According to the selfish gene theory, it seems like because identical twins sometimes get produced, a mutation to a gene that says, "if you have an identical twin, be fully altruistic towards them" would get selected for when ever it happens to arise in an identical twin. After humans evolve to be fully altruistic towards identical twins, wouldn't a mutation to a gene that makes a woman more likely to produce identical twins also be selected for because if it happens to appear in a a child who has a non-identical sibling, that gene might be discontinued from reproduction by the partial selfishness of the sibling but if it appears in an identical twin, the other identical twin will preserve that gene with their altruism.
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$\begingroup$ Can we even assume that it's possible for a gene to say "be altruistic to your twin"? Certainly we don't see any sort of exclusive altruism between siblings. $\endgroup$– jamesqfFeb 28, 2017 at 20:02
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$\begingroup$ One issue is twin baby's are difficult to carry to full term. So it's not a good idea for a twin birth to be preferential over a single birth $\endgroup$– user22339Aug 5, 2017 at 3:02
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3 Answers
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What you describe could have happened under the right conditions. However, there are a few things you haven't considered.
- Because humans are especially altricial, always having twins would double the cost of children on parents.
- The benefit of sexual reproduction is immune diversity. So a population like this could be far more vulnerable to disease.
So as far as this thought experiment goes, genes can't be too selfish or they dramatically reduce their own fitness. It's something worth simulating computationally to see what happens.
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$\begingroup$ Although I didn't want to making the question very long to explain it, what I meant was that should be an evolutionary advantage for a mother to reproduce 2 identical twins only once than to get pregnant and reproduce one kid then get pregnant again and reproduce another kid when she only has enough food to feed 2 kids. I'm assuming the tendency for a mother to choose not to reproduce once she already has 2 kids means having identical twins won't cause her to reproduce more than 2 children. $\endgroup$– TimothyAug 20, 2015 at 21:25
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$\begingroup$ 2 children require twice the resources of one, during gestation those resources come directly from the mother, twins are more likely to have complications because of this higher demand on the mother's body. Twins are often underweight or premature and have higher occurrence of birth defects. ncbi.nlm.nih.gov/pubmed/11321243 $\endgroup$– JohnAug 5, 2017 at 3:57
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$\begingroup$ Maybe it was an evolutionary disadvantage to produce two kids at the same time because when two kids get produced at the same time, by the time they're nearly finished getting reared, two children both have a high energy demand at the same time and the parents can't keep up with providing both of them such a high energy demand. Maybe the parents also were unable to simultaneously defend two babies against predators. $\endgroup$– TimothyApr 8, 2019 at 23:13
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From jzx's answer, I thought of a possible answer to my own question. Maybe not producing identical twins all the time is a paradoxical evolutionary stable strategy for the following reason:
Until recently in evolutionary history, the population remained constant when parents only had enough food to feed 2 children. Siblings didn't evolve to be fully altruistic towards each other so if a parent had 3 children in case of the small chance of having enough food to feed 3, none of them would willing let themself starve to make sure the others survive, risking making only one survive, so the parents would have had an evolutionary advantage in only producing 2 children in the first place. Although definitely producing identical twins is an evolutionary advantage, a mutation that makes the chance of producing identical twins slightly higher than it already is is an evolutionary disadvantage because a woman with an increased chance of producing identical twins might have produced 1 child on her first pregnancy and identical twins on her second pregnancy, giving a total of 3 children and increasing the risk of only rearing 1.
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If we produced identical twins all the time then we'd completely lose genetic diversity which would, I assume, overwhelmingly compromise our adaptive capabilities. Don't forget, those identical twins then need to reproduce with other identical twins of others families. Moreover, and this is the more obvious explanation, as a consequence deleterious recessive disorders would rapidly increase in rate.
For more, see https://en.wikipedia.org/wiki/Inbreeding_depression http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0109585
You might also like to study frequency dependent selection for why it is sometimes advantageous to have identical twins, albeit at a low rate.
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1$\begingroup$ I think that you may be confusing identical twinning with reproductively closed populations. Also, with the mix and match of chromosomes and crossover events during Meiosis, there will still be a good deal of diversity, even in populations that are relatively isolated. Even if two identical twin males mated with two identical twin females and produced the same number of offspring, it is highly unlikely that any of the offspring of different births would have an identical genetic make up. $\endgroup$– AMRAug 20, 2015 at 20:03
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1$\begingroup$ I don't see a reason we'd lose genetic diversity because the identical twin of the spouse of somebody wouldn't be the same person as the spouse of their identical twin. $\endgroup$– TimothyAug 20, 2015 at 21:27
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$\begingroup$ But by definition you restrict the diversity by at least a factor of 2. i.e. imagine you had two non-identical twins v two identical twins. $\endgroup$ Aug 20, 2015 at 22:01