once you have hermaphrodites, there can be benefits to specializing in being “male”; fertilize others’ eggs, but don’t bother producing any of your own. As males become more frequent, the utility of male reproductive organs for the rest of the species decreases, encouraging them to become “female”. As the frequency of sex-specific individual increases, the utility of hermaphrodism decreases. The fact that half the population is incapable of childbearing isn’t really relevant; evolution works on the individual level, not species.

      • Posted July 29, 2012 at 1:33 am | Permalink

        Ingenious, but I don’t think it works. For in the hermaphrodite population, you already have equal numbers of males and females, which is, if I understand correctly, the evolutionarily stable situation.

        • RF
          Posted July 29, 2012 at 6:19 pm | Permalink

          The evolutionary stability of equality of sexual percentages is predicated on each act of reproduction requiring exactly one member of each sex. If some members of the species are hermaphrodites, then that premise does not hold.

          • Posted July 31, 2012 at 3:38 am | Permalink

            RF, true indeed, but would that alter the statistics? each act still requires one male and one female, even if for part of the population the female is also a male and the male is also a female.

            How would things work out if you had, say, 90% hermaphrodites and 10% true-breeding pure males? Would the offspring of a pure male and a hermaphrodite all be pure males, or half pure males and half hermaphrodites? Under each of these alternatives, would pure maleness spread, or would it go extinct, as I naively suggested? The answer should be obvious, although not,as yet, to me.

  3. Kevin Meredith
    Posted July 28, 2012 at 10:31 am | Permalink

    Short of perfection, I would assert, the highest virtue is the ability to improve, a principle particularly relevant on a planet with rapidly-changing weather, geology and competing creatures, pathogen or otherwise. So if sexual reproduction is the best path to mutability (is it? I’m not sure), it seems it would find favor despite the drawbacks.

  4. Stonyground
    Posted July 28, 2012 at 10:41 am | Permalink

    Presumably quite a bit of time and effort goes into finding a mate that could be spent on finding food and avoiding being eaten. I had always assumed that it was the ability to shuffle genes that was the answer, particularly when it came to virulent diseases. Your hypothetical asexually reproducing females run the risk of having their descendants completely wiped out due to their lack of genetic variation. Even if all my descendants die in an epidemic, my genes will survive via my siblings and cousins.

    • Posted July 28, 2012 at 10:49 am | Permalink

      Indeed; but this does not explain why most vertebrates have distinct sexes, rather than hermaphrodism, with measures against self-fertilisation, as in tunicates and in many plants.

      • andreschuiteman
        Posted July 28, 2012 at 11:13 am | Permalink

        If sexual selection leads to increased fitness then hermaphrodism in animals may be under negative selection pressure.

      • Scott near Berkeley
        Posted July 28, 2012 at 11:37 pm | Permalink

        Perhaps the immobility of plants versus the mobility of animals is a factor.

        • Posted July 29, 2012 at 10:15 am | Permalink

          Linnaeus was probably the first to suggest this (Praeludia Sponsaliorum Plantarum, 1729). A bit more recently, Eppley & Jesson (2008) found a strong correlation; sessile organisms are more likely to have combined sexes (Journal of Evolutionary Biology 21: 727-736. DOI: 10.1111/j.1420-9101.2008.01524.x)

  5. matunos
    Posted July 28, 2012 at 11:13 am | Permalink

    Security through diversity: not just for computer science.

    • Nicolas Perrault
      Posted July 28, 2012 at 4:43 pm | Permalink

      Or finance.

  6. Posted July 28, 2012 at 11:25 am | Permalink

    Is there an advantage for sexual reproduction in a resource constrained environment if reproduction is constrained just a bit? For example, birds that limit themselves to one chick are sometimes more successful than broods that push the envelope with more mouths to feed. They are able to efficiently keep some offspring alive, especially in times of dearth. Asexual reproduction, seems to me, would lead to runaway reproduction, overcrowding and susceptibility to extinction in times of resource nadir. Sexual reproduction may not be as prolific but may be more “efficient” than asexual reproduction.

  7. gbjames
    Posted July 28, 2012 at 11:35 am | Permalink

    Sub

    • jimroberts
      Posted July 29, 2012 at 4:14 am | Permalink

      sub

  8. Torbjörn Larsson, OM
    Posted July 28, 2012 at 12:25 pm | Permalink

    One promising explanation involves parasites and pathogens.

    So we have this weird trait (not easily explained) because of parasites, which have weird traits (simplified body plans but many-faceted life cycles)?

    I feel so dirty now.

  9. Matthew Cobb
    Posted July 28, 2012 at 12:55 pm | Permalink

    I have been a scientific advisor on the upcoming BBC series Wonders of Life, presented by the particle physicist Brian Cox. The producers/writers wanted to have an episode on sex, but I argued against it because it is very complicated and confused, and as Jerry points out, just because something apparently gives an advantage now doesn’t mean that’s why it evolved. My feeling was that explaining lots of different theories and ending by shrugging our shoulders would not make for good TV. They ended up agreeing.

    • Posted July 28, 2012 at 2:18 pm | Permalink

      Perhaps there’s scope for a future BBC series, The Wonders of Sex … with Brian Cox …

      /@

      • Posted July 28, 2012 at 9:37 pm | Permalink

        Where does the line form?

        • lisa
          Posted July 29, 2012 at 12:26 am | Permalink

          Do you have a picture?

  10. Rich
    Posted July 28, 2012 at 1:12 pm | Permalink

    This is wrong:

    the vast majority of species on Earth reproduce sexually

    • Greg Fitzgerald
      Posted July 28, 2012 at 2:41 pm | Permalink

      According to this BBC article (http://www.bbc.co.uk/news/science-environment-14616161), which summarizes a PLoS Biology paper about the number of known species, Jerry’s statement is correct.

      Although there are many species yet uncatalogued, the current data suggest that most are sexually reproducing.

      • Posted July 28, 2012 at 5:39 pm | Permalink

        The BBC article notes that: “The figure excludes bacteria and some other types of micro-organism.” Leaving aside the difficulties in defining a bacterial species, there are certainly millions of distinct groups of micro-organisms which reproduce asexually. So Rich is right; to say “the vast majority of species on Earth reproduce sexually” is not correct.

        I guess the author means “the vast majority of eukaryotic species.”

        • Gerdien
          Posted July 30, 2012 at 1:15 am | Permalink

          Facultative sexual reproduction is thought to be plesiomorphic (original) for eukaryotes.

  11. RF
    Posted July 28, 2012 at 1:30 pm | Permalink

    “In the next generation, such asexual females would produce two females instead of one, meaning that our asexual females would effectively have double the reproductive rate of their sexual neighbors.”
    That phrasing isn’t quite accurate. The asexual *trait* has double the reproductive rate, but the female herself has the same reproductive rate as before. Moreover, she’s either raising these children herself, or she’s somehow getting a male to help her. (in which case males can’t be crowded out, since they’re necessary for this strategy). If males can reproduce without contributing to the raising of the children, then that’s a significant advantage to having male children.

    “but it’s really the cost of producing males who can’t bear children.”
    Huh? What? How is not bearing children a cost? Evolution is driven by how many children one contributes genetic material to, not by how many one gives birth to.

    “There must be some big evolutionary advantage of sex that offsets the cost of producing those useless males.”
    You seem to be operating under some implicit group-selection mindset. To whom are the males useless? They certainly aren’t useless to their mothers, since they pass on their mother’s genes. In some species, (humans most certainly not being one of them) males may not significantly contribute to the well-being of the herd, but so what? That’s irrelevant to individual selection.

    • Gregory Kusnick
      Posted July 28, 2012 at 1:45 pm | Permalink

      The reproductive bottleneck is the egg, not the sperm. In each generation there’s an effectively unlimited number of sperm, but only a finite number of eggs. The female who produces only egg-bearing children will therefore own a larger fraction of that generation’s reproductive potential than one who divides her output between egg-bearers and inseminators.

      • Posted July 28, 2012 at 2:35 pm | Permalink

        Is that really true? At least in the case of equal numbers of males and females, and random fertilization, even if the total sperm supply is very large, the expected reproductive success of each male would still be equal to that of a female, because each male contributes equally to the sperm supply. The average success rate per egg (or per clutch if there is a “winner takes all” mating strategy) would always be 1/N where N is the number of males contributing (equally) to the sperm cloud. If there are Y females which each produce k eggs, the expected number of offspring per male would be the total number of eggs (kY) times the chance that this male fertilizes a given egg (1/N), which equals kY/N. If the number of males equals the number of females, this is exactly k, the same expected number of offspring as the females produce.

        • Posted July 28, 2012 at 2:41 pm | Permalink

          “… as each female produces.”

        • Gregory Kusnick
          Posted July 28, 2012 at 5:13 pm | Permalink

          But remember the problem is to explain how sex gains a selective advantage in an asexual population. So you can’t assume equal numbers of females and males, or that every egg requires a sperm.

          • Posted July 28, 2012 at 7:45 pm | Permalink

            I was replying directly to your comment that “The female who produces only egg-bearing children will therefore own a larger fraction of that generation’s reproductive potential than one who divides her output between egg-bearers and inseminators.” I don’t see that your statement is generally true.

      • RF
        Posted July 29, 2012 at 6:31 pm | Permalink

        “The reproductive bottleneck is the egg, not the sperm”
        Again, this is a SPECIES argument. For the SPECIES, the bottleneck is the egg. Evolution does not work on the species level. You need to stop trying to analyze evolution on that level. And the bottleneck isn’t really eggs at the species level, either. In some species, a female can lay hundreds of eggs.

        “But remember the problem is to explain how sex gains a selective advantage in an asexual population.”
        No, that’s not the problem. The post that you were allegedly replying to was a response to the question of why an asexual female wouldn’t take over a sexual population.

  12. emmageraln
    Posted July 28, 2012 at 2:12 pm | Permalink

    Reblogged this on emmageraln.

  13. Posted July 28, 2012 at 3:38 pm | Permalink

    Thank you for these posts, they are extremely helpful for me.

    I am currently trying to work my way through the evolutionary journey on my own blog, and your blog is very handy for directions to take.

  14. Bob Carlson
    Posted July 28, 2012 at 3:46 pm | Permalink

    In fact, it turns out that reproducing asexually has a big evolutionary advantage.

    It appears, though, that having it both ways may provide an even greater evolutionary advantage, and some species of the insect order Hymenoptera have it both ways. In this order, females are diploid and males are haploid, and some species, in some situations, especially when invading non-native habitats, dispense with males altogether. One such species, is the ichneumon wasp, Venturia canescens, for which males are unknown in North America. Because this species is easy to raise in the laboratory, it is the most studied species in the family Ichneumonidae. As it were, one the papers about the species concerns gene flow between sexual (arrhenotokous) and asexual (thelytokous) populations. But the last two sentences of the paper indicate that the study didn’t seem to help in resolving the reasons why the species doesn’t dispense with males altogether:
    If thelytokous females can occasionally benefit from incorporation of genes of arrhenotokous individuals, this advantage of sexuals diminishes, making it harder to explain the persistence of sexual populations of V. canescens. Studies comparing life histories of both modes may shed light on this problem.

  15. hossiet
    Posted July 28, 2012 at 4:39 pm | Permalink

    I thought Muller’s Ratchet basically solved this question? I.e. asexual species accumulate deleterious mutations and can’t be removed from their subsequent progeny… Also, it is assumed that producing males is costly, when there are possible inclusive fitness advantages to producing males (e.g. when they help rear offspring).

    • Greg Fitzgerald
      Posted July 28, 2012 at 5:08 pm | Permalink

      I think you mean *is NOT costly*

      • hossiet
        Posted July 28, 2012 at 5:15 pm | Permalink

        Yes, sorry if that was unclear. What I meant was that assuming males are costly underestimates the possible contributions to fitness (i.e. via the effects of inclusive fitness) of having the male sex.

  16. Gary W
    Posted July 28, 2012 at 5:41 pm | Permalink

    Also, why there only two sexes, rather than more? Why does sexual reproduction involve a giant egg fertilized by a tiny sperm rather than two similar packages of genes that combine to form a new cell? Are there clear and generally accepted answers to these questions, or are they also a bit of a mystery?

    • Posted July 28, 2012 at 9:44 pm | Permalink

      Breathes there a man with soul so tough
      who thinks two sexes aren’t enough?

      – Ogden Nash? Dorothy Parker?

      • candyliz2003
        Posted July 29, 2012 at 12:20 pm | Permalink

        Samuel Hoffenstein. Also wrote several famous scripts from the 30’s and 40’s.

    • Posted July 28, 2012 at 9:48 pm | Permalink

      If we assume that it began with two of about equal size, as soon as their roles began to diverge wouldn’t there be a natural tendency for the more motile one to get both smaller and still more motile and the other perhaps to get larger? There’s a basic instability, like two bubbles connected by a tube.

    • infiniteimprobabilit
      Posted July 29, 2012 at 12:02 am | Permalink

      Suppose it required three sexes to mate. That seems to me to unnecessarily complicate matters. Assuming the advantage of sex is that it shuffles genes, that’s achieved by having two sexes, not sure three would do it any better. And then there’s the trouble of getting all three together at the right time. Considering the trouble two-sex heterosexuals frequently have in finding a mate, the odds against finding a third (by which time one of the original two has developed a headache) seem to be quite forbidding.

      • Gregory Kusnick
        Posted July 29, 2012 at 12:28 am | Permalink

        I recall a cartoon in National Lampoon decades ago about date night on the planet of 12 sexes. It was kind of like organizing a game of Ultimate Frisbee.

        That said, they wouldn’t necessarily all have to get together for an N-way mating. You could have pairwise matings between A and B, A and C, A and D, etc, with A collecting all the gametes and combining them into a polypoid zygote or whatever.

        But I agree that this is still a lot more machinery than you need to shuffle genes, and therefore a lot more ways for things to go wrong, without adding much if any incremental benefit over two sexes.

      • gbjames
        Posted July 29, 2012 at 7:25 am | Permalink

        It seems to me that from the viewpoint of gene shuffling, the same results can be achieved more efficiently via promiscuity.

        • infiniteimprobabilit
          Posted July 30, 2012 at 12:11 am | Permalink

          🙂

          I _must_ remember that next time I encounter one of those tiresome puritans…

          “Sorry, I have to go and find someone to help me shuffle genes.”

      • HaggisForBrains
        Posted July 29, 2012 at 8:43 am | Permalink

        Isaac Asimov has an interesting take on three sexes in “the Gods Themselves”

        Colin.

    • Thanny
      Posted July 29, 2012 at 1:06 am | Permalink

      Slime molds have many more than two sexes.

  17. Wayne Tyson
    Posted July 28, 2012 at 8:47 pm | Permalink

    That which is adaptive persists; that which isn’t does not. It doesn’t matter whether reproduction is sexual, asexual, or both.

    (Several posts have, I believe, made this point, but I thought I would take a crack at saying it another way, as briefly as I could.)

    Organisms simply do what they can, when they can, where they can.

  18. infiniteimprobabilit
    Posted July 28, 2012 at 11:57 pm | Permalink

    “Sex is, in fact, a surprisingly tricky problem.”

    In all sorts of ways…

    (Sorry, just couldn’t resist) 🙂

  19. Thanny
    Posted July 29, 2012 at 1:02 am | Permalink

    It’s important to remember that sex first evolved in single-celled organisms, with no differentiation at all (i.e. sex but no sexes).

    Trying to figure out why sex came into being by examining modern multi-cellular lifeforms is futile.

    Honestly, while I certainly can’t explain exactly what selective pressures were in play, I don’t find the development of sex to be very mysterious. In the context of a single-celled organism that exchanges genes every so often, parasitism alone is enough to provide the selection pressure. And it certainly seems strong enough in most cases to preserve sexual reproduction going forward.

    • Gregory Kusnick
      Posted July 29, 2012 at 9:01 am | Permalink

      But can we say with confidence that sex evolved just once? Is metazoan sex really a lineal descendant of bacterial gene exchange? Seems to me (as a non-expert) that the mechanisms are different enough that they could be independent inventions.

      • Thanny
        Posted July 29, 2012 at 6:35 pm | Permalink

        It probably did evolve independently more than once. But in single-celled organisms.

        Modern-day trading of plasmids among bacteria (and possibly archaea) and the sexual shuffling of whole genomes are, indeed, very different mechanisms. I don’t see why you’d assume that the evolution of the latter has to resemble the former, just because we’re talking about single-celled organisms in both cases.

        We know that eukaryote cells can fuse together, so that several nuclei share a single cell body. It shouldn’t be too hard to imagine genes getting shuffled in such a scenario, before separate cells (with modified genomes) bud off again.

  20. bernardhurley
    Posted July 29, 2012 at 2:27 am | Permalink

    I often wonder if sex itself originated as a parasitic relationship where one organism has tricked another into doing its reproduction for it.

  21. Fré Hoogendoorn
    Posted July 29, 2012 at 5:27 am | Permalink

    I very much like the explanation for sex that Mark Ridley put forward in Mendel’s Demon: that sex was required to overcome the copying error rate in the duplication of genes, which is required for genomes to increase in size beyond a certain limit. See http://www.nytimes.com/books/first/r/ridley-gene.html for a description of the book by the author.

  22. Lurker111
    Posted July 29, 2012 at 6:17 am | Permalink

    Fascinating article. I’ve always wondered why single-gender organisms (like those all-female lizards in the desert SW) hadn’t gone extinct–no way to compensate for deleterious mutations, no way to shuffle genes to adapt to parasites, etc.

    BTW, when you say, “Sexuality gives you a leg up, however,” don’t you mean a “leg over”?

  23. Posted July 29, 2012 at 7:28 am | Permalink

    Interesting….. Women through the evolutionary
    event as u say… did not rise to the top. Why?

    • gbjames
      Posted July 29, 2012 at 8:19 am | Permalink

      Huh?

      • Posted July 29, 2012 at 1:36 pm | Permalink

        HUH??

        • gbjames
          Posted July 29, 2012 at 4:49 pm | Permalink

          The question you asked is unintelligible. If you have an actual question it will need to formed clearly enough to be answered.

  24. Posted July 30, 2012 at 3:00 am | Permalink

    “If a female had a mutation that allowed her to reproduce asexually, she would have a tremendous evolutionary advantage. In the next generation, such asexual females would produce two females instead of one, meaning that our asexual females would effectively have double the reproductive rate of their sexual neighbors.”

    I doubt your premises, Jerry. Like Williams and Maynard Smith, you’ve tacitly taken an all-else-equal assumption on board without specifying it. Of course, you mean that the asexual mutant female will have as many offsring, on average, as a sexual one.

    However, all else equal may mean that the asexual mutant female is as attractive to and as attracted to males as the normal sexual females. It may mean that the mutation really does not do anything except start the parthenogenetic development of her eggs. In turn, mating may readily occur between this asexual mutant female and males and her parthenogenetically developing eggs may get penetrated by sperm cells rendering them sterile.

    A host of other things are possible as well. For example, the asexual mutant female may be repelled by males, mating may not occur for some other reasons, or mating may not be detrimental to her fitness, parthenogenetic eggs may be resistant against sperm penetration, etc., etc.

    I think that specifying all these cases will whittle the number of actually paradox cases of the maintenance of sex down a grat deal. I twill also go a long way towards getting the explanation that’s wanting IMHO.

  25. Posted July 30, 2012 at 10:46 pm | Permalink

    “If a female had a mutation that allowed her to reproduce asexually, she would have a tremendous evolutionary advantage.”

    Virgin Mary? Had a son who got himself crucified, so that wasn’t so advantageous.

    On the other hand, maybe she also had a ton of daughters who kept quiet and their asexually reproductive clones walk among us now. Aren’t we due for a new conspiracy theory?

  26. Leo
    Posted August 16, 2012 at 11:41 am | Permalink

    I majored in Biological Sciences in college and it is my understanding that through sexual reproduction a species attains greater diversity. Greater diversity ensures species survival after major changes in the environment. Furthermore, meiosis (production of eggs and sperm) allows for a greater degree of “crossing over” in which alleles of genes are mixed up from their original connections with other genes. Basically the answer is diversity and the greater diversity the greater chances of survival.


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