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Can current evolutionary theory explain identical twins and intersex babies?

  1. Feb 4, 2010 #1
    Phew, I made the mistake of posting these questions on Yahoo Answers and got blown away by people who wanted to convert me to Christianity. I had forgotten about this forum. Thank goodness it's here.

    So it's actually two questions:

    #1: Intersex

    I've just discovered some literature on intersex people. It says that being born with not entirely obviously sexed genitalia occurs at about the same frequency as twins. That seems really, really high for it to be coincidental. If it's true, then has anyone yet tried to explain this frequency using the latest version of evolutionary theory? I've searched with Google but found nothing. If it's not true, can anyone point me to some better literature?

    #2: Identical twins

    I can understand that fraternal twins result more-or-less from an accident of the woman's body. Still, interesting to see that natural selection does seem to place a typical limit on the "litter size" for humans. But the fact of multiple eggs seems coincidental enough to me not to require an explanation.

    What I'm not so sure of is whether identical twins are so obviously coincidental. As I understand it, they're a mistake of the doubling process after the egg is already fertilized. A totally different kind of mistake from fraternal twins. For an egg to make a random mistake at the cell-division level seems like it would be catastrophic, so the fact that it works so well now seems to suggest that natural selection has had a hand in making it safe. But if that's the case, then doesn't that imply that identical twinning has some survival value? Some survival value that is not, or at least not necessarily, conferred on fraternal twins?

    Am I totally missing some obvious point, or wading too far into the deep end?

    P.S. You really have to check out the preposterous answers I received on Yahoo Answers. Read these when you need a laugh:

    Last edited: Feb 4, 2010
  2. jcsd
  3. Feb 6, 2010 #2
    Really, not one person here wants to weigh in? (Identical Twins and Intersexuals)

    I am really, really baffled. I posted this question a while back and got 190 views but not a single response. I'm a bit worried that it's because my questions were so stupid that they didn't deserve an answer. I ask a favor of the group. A few of you, please respond and give me some idea of why you have no response. If I can just get an idea of what the community DOESN'T know, I might be able to explain something at least to myself.
    Last edited by a moderator: Feb 6, 2010
  4. Feb 6, 2010 #3


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    You are making the mistake to think that if it occurs in a population, that it must have an evolutionary advantage. Inter-sex anatomy can occur for many different biological reasons, such as chromosomal abnormalities or hormonal imbalances/insensitivity's. There is no evolutionary advantage to being inter-sex, it can be a disadvantage when it leads to sterility.

    You appear to believe that identical twinning is an efficient process and must thus have been positively selected upon. In fact it is not safe, I'm sure you are aware of conjoined/Siamese twins. In the cases where it goes catastrophically wrong the embryo/fetus will abort or be still-born.
  5. Feb 6, 2010 #4
    Well, not exactly. I think that if it occurs frequently in a population then one might reasonably expect there to be at least an explanation as to why it hasn't been selected against.

    Exactly my point: if it leads so easily to sterility, why does it show up as often as twinning shows up?

    I know all of this, but it just strengthens my point: most faulty egg cell division goes really badly. My point is that there are an awful lot of successful identical twin births (and I assume here that our far distant ancestors' similarly faulty egg cell division went badly almost every time); it seems that there must be an explanation for why it goes right so often.
  6. Feb 6, 2010 #5


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    We have many deleterious conditions in our population that don't necessarily have any advantage. What is the advantage of cancer, of Down syndrome, of Achondroplasia? Some disorders might give an evolutionary advantage under certain conditions, such as sickle cell anemia, but these are exceptions.

    Are you suggesting that there is any connection between twinning and inter-sex anatomy?

    It is not known how successful identical twinning is, so you cannot make any conclusions about the fidelity of the process.
  7. Feb 6, 2010 #6
    Hmm, you probably have more data than I do, but I thought it safe to assume that cancer in particular would show up far less frequently than twinning if you consider only cancer that reduces progeny (i.e., occurs young).

    However, I do get your point, and I do see that there is one assumption I'm making that may not be safe: that intersexuality occurs far more frequently than other deleterious conditions. I say far more frequently because of the comparison I read, that the frequency is about that of twins, which seems extremely frequent for an often-sterilizing condition. Certainly twins aren't clucked over because of an excess of sterility.

    None at all. The only correlation between the two is that when I looked up intersexuals, the article I read said that they occur at about the same frequency as twins. This brought up two questions, unbidden to tell the truth; they just popped into my mind: why do intersexuals occur so frequently if it tends to be a sterilizing condition, and why do identical twins occur so frequently when it seems that the kind of cell division necessary is clearly quite risky.

    That is a superb point. Thanks. Also, just in general, thanks for responding. The board was silent on my question and I was afraid that my questions were "not even wrong," as they say. Plus, thanks for fixing my double-post faux pas. Now I know what to do if I wait too long for an answer.
  8. Feb 6, 2010 #7
    Wait, this is a good point, but I think I have a counterpoint. Please let me know if I'm just making some sort of logical fallacy like circular reasoning or something. I see that I'm making another assumption implicitly: how can I say it...I'm going to naively assume that at least the vast majority of identical twinning occurs due to the same basic category of error in cell division. If that's reasonable, then I want to say this: it obviously works really, really well quite often now. I'm assuming, and now making the assumption explicit, that back in the days when this same basic category of error first started happening, the vast majority of the eggs would fail catastrophically. So I'm assuming, perhaps really stupidly, that just the fact that we see identical twins at all seems to suggest that natural selection has made us at least pretty darned good at it. Am I really stretching it too far?
  9. Feb 6, 2010 #8


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    It really is not understood how and why embryos split into two embryos. Based on the morphology of the developing embryos (whether they share a chorion/placenta or amnion/amnionic sac) it can be deduced at what stage the splitting occurred.

    About 33% of identical twins have two complete and separate chorions, which indicates that they split before day 5 (before the trophoblast tissue was formed). About 66% share the chorion, but have separate amnions, indicating that they split after day 5, but before day 9. A small percentage of embryos share both the chorion and amnion, suggesting that they split after day 9 (this group has a high risk for being conjoined twins). Clearly the process is not reproducibly the same.

    There is absolutely no basis for this assumption.
    Yes, that is absolutely incorrect logic. It could very well be that the fact that we have identical twins, is a side-effect (!) of another biological process that is present. During the early stages of human embryonic development the embryonic mass is pluripotent: the mass consists of stem cells that have the ability to differentiate in many different cell types. The fact that the cell mass is pluripotent allows for the splitting and subsequent development of normal embryos. As said, the process of splitting is not understood and could very well be a stochastic process. Once the embryonic mass starts to differentiate, you can no longer split the embryo without causing gross developmental problems.
  10. Feb 6, 2010 #9


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    Other note: the frequency of monozygotic twins is the same throughout the world (unlike the frequency of dizygotic twins). This suggests that the process is not sensitive to environmental or genetic cues, but is a direct result of embryonic development.
  11. Feb 6, 2010 #10


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    Remember that natural selection only favours one sub-population that is statistically significant over some other sub-pop. that is statistically significant.

    Mistakes are made over and over again, but natural selection prevents the frequency of such genetic mistakes from ever passing much over the level of statistical significance (they get pared down to that level).

    That doesn't mean such mistakes can't exist at a (very) low frequency.
  12. Feb 6, 2010 #11
    Hey, thanks for stepping up to the plate. I think I'll have more questions but I have to let all of your answers percolate for a while.
  13. Feb 6, 2010 #12

    Doug Huffman

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    Consider the vast number of ova that are not viable beyond critical points. Consider the vast number of sperm and that some fraction are just effective (contra defective) enough to quicken an ova. I recall a number from 40 years ago, that 600 instances of coitus are required on average for a single pregnancy, not birth.
  14. Mar 23, 2010 #13
    Wonderful answers from everyone. Thanks a lot Monique for hanging in there with me past my naivete! I'll be processing all of these thoughts for a long time.
  15. Jun 14, 2011 #14
    I know this is an old thread but I had the same question today and Google transported me here.

    In Monique's 2nd to the last post above, she mentioned that the cause is unknown, and in her next post she said it does not seem to be a result of "genetic cues". If this implies it's not heritable, then it obviates my thoughts. Because if there is no genetic cause during the embryonic development, then there will be no heritability of the trait, and it's entirely driven in the embryonic processes. I've certainly heard "twins run in our family" during my life, but that doesn't mean those people were correct.

    However, if a predilection IS it is a heritable trait, then the reason it would confer an evolutionary advantage is statistical: If one birth produces two offspring that have evolutionarily successful traits in the current evolutionary environment, then you've doubled your chances of that trait being passed on. Again, this is contingent upon "twinning" being a heritable trait. But if so, then this is just natures way of "doubling down".

    The flip side of this being an evolutionary advantage, is that it's much harder on the mother to carry twins and deliver them. At least, it seems to me, prior to the aid of 20th century medicine, much more difficult. However, there might be a flip side...to this flip side. :P

    Our species produces giant-headed babies with soft skulls that can't even walk for 18 month. We are born prematurely, because our brains give us an overall advantage as a species. BUT, unlike other animals that can walk hours after birth, we take 18 months. Our skulls still leave vulnerable our most important organ because a solid skull would kill the mother.

    Given that we're a social species that could defend our otherwise non-viable young until they can hold that spear or torch -- this works for us. So this "flip side to the flip side" is that if in any way twins might tend to be born a little (not a lot) prematurely, their heads would be smaller and less of a risk to the mother. I'd even be a little inclined to see if the skulls of twins are more "complete" than single births.

    If twins, at least in the evolutionary environment, were born a little prematurely, this would reduce the risks of the mother dying during birth, though she'd still have the attendant risks of providing material sustenance for two embryos.

    Again, even if one of the twins dies as a result of some premature-birth complication, because nature "doubled down", the trait would still be passed on, if it's a genetic pre-disposition to have twins.
  16. Jun 14, 2011 #15


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    You need to do some research and any claims need to be backed by linking to an approved source. Most toddlers can walk by their first birthday, some much earlier. Of course some children can take longer.


    Perhaps this article will answer some questions you have about twins.

    Last edited: Jun 14, 2011
  17. Jun 15, 2011 #16
    Not only does the mother have to carry the baby but also raise her offspring after giving birth to them. The mother, and in many cases, together with the father, has to spend a lot of time and effort in bringing up children. That is why birds and mammals usually have a limit as to how many offspring they can have at a time (differs between species).

    Incorrect usage of the word "premature birth".

    Define "complete skull". (I don't know what you mean by that. Unless we are talking about fossils.)
  18. Jun 15, 2011 #17


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    He's mistaking the soft spot on a newborn's skull with it actually missing. The part he posts that twins should be born more prematurely than usual (~60% of twins are premature) so that their heads are smaller, therefor less injurious to the mother makes absolutely no sense unless he thinks that both babies try to come out simultaneously.
  19. Jun 15, 2011 #18
    Not only twins, he also implies that humans in general are born prematurely.

    The soft spot is not at as vulnerable as it seems. There is a layer of tough tissue that does its job of protecting the brain.
  20. Jun 15, 2011 #19

    No, you're missing the point. If a child is born prematurely, with a smaller skull, then any attendant risks regarding the mother dying from complications that might result due to the big-head/small-birth canal issues are lessened. Has nothing to do with twins being born simultaneous.
  21. Jun 15, 2011 #20
    No, it's a correct use of the word "premature birth". I wasn't being literal as in "when do humans usually get born". I was comparing us to other species. I didn't make up that phrase on my own, it comes from a Dawkins, Matt Ridley, Robert Wright or some other evolutionary biology author.

    It means exactly what I intended it to mean. Compared to other species, (and when you graph gestation time versus life span) we are way out of line with the rest of the animal kingdom. Why, because of our brains.

    A little Google work using the word "premature" will show you many people refer to the process exactly so. Here's one:

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