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Why do women menstruate?

Menstruation is a peculiar phenomenon that women go through on a roughly monthly cycle, and it’s not immediately obvious from an evolutionary standpoint why they do it. It’s wasteful — they are throwing away a substantial amount of blood and tissue. It seems hazardous; ancestrally, in a world full of predators and disease, leaving a blood trail or filling a delicate orifice with dying tissue seems like a bad idea. And as many women can tell you, it’s uncomfortable, awkward, and sometimes debilitating. So why, evolution, why?

One assumption some people might make is that that is just the way mammalian reproduction works. This isn’t true! Most mammals do not menstruate — they do not cycle their uterine linings, but instead only build up a thickened endometrium if fertilization occurs, which looks much more efficient. Of the mammals, only most primates, a few bats, and elephant shrews are among the lucky animals that menstruate, and as you can see from the phylogeny, the scattered diversity of menstruating mammals implies that the trait was not present ancestrally — we primates acquired it relatively late.


Phylogeny showing the distribution of menstruation in placental mammals and the inferred states of ancestral lineages. Menstruating species/lineages are colored in pink, non- menstruating species/lineages in black. Species in which the character state is not known are not colored, and lineages of equivocal state are represented with black lines. Monodelphis represents the outgroup. Inference of ancestral states was performed in MacClade 4 by the parsimony method. Note that there is strong evidence for three independent originations of menstruation among placental mammals.

I suppose we could blame The Curse on The Fall, but then this phylogeny would suggest that Adam and Eve were part of a population of squirrel-like proto-primates living in the early Paleocene. That’s rather unbiblical, though, and what did the bats and elephant shrews do to deserve this?

There are many explanations floating around. One is that it’s a way to flush out nasty pathogens injected into the reproductive tract by ejaculating males — but that phenomenon is ubiquitous, so you have to wonder why only a few species bother. Another explanation is that it’s more efficient to get rid of the endometrium when not using it, than to maintain it indefinitely; but this is a false distinction, because other mammals don’t maintain the endometrium, they just build it up in response to fertilization. And finally, another reason is that humans have rather agressive embryos that implant deeply and intimately with the mother’s tissues, and menstruation “preconditions” the uterine lining to cope with the stress. There is, unfortunately, no evidence that menstruation provides any boost to the ‘toughness’ of the uterus at all.

A new paper by Emera, Romero, and Wagner suggests an interesting new idea. They turn the question around: menstruation isn’t the phenomenon to be explained, decidualization, the production of a thickened endometrial lining, is the key process.

All mammals prepare a specialized membrane for embryo implantation, the difference is that most mammals exhibit triggered decidualization, where the fertilized embryo itself instigates the thickening, while most primates have spontaneous decidualization (SD), which occurs even in the absence of a fertilized embryo. You can, for instance, induce menstruation in mice. By scratching the mouse endometrium, they will go through a pseudopregnancy and build up a thickened endometrial lining that will be shed when progesterone levels drop. So the reason mice don’t menstruate isn’t that they lack a mechanism for shedding the endometrial lining…it’s that they don’t build it up in the first place unless they’re actually going to use it.

So the question is, why do humans have spontaneous decidualization?

The answer that Emera suggests is entirely evolutionary, and involves maternal-fetal conflict. The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment. The fetus, for instance, manipulates the mother’s hormones to weaken the insulin response, so less sugar is taken up by mom’s cells, making more available for the fetus.

Within the mammals, there is variation in how deeply the fetus sinks its placental teeth into the uterus. Some species are epithelochorial; the connection is entirely superficial. Others are endotheliochorial, in which the placenta pierces the uterine epithelium. And others, the most invasive, are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.

That’s a hint. Menstruation is a consequence of self-defense. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta. In species with especially invasive embryos, it’s too late to wait for the moment of implantation — instead, they build up the wall pre-emptively, before and in case of fertilization. Then, if fertilization doesn’t occur, the universal process of responding to declining progesterone levels by sloughing off the lining occurs.

Bonus! Another process that goes on is that the lining of the uterus is also a sensor for fetal quality, detecting chromosomal abnormalities and allowing them to be spontaneously aborted early. There is some evidence for this: women vary in their degree of decidualization, and women with reduced decidualization have been found to become pregnant more often, but also exhibit pregnancy failure more often. So having a prepared uterus not only helps to fend off overly-aggressive fetuses, it allows mom a greater ability to be selective in which fetuses she carries to term.

The authors also have a proposed mechanism for how menstruation could have evolved, and it involves genetic assimilation. Genetic assimilation is a process which begins with an environmentally induced phenotype (in this case, decidualization in response to implantation), which is then strengthened by genetic mutations that stabilize the phenotype — phenotype first, followed by selection for the mutations that reinforce the phenotype. They make predictions from this hypothesis. In species that don’t undergo SD, embryo implantation triggers an elevation of cyclic AMP in the endometrium that causes growth of the lining. If genetic assimilation occurred, they predict that what happened in species with SD was the novel coupling of hormonal signaling to the extant activation process.

If either of these models were correct, we would expect an upregulation of cAMP- stimulating agents in response to pro- gesterone in menstruating species like humans, but not in non-menstruating species such as the mouse.

Results from experiments like those described above will elucidate the evolutionary pathway from induced to spontaneous decidualization, allowing us to answer long-unanswered questions about the evolutionary significance of menstruation. In addition, they will provide mechanistic insights that might be useful in the treatment of common reproductive disorders such as endometriosis, endometrial cancer, preeclampsia, and recurrent pregnancy loss. These disorders involve dysfunctional endometrial responses during the menstrual cycle and pregnancy. Thus, clarifying mechanisms of the normal endometrial response to maternal hormones, i.e. SD, will facilitate identification of genes with abnormal function in women with these disorders. An analysis of how SD came about in evolution can aid in identifying these critical molecular mechanisms.

Evolution, genetic assimilation, a prediction from an evolutionary hypothesis, and significant biomedical applications … that all sounds powerful to me.


Emera D, Romero R, Wagner G (2011) The evolution of menstruation: A new model for genetic assimilation: Explaining molecular origins of maternal responses to fetal invasiveness. Bioessays 34(1):26-35.

(Also on Sb)

P.S. The maternal-fetal conflict is also a conflict between males and females: it is in the man’s reproductive interests to have his genes propagated in any one pregnancy, while it is in the woman’s reproductive interests to bail out and try again if conditions aren’t optimal for any one pregnancy. This conflict is also played out in culture, as well as genetics — pro-choice is a pro-woman strategy, anti-abortion is a pro-man position. Sometimes, politics is a reflection of an evolutionary struggle, too.

Comments

  1. Dhorvath, OM says

    So the embryo is being ‘guarded’ against by a thick endometrial lining? Human pregnancy is so hard that it costs even when one isn’t pregnant. Savage parasites, aren’t they?

  2. says

    Overall, I’ll take the suboptimal compromise of external gonads over the suboptimal compromise of menstruation. Thanks for a fascinating read!

  3. The Lorax says

    The science is strong with this one.

    If you keep this up, this may become my second-favorite science/freethought blog after the Bad Astronomer.

  4. browneyedgirl65 says

    I don’t know about all that.

    I do know dogs. They have an estrus cycle, meaning that twice a year, their bodies prepare for a possible pregnancy. What’s interesting about this is that their bodies go through the entire hormonal profile of a pregnancy for the next nine weeks after ovulating, whether or not fertilization occurs. In fact, it is only relatively recently we even have a pregnancy test for dogs (one for women was available for decades before, since we only go through the hormonal profile associated with pregnancy once fertilization occurs).

    An estrus cycle means the dog is only fertile twice a year (or whatever the estrus cycle is — obviously this can differ by bitch and also by species). Eg, foxes, wolves tend to do so once a year, and seasonally so as to have good weather for the young.

    A menstrual cycle, on the other hand, pretty much makes a woman fertile most of the time as compared to the estrus cycle. Now maybe that’s what makes the mother/fetus relationship more “adversarial” but when I read that part above, all I could think of was — well fuck, what about the bitch that’s carrying 12 puppies?!

    But perhaps because she can guarantee this happens at most twice a year it’s not as big a deal? Scaling up for humans — considering nine weeks to nine months — that would be like a woman’s cycle guaranteeing she wouldn’t get pregnant more than once every four years. On the other hand a cow also has a nine month pregnancy and estrus once a year so… ?

    Another note: women reabsorb about 2/3 of the lining each month anyway, so they’re not throwing out quite that much as might be thought.

    Quite a bit of food for thought, there. What were the evolutionary advantages to constant reproduction? Faster population spread, for one thing. Um, constant sex instead of sex just when the estrus cycle begins.

    But the estrus method seems far more pervasive among mammals. More research, I guess.

  5. carlie says

    When I first saw the post title in my blog reader I thought it said “Why do women masturbate?” and that this was going to be a whole different discussion.

  6. capnxtreme says

    Everything you wanted to know about mammalian menstrual cycles, but were afraid to ask. All right, maybe not everything, but truly fascinating stuff all the same.

    I once made the mistake of referring to a hypothetical developing fetus as a “sort of parasite” during an abortion debate and was verbally berated for it by both sides. Imagine my surprise when I read this article and discovered how close I was to the truth! They really are tenacious little bastards.

  7. alysonmiers says

    Bonus! Another process that goes on is that the lining of the uterus is also a sensor for fetal quality, detecting chromosomal abnormalities and allowing them to be spontaneously aborted early. There is some evidence for this: women vary in their degree of decidualization, and women with reduced decidualization have been found to become pregnant more often, but also exhibit pregnancy failure more often. So having a prepared uterus not only helps to fend off overly-aggressive fetuses, it allows mom a greater ability to be selective in which fetuses she carries to term.

    Ooooh! Will you later show us the info about the mechanisms for sensing fetal quality?

  8. Psych-Oh says

    That is a very different way of thinking about menstruation- I really want to read more. I can’t wait for this to be incorporated into Sex Ed.

  9. rowanvt says

    @#6, TMI warning:

    If I’m absorbing 2/3 of what’s there, I don’t think I’ll be able to have anything other than an ectopic pregnancy for sheer lack of room. When I was a teen, it was 8 days of cramping hell and having to change the pad every 2 hours from day 1 to day 8. It’s shorter now that I’m nearing 30, but I still bleed a hell of a lot more than average.

    I’ll take this over the canine method though where the constant false pregnancies (and even real ones) cause the uterine lining to develop a cobblestone texture with deep fissures that are incredible havens for bacteria. And that in turn leads to a lovely condition called a pyometra and the more heat cycles they go through, the greater the risk. Nothing quite like watching the surgery to remove a uterus with 3 pounds of pus in it.

  10. says

    Science, schimence. The correct explanation of the “phylogenetic” tree, is that since primates are mostly frugivores we were the only species to eat the apple and get kicked out of the garden. Also Glossophaga bats must have pollinated the tree of knowledge and once the apples fell to the forest floor, Elephant shrews must have scavenged the fruit.

  11. raven says

    . The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment.

    Intelligent design is rather laughable here.

    I’ve long thought that males and females also have a partly adversarial reproductive relationship. Their interests in maximizing surviving children don’t perfectly coincide.

  12. leahr says

    This conflict is also played out in culture, as well as genetics — pro-choice is a pro-woman strategy, anti-abortion is a pro-man position. Sometimes, politics is a reflection of an evolutionary struggle, too.

    Mind = blown.

    This could be the start of revolutionizing the way we approach the abortion debate.

  13. F says

    Neat. I’ve never seen anything like an explanation as to why only a few mammalian species menstruate. (I didn’t know there was a shrew that menstruated, either.)

  14. nataliebaldwin says

    Wait…wait… hold up. Presumably, a mother surviving to have several subsequent pregnancies is better for the species as a whole, so why would an aggressive embryo/fetus be selected for in the first place? I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species. ????

  15. Brother Ogvorbis, OM . . . Really? says

    Fascinating. Just, wow.

    This could be the start of revolutionizing the way we approach the abortion debate.

    Except that there is no real debate. On one side is a group which alternates what-if horror stories (lies) and cherry-picked idiocy from their book of myths, and on the other side a group which has real-life, biology, evolution, and honesty. Not really a debate.

  16. SallyStrange, Spawn of Cthulhu says

    why would an aggressive embryo/fetus be selected for in the first place?

    Because it is more likely to implant and complete gestation in the first place. Even if the pregnancy ends badly for the mother, thereby increasing the chances that the infant won’t survive, embryos that fail to implant at all are still less likely to survive and reproduce than a born infant whose mother is injured or dead.

  17. magistramarla says

    Browneyedgirl,
    Here is something interesting that our culture doesn’t usually take into account. If the mother TOTALLY breastfeeds for six to eight months (no solids, no supplements, no pacifier), the menstrual cycle is suppressed, thus keeping the mother from conceiving again while the baby most needs her resources. If the Mom is lucky, this suppression might last well over a year. Unfortunately, our culture has ruined this with artificial feeding. It certainly worked for me – I was pregnant, nursing or both for thirteen years. I had five or six menstrual periods in that time, and my five babies were spaced about two years apart.
    On the down side, my body still has an abundance of eggs, and at the age of 54, I still haven’t finished with menopause. It’s a pain, but then again, I’m told that still maintaining a high estrogen level is better for my heart, skin and overall health.
    Also, breastfeeding has been linked with preventing breast cancer in the mother – I call it “use it, or lose it”.
    So, it seems to me that the little parasite does some favors for its host.

  18. SallyStrange, Spawn of Cthulhu says

    Reading about menstruation reminds me that it’s been a year and a half since I last menstruated and I feel great. Thanks small dose hormonal birth control!

  19. Luc says

    I’m not seeing why fucking whatever hole is beneficial for the man. It’s not only about getting a lot of pregnancies, it’s about making healthy children with good chances of survival so they are part of the next generation of reproductors. It’s not good for your genes if you impregnate a pre-teenager who won’t care for the child and then just walk away. Chances are you’re wasting time you could be spending doing something more productive, like eating or trying to have children with someone who will ensure the kids grow fat and robust.

  20. bromion says

    I’m wondering why you support this kind of evolutionary explanation of a phenomenon that is difficult to prove directly (we can’t observe an evolutionary fossil record here), but dismiss the entire field of evolutionary psychology as “just so” stories. While there’s a lot of well-publicized bad EP, it seems that this hypothesis is just as “just so” as EP hypotheses. Can you explain the distinction?

  21. ringo says

    So here’s PZ being anti-trans again, ignoring the menstruating FTMs. :p Just kidding!
    I am currently a menstruating man, but I don’t expect any real recognition for it, most FTMs don’t menstruate normally anyway.

  22. littlejohn says

    I, too, read it as “Why do women masturbate.” I was about to give an answer making reference to my lousy technique in bed. That would have been an embarrassing admission. Good thing I didn’t have to do it. Wait a minute….

  23. Nerd of Redhead, Dances OM Trolls says

    Can you explain the distinction?

    Can you explain the distinction between a physical process and a psychological process, on a “plastic” brain? Then you will have answered the question.

  24. Azkyroth says

    Wait…wait… hold up. Presumably, a mother surviving to have several subsequent pregnancies is better for the species as a whole, so why would an aggressive embryo/fetus be selected for in the first place? I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species. ????

    *facepalm*

    Evolution does not work that way.

  25. Azkyroth says

    I’m wondering why you support this kind of evolutionary explanation of a phenomenon that is difficult to prove directly (we can’t observe an evolutionary fossil record here), but dismiss the entire field of evolutionary psychology as “just so” stories. While there’s a lot of well-publicized bad EP, it seems that this hypothesis is just as “just so” as EP hypotheses. Can you explain the distinction?

    Did you miss the part where they produced a testable hypothesis?

  26. bromion says

    Can you explain the distinction between a physical process and a psychological process, on a “plastic” brain? Then you will have answered the question.

    No, I cannot, because the brain has evolved just as other soft tissues have, and psychology is a function of the brain interacting with the environment, just as other biological processes are. Unless we discover some deep, new science that allows for mind/brain dualism, this seems clear.

    Did you miss the part where they produced a testable hypothesis?

    Indeed, and good EP researchers also produce testable hypotheses. The typical complaint isn’t the lack of a testable hypothesis, but the source of said hypothesis — namely, that the mechanism evolved and here’s why. This write-up implies the same thing. We have observations of a phenomenon, proposed mechanisms, confirming studies, and an evolutionary framework in which it’s cast.

  27. raven says

    I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species. ????

    Evolution is blind and nonthinking. It doesn’t plan for the long term.

    Selection is mostly at the level of the individual.

    The long term advantage of tool using intelligence has yet to be demonstrated. But evolution will neither know or care.

  28. Brother Ogvorbis, OM . . . Really? says

    I’m wondering why you support this kind of evolutionary explanation of a phenomenon that is difficult to prove directly (we can’t observe an evolutionary fossil record here),

    I’m just an historian, so I’m sure someone will be able to answer your leading question much better than I, but evolutionary biology is not just about the fossil record. Using cladistics, we can see that the extant primate family, from most primitive to most derived, shares a particular trait: in this case, hemochorial feti. By observing extant species (y’know, the living ones), it is not that difficult for a biologist (well, maybe it is difficult, but I gaurantee it is far easier for a biologist than a public historian) to hypotheosize about why based on the what. And there is a big difference between some of the suspect evolutionary psychology hypotheoses out there and biological evolutionary based hypotheoses — the biologist can actually observe animals in which the trait manifests, and animals in which it does not, and observe such things as behaviour, mating and reproduction. Please note that the authors of this paper did not say ‘primates menstruate, therefore x,’ but rather ‘here is why primates menstruate, and here is how x may be explained using this information.’

  29. says

    I’m wondering why you support this kind of evolutionary explanation of a phenomenon that is difficult to prove directly (we can’t observe an evolutionary fossil record here), but dismiss the entire field of evolutionary psychology as “just so” stories.

    This hypothesis has the advantage of being based on actual comparative data and the physiology of decidualization. It has an explanation based on the cell and molecular mechanism, not just derived from a phenotype. And it makes a specific, testable prediction about how the shift could have been made.

    It isn’t about looking up the fossil record (fossils suck for evaluating most evolutionary hypotheses), but about using the molecular evidence to evaluate the answer. We have molecular evidence. We can also get more much more readily than we can dig up a fossil uterus.

  30. raven says

    I’m wondering why you support this kind of evolutionary explanation of a phenomenon that is difficult to prove directly (we can’t observe an evolutionary fossil record here),…

    Well, true, we don’t have any fossilized squirrel sized tampons from the late Cretaceous.

    But there was a lot of phylogentic data and biology data presented in the OP. Try reading it again slowly.

  31. noastronomer says

    @luc #22

    But for all mammal species the actual energy investment in reproduction is so small that ‘fucking whatever hole’ *is* a viable strategy.

    The female’s energy commitment is so large however that she is essentially committed to raising the child. Even Mrs Duggar has only managed to produce 19 children in her lifetime. A male could easily father ten times that many, or more, in a single year.

    Mike.

  32. Happiestsadist says

    SallyStrange, I am so jealous. TMI: With the BC pills I’m taking, they’re reduced me to only bleeding three out of four weeks a month (and no, those off days are non-consecutive, and sex of any kind can take those out too.) Pity what the pills did to my boobs, though, the added gender dysphoria isn’t my favourite side effect.

    On the upside, I finally got a date pinned down for surgery to fix all that pelvic malfunction like literally minutes ago.

    My sympathies to the other primates, bats and shrews.

  33. bromion says

    I’m just an historian, so I’m sure someone will be able to answer your leading question much better than I, but evolutionary biology is not just about the fossil record. Using cladistics, we can see that the extant primate family, from most primitive to most derived, shares a particular trait

    This seems a reasonable distinction — that in certain cases, we can compare other species which have branched off the ancestral tree at different times, giving an observable indication of how a trait has evolved. When humans are the only observable for a trait, that makes the proposition more dicey.

    That said, there is a lot of discussion in the write-up about why a particular mechanism evolved — maternal-fetal conflict — that sounds much like an EP explanation for why a particular psychological trait may be prevalent in the population. The only difference in this part of the theory is the complexity of the expression (menstruation happens or doesn’t, whereas behavior is more complex).

  34. Azkyroth says

    This hypothesis has the advantage of being based on actual comparative data and the physiology of decidualization.

    As opposed to an “everyone knows” generalization or a bar joke, say…

  35. pj says

    @browneyedgirl65

    Adversalial mother-fetus relationship is a fact in every mammal species. It is simply a matter of non-identical interests between the individuals in question (thus being part of the general parent-offspring conflict). What you probably missed was that the measure of the degree of adversiality was the invasiveness of the implantation of the fetus into the endometrium. This part:

    And others, the most invasive, are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.

    I.e. it’s not the biomass you got to carry to term that is relevant to spontaneous decidualization. Relevant is the *damage* the embryo(s) do to the endometrium.

    @nataliebaldwin

    Natural selection does *not* work for the benefit of the species. That misconception is still quite common even if it was shown to be false as early as in the 1960s.

  36. Brother Ogvorbis, OM . . . Really? says

    When humans are the only observable for a trait, that makes the proposition more dicey.

    Did you even read the original post? Just at a glance, we have Pan paniscus, Pongo pygmaeus, Hylobates concolor etc. in the phylogenetic chart. And, save for the least-derived primates (lemurians), all primates have the same trait. And we know of four other mammals, spread out across the phylogenetic tree, which have the same trait. And the same homocharial feti which really do compete, inside the female’s body, for the same nutrients, fats, proteins, whatever. The

    maternal-fetal conflict

    that you point to as being a psychological adaptation is a literal description of what can be observed within multiple species, all of whom menstruate.

  37. Brother Ogvorbis, OM . . . Really? says

    Damn. I submitted too soon.

    And “maternal-fetal conflict” is not a psychological term. It describes something that is physical and observable. Just because the term is used in psychology does not mean that the term has the same meaning in a different science.

  38. says

    @Ringo:

    And clearly he’s marginalizing all us non-menstruating trans women, too! PZ IS CLEARLY A BIO-ESSENTIALIST CISSEXIST AGENT OF THE BINARY. ;)

    All things considered, the lifelong infertility sucks, but the lack of menstruation? Not such a bad bargain.

  39. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says

    Count me the third who read the title as referencing masturbation.

    Of course, I’m completely over the moon about Ms. Crush right now, and we can’t get our hands on each other until the first weekend of January (16 days, not that I’m counting), so that might have something to do with where my brain went….

    Okay, time to read the OP and actually respond to the content

  40. bromion says

    Did you even read the original post? Just at a glance, we have Pan paniscus, Pongo pygmaeus, Hylobates concolor etc. in the phylogenetic chart. And, save for the least-derived primates (lemurians), all primates have the same trait.

    You misunderstood my meaning. I agree that THIS hypothesis has multi-species support, as you cite, but EP hypotheses ONLY have observations from one species — humans.

    As for the maternal-fetal conflict, I am not citing that as a psychological adaptation! I am pointing to that as an evolutionary-inspired explanation for an observed phenomenon.

  41. Azkyroth says

    As for the maternal-fetal conflict, I am not citing that as a psychological adaptation! I am pointing to that as an evolutionary-inspired explanation for an observed phenomenon.

    But it’s really not even that.

  42. Brother Ogvorbis, OM . . . Really? says

    Does anyone know a good reading tutor? Apparently I need one. Bad.

  43. KG says

    Scaling up for humans — considering nine weeks to nine months — that would be like a woman’s cycle guaranteeing she wouldn’t get pregnant more than once every four years. – browneyedgirl65

    IIRC, that’s about the inter-birth interval in gatherer-hunter societies: maintained partly by breastfeeding as magistramarla explained, partly by borderline malnutrition, partly by norms forbidding sex with a recent mother.

  44. says

    @pj…

    Natural selection doesn’t work for the good of the species?

    Well I know that in the sense that natural selection doesn’t really care about the “good” of anything, and isn’t actually “working for” anything either, but doesn’t that process generally favour traits that lead to the perpetuation of the species, over the immediate benefit to individuals? Like… you know, the numerous species that die after mating. Or the absolutely HORRIBLE life of the male angler fish. Life finds a way, but life doesn’t give a damn about its individual members.

    Sometimes a species or trait screws up, and is advantageous only for a short-term, at great cost to the long-term. Like a virus, bacteria or parasite that harms its host so badly the hosts eventually die off, and the virus / bacteria / parasite dies off too, if it is unable to adapt quickly enough to find a new host organism.

    But still, due to the fact that such species GO EXTINCT is exactly one of the ways that over the arc of deep history, such strategies aren’t favoured.

  45. bromion says

    IE — the explanation “the maternal-fetal conflict is a situational fact that influences the development of the menstruation mechanism in some species” (per the write-up, paraphrased) sounds a lot like EP explanations for psychological phenomenon (not implying that the maternal-fetal conflict is at all psychological in nature).

  46. Nerd of Redhead, Dances OM Trolls says

    No, I cannot, because the brain has evolved just as other soft tissues have, and psychology is a function of the brain interacting with the environment, just as other biological processes are.

    Well, there’s your problem. Psychology is not dependent totally on genes, but also has a social component built in. Anybody in the field would know that. The social environment creates a large, possibly insurmountable amount of noise for the genetic psychology signal. Which is the problem for evolutionary biology, and why it is in such a bad state.

  47. Beatrice, anormalement indécente says

    Count me the third who read the title as referencing masturbation.

    And me as fourth.

  48. Brother Ogvorbis, OM . . . Really? says

    the maternal-fetal conflict is a situational fact that influences the development of the menstruation mechanism in some species

    An evolutionary influence is one that helps an individual have more offspring. That’s it. If, because the fetus is able to use the female’s blood supply directly, this puts extra stress on the pregnant female, then an adaptation (in this case, menstruation) which minimizes or mitigates the stress would, presumably, allow a female to have more offspring thus passing the trait on to her female progeny. This is not like a psychological argument as comparisons can be made between species with the same trait, between species with other types of maternal-fetal conflict, and even between individuals within the same species. I still fail to see how an observable phenomena, observable in multiple species (even multiple families!), with observable and testable results, is “a lot like EP explanations for psychological phenomenon”. It is not. Psychology is useful, but trying to use evolutionary theory to show why we think the way we do has too much background noise to make it consistent (and no, I am not dumping on psychology, I am merely pointing out my useless point of view regarding an immature science).

  49. KG says

    doesn’t that process generally favour traits that lead to the perpetuation of the species, over the immediate benefit to individuals? Like… you know, the numerous species that die after mating. Or the absolutely HORRIBLE life of the male angler fish. – nataliebaldwin

    No, there’s no general reason to expect that traits favouring the long-term continuation of the species will be selected for: selection at the individual level (or at the level of genes – the two approaches can be formulated more or less equivalently) is so much faster than processes of species survival and extinction over evolutionary time. Organisms that die after spawning are those that reproduce once in their life: they put all their accumulated resources into one maximum effort – and presumably, that produces more descendants than any alternative that has arisen in their recent evolutionary past. I don’t suppose male angler fish mind their life, but even if they do, natural selection certainly doesn’t favour happiness over misery (or vice versa). Their reproductive strategy* is easy to understand: finding a potential mate is very chancy, so once you do, don’t let go!

    *Of course I don’t mean they plan this out – they just react automatically to the proximity of a female of the same species by grabbing on and holding tight.

  50. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says

    Brother Og –

    while I, too, find Bromion’s defense of EP falling flat, your critique in #39:

    Did you even read the original post? Just at a glance, we have Pan paniscus, Pongo pygmaeus, Hylobates concolor etc. in the phylogenetic chart. And, save for the least-derived primates (lemurians), all primates have the same trait.

    isn’t fair. Bromion was admitting that EP has the more formidable challenge of examining only a single extant species and thus lacks the information about phylogenetic pathways available to those studying menstruation in this study.

    Bromion is saying that, yes, this study gets multiple species to examine and EP only has one and that is a point in favor of PZ drawing a distinction. Bromion goes on to argue that this isn’t sufficient for the harsh treatment given EP, but on the specific point you made, you & Bromion are already in agreement.

  51. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says

    ooo, sorry for piling on, Og. I swear when I started to write those other posts weren’t visible to me yet!

    On another topic, I see a potential for a good discussion of jilling off in TET since so many of us apparently have it on our minds just now…

  52. says

    When I first started looking at birth control in my late teens I looked for types that would reduce or eliminate menstruation. I’m one of those unlucky women with nearly incapacitating periods heavy enough to make me anemic. Used depo provera for a while and now have an IUD (which I definitely recommended for any woman not looking to have kids for a while). I haven’t had a period in close to 10 years and since I don’t want children plan to keep this up until menopause takes care of it naturally.

    It’s another example of how evolution is comprised of a lot of trade-offs and that conflict drives much of it. Mother-fetus, predator-prey, species-climate, etc. it’s all very adversarial if you indulge in anthropomorphizing the process a little bit.

  53. Ms. Daisy Cutter says

    Nerd: “Psychology is not dependent totally on genes, but also has a social component built in. Anybody in the field would know that.”

    I suspect that the difficulty EPers have with grasping this relates to how heavily conservatives and libertarians are represented among their ranks.

  54. csmiller says

    nataliebaldwin: I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species. ????

    raven:
    Evolution is blind and nonthinking. It doesn’t plan for the long term.
    Selection is mostly at the level of the individual.

    I suspect that Natalie Baldwin was confused by the mantra that “individuals don’t evolve, the species does”. Which is true, in a way, but its the individuals that can produce the most children that affect the direction of evolution. For example, it would probably be better for lions if the males didn’t kill the ousted male’s cubs, especially as the males are replaced every 2-4 years. However, it is in the male’s interests not to raise the previous’s cubs, but to force the females to enter heat, and carry his.

    Apropos of this, in lions (or it might be tigers), the males try to make the fœtus as large as possible (the fœtus releases hormones to grow quickly), the females try to stop this (by releasing growth-suppression hormones).

  55. colubridae says

    The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment.

    It’s so easy to make this mistake. Mom has no interest in controlling her resources for the long haul. The frequency of mom’s genes will increase if (amongst other things) her uterus follows the described pattern.

    I’ve done it as well. Even “if her uterus follows the described pattern…” is false.

    It is extremely difficult to remove anthropological phraseology from evolution, without cumbersome circumlocutions.

    Dawkins describes the fetus/mother ‘battle’ brilliantly in The Extended Phenotype”.

  56. Brother Ogvorbis, OM . . . Really? says

    isn’t fair. Bromion was admitting that EP has the more formidable challenge of examining only a single extant species and thus lacks the information about phylogenetic pathways available to those studying menstruation in this study.

    And I already admitted that my reading comprehension had failed. Really.

  57. says

    @nataliebaldwin

    You’re confusing “natural selection as an evolutionary mechanism” with “selection for or against a trait in an individual”. Or, perhaps more accurately, you’re incorrectly assuming that they’re the same thing – they’re not. Selection on the level of the individual, as has been pointed out, is a direct result of some trait (or the change to some trait) conferring a reproductive advantage onto that individual – this would be “selection for or against a trait in an individual”. Over time, that reproductive advantage (or, again more precisely, probably the sum of several reproductive advantages) spreads to more and more members of the species (through the increased number of offspring to which the traits are passed) and the variation curve for the trait shifts accordingly (which is “natural selection as an evolutionary mechanism”, moving the characteristics of a species in one direction or another). The second naturally results from the first, but they are separable events, and can be analyzed individually.

  58. ChasCPeterson says

    EP hypotheses ONLY have observations from one species — humans.

    But of course that’s not true. Many hypotheses of evolutionary psychology–those dealing with social behavior, at least–are direct extrapolations from the well-known behavioral ecology of other species.
    Just as uteruses, bones, muscles, and brain-parts are homologous in humans and other mammals, the argument goes, so too can behavior be. It’s eminently logical for those not blinded by ideological denialism.
    But the term ‘Evolutionary Psychology’ has long been a magical talisman that summons forth the stupidest comments from pharyngulistas. Look, here’s one now:

    the difficulty EPers have with grasping this relates to how heavily conservatives and libertarians are represented among their rank

    [citation needed]

  59. Illuminata, Genie in the Beer Bottle says

    but dismiss the entire field of evolutionary psychology as “just so” stories. While there’s a lot of well-publicized bad EP, it seems that this hypothesis is just as “just so” as EP hypotheses. Can you explain the distinction

    Can someone please tell me where this mythical “good” EP is? Someone says this every fucking time EP comes up and not one of them ever provides this “good” EP research.

    On topic: I don’t care why I menstruate – I just wish I could stop it. It really fucking hurts and I get so fucking tired eye will not stay open – damn near comatose. No IUD for me though, since I don’t have kids, no doc will give me one.

  60. Illuminata, Genie in the Beer Bottle says

    Chas – are you suddenly new here? Or have you somehow never seen the glibertarian and plain old rightwing bigot influx that coincides with Ep discussions?

  61. says

    Okay… I think I’m following. Basically, the stuff that is advantageous to a species would be selected for over a longer stretch of time than traits that would be advantageous to particular individuals within a given species? And that’s why we might end up with situations where a species-favouring trait (survival of the mother) can be in conflict with an individual-favouring trait (survival of the embryo/fetus)?

    But still… it’s only through reproduction that the traits get passed on. So if all them babbies that are formed in the direction of being aggressive pass on those traits to their babbies, and those babbies end up killing the mommies in the process of babby-forming… wouldn’t that theoretically result in fewer offspring than the babbies that were initially less aggressive as embryos, and then pass on those less aggressive traits to their babbies, which don’t kill the mommies and the mommies get to go on to have more babbies?

    Or would this only really work out in a primarily monogamous, long-term pair-bonding species, which humans don’t necessarily behave as in a “natural” setting?

    I don’t know. I hope I’m not coming across like too much of an idiot. I’m just curious.

  62. Azkyroth says

    It’s eminently logical for those not blinded by ideological denialism.

    Ah, yes. People point out that you haven’t made your case for specific claims and that there are serious problems with some of the basic assumptions underlying those claims, and you respond by insisting they’re only denying your conclusions because they have a prior ideological commitment.

    Evo-phrenologists do it. Creationists do it. Most quacks do it. It’s like a pseudoscience smoking gun…

  63. Azkyroth says

    …now, what I want to know is, is this a conserved trait from the ancestral Kook lineage, or did it evolve independently in multiple sub-lineages?

  64. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says

    Illuminata:

    Can someone please tell me where this mythical “good” EP is? Someone says this every fucking time EP comes up and not one of them ever provides this “good” EP research.

    More to the point: if good EP exists, then how does so much bad EP get published in the “good” EP journals?

    If they are doing good science, then that means, at the least, that they have a way to ID good quality EP science. Otherwise studies that are well situated to give good data would not have been **designed** to give good data because one couldn’t know in advance what is needed in good EP.

    If they have a way to determine what is or isn’t good quality EP science, then why do they throw those methodological restraints overboard just when someone submits a totally batshit study…every other week…to a major EP journal?

    You can’t convince me that the science is sound if the science is unable to catch the batshit stuff and it’s left to scientists OUTside of EP to call out EP junk science.

    So go on, EP defenders, please explain why and how junk keeps getting through if there exist clear ways of IDing good EP science. And if such clear methodological constraints don’t exist, then how the heck do you know that there is any good EP science?

    Just because someone arrives at a plausible or even true answer doesn’t mean the science was good. I can flip a coin with 1 painted on one side and 2 painted on the other. Then I can go through a 3rd grade math text and flip the coin for every problem. Sometimes when the right response is 1 or 2 I’ll correctly answer the question. That doesn’t mean I was doing good math when I got the answer right and bad math when I got the answer wrong.

    So let’s have it: what is good EP science and why do the peer reviewers for major journals have such a lousy track record distinguishing the good EP from the bad?

  65. says

    @Illuminata: Go to Planned Parenthood or another clinic that specializes in family planning, they will do it. I’ve never had children and had no problem getting one put in.

  66. Pierce R. Butler says

    Of more daily consequence, humans seem to be the only primates without a significant estrus cycle (which is to say, reproductive-hormonal influences continue, but are routinely overridden by the individual and/or her circumstances).

    It seems to (the utterly amateur) me that the phenomena of menstruation, “hidden” estrus, atrophied pheromonal systems, and large brains (making childbirth much more difficult and dangerous than in other primates, especially when combined with bipedalism) must be evolutionarily interrelated.

    Some four decades back, pop-sci evolution books such as The Naked Ape dwelt regularly on such questions, with particular emphasis on the hymen (also apparently unique to humans). I suspect it was the embarrassing sexism and just-so-story qualities of such speculations that led authors to find other topics to play with, but a ripe field of inquiry apparently remains.

    So, a question and a suggestion:

    * Can anyone recommend up-to-date and well-grounded readings for non-scientists on human reproductive evolution?

    * An extremely good, and relevant, book – though with little to say about menstruation or estrus – is Sarah Blaffer Hrdy’s Mother Nature: A History of Mothers, Infants, and Natural Selection.

  67. jacquelinesexton says

    Fantastic post! My brain was overflowing with ideas concerning pro-choice vs pro-life even before reaching the postscript.

    Though patterns/themes in evolution may not help us make the most moral choices, it’s important to note the patterns which work–If a mother feels as though she is unable to care for a child at a specific time, it is probably in the best interest of both lives (of the genes) for a pregnancy to be avoided/terminated.

  68. raven says

    But still… it’s only through reproduction that the traits get passed on. So if all them babbies that are formed in the direction of being aggressive pass on those traits to their babbies, and those babbies end up killing the mommies in the process of babby-forming… wouldn’t that theoretically result in fewer offspring than the babbies that were initially less aggressive as embryos, and then pass on those less aggressive traits to their babbies, which don’t kill the mommies and the mommies get to go on to have more babbies?

    A lot of evolution is tradeoffs. Big brains are advantageous but the human brain is a huge energy drain, 20% of the glucose and hard to get through the pelvis.

    In the case of maternal-fetal conflicts, what would be selected for is maximum female survival with offspring and maximum aggressive burrowing in fetuses. What results is an equilibrium between the two. These equilibriums can shift one way or another depending on environmental and other changes.

  69. ringo says

    @nataliebaldwin

    Haha! You rock. :)

    I’m lucky enough to have my gonads on the inside where they don’t usually bother me (except when I miss a dose of testosterone, argh) so I’m keeping them. While I’d rather not host a fetus, I do want kids and this seems like the cheapest option. On days where I do miss the T though… I go visit my friend’s kids so I remember why I’m putting myself through this.

  70. says

    @nataliebaldwin

    Raven hit the nail on the head with the idea of trade-offs. My students really balk when I explain pregnancy as a parasitic relationship, but it’s (as PZ’s post says) an accurate picture. And, like all parasitic relationships, if a parasite experiences a mutation such that the next generation kills the host before the parasite is ready for that to happen (whenever that may be – parasite life cycles vary pretty impressively, and while some kill the host eventually, not all do), then that mutation is deleterious, and that individual trait doesn’t get passed into the population. The same is true for an overly aggressive fetus – if it kills the mother in utero, then it doesn’t get to survive, either (usually), and that hyper-aggression trait never makes it to the population for further transmission.

  71. says

    It was vindicating to read all the same questions I have been asking for years – how on earth did this evolve? What benefits does it bring? While it’s intriguing to have these suggested answers, it’s also downright creepy to consider, particularly as someone who’s interested in incubating a fetus in the next few years. The final notes about the mother vs. fetus biological conflict made me think about the terrible new Twilight movie that apparently depicts the fetus destroying its mother from the inside…ugh!

    My mind was also blown by the framing of the abortion debate in evolutionary/biological terms. Lots of good food for thought here.

  72. Amphiox says

    Basically, the stuff that is advantageous to a species would be selected for over a longer stretch of time than traits that would be advantageous to particular individuals within a given species?

    It is as yet unclear whether anything at all that is advantageous to a species but disadvantageous to the individual can be selected for at all, regardless of how much time there is.

    So far, the only traits that are advantageous to the species that we know for certain do get selected for are traits that are advantageous to individuals, and benefit the species only indirectly by benefiting the individual members of the species, and only over the short term of a single, or at the very most, a few, generations.

    This is the heart of the evolutionary controversy over group selection. And so far, the only cases where we know for sure that group selection works, are special exceptions wherein one can clearly see the group functioning as, and taking on the characteristics of, an individual. (Think of the social insects as one giant individual in multiple parts, with the individuals acting like the cells of a multicellular organism. The queen is the gonad/germ line/stem cell, and the workers are the specialized, differentiated soma cells. The colony is the organism/organ.)

    doesn’t that process generally favour traits that lead to the perpetuation of the species, over the immediate benefit to individuals? Like… you know, the numerous species that die after mating.

    What the process really favours are the traits that lead to the perpetuation of the GENES. It favors the individuals only indirectly in that individual survival favors the genes the individuals carry, and the species only indirectly in that the success of the species means success for the individuals that make it up. For the numerous species where individuals die after mating, the GENES benefit from being copied multiple times into a new generation. (And when speciation occurs, most of the genes between original and new species don’t change, so the genes don’t particularly care if one of those species goes extinct).

    Sometimes a species or trait screws up, and is advantageous only for a short-term, at great cost to the long-term.

    But it isn’t just sometimes. It’s virtually all the time. The situations wherein a population merrily selects itself straight to oblivion, are legion. Consider the infanticidal instincts of the male lion. Today with reduced populations and restricted ranges, every lion cub so killed is a major blow to the potential long-term survival of the species. But natural selection is still merrily favoring the males that kill cubs that are not their own, and will do so to the last lion.

    Like a virus, bacteria or parasite that harms its host so badly the hosts eventually die off, and the virus / bacteria / parasite dies off too, if it is unable to adapt quickly enough to find a new host organism.

    In epidemics we see this all the time. Disease organisms go through boom/bust cycles. Booming when the hosts are plentiful, busting themselves by killing most of their hosts. They persist because bust is almost never total, a tiny reservoir always remains, somewhere, for the parasite to persist. But this, in fact, is arguably the MORE USUAL pattern.

    But still, due to the fact that such species GO EXTINCT is exactly one of the ways that over the arc of deep history, such strategies aren’t favoured.

    Here we’re kind of touching on the idea of Clade Selection, which in itself remains controversial. But, if Clade Selection really does exist, just as in individuals where the target of selection is reproductive success and not long-term survival (ask any mayfly…), the target of selection would be speciation potential, and not species persistence, except inasmuch that longer persistence may give more opportunities to speciate. But species wherein selection for individual success tends to lead to earlier extinction of the population as a whole will continue to appear so long as those populations are able to speciate into new varieties before they go extinct. And there’s a whole smack of things that can conceivably affect speciation rates.

  73. says

    Sorry for the slight derail, but as I am new here (and consider myself a libertarian), can someone explain to me the link between evolutionary psychology and libertarianism? I’m not all that familiar with EP, but a cursory Google search turned up several results referencing “Social Darwinism”, a term that causes me to twitch with the desire to make angry posts about not applying natural selection principles to socialization behaviors.

  74. says

    But it isn’t just sometimes. It’s virtually all the time. The situations wherein a population merrily selects itself straight to oblivion, are legion. Consider the infanticidal instincts of the male lion. Today with reduced populations and restricted ranges, every lion cub so killed is a major blow to the potential long-term survival of the species. But natural selection is still merrily favoring the males that kill cubs that are not their own, and will do so to the last lion.

    You can also look at any of the evolutionary lines that are significantly diminished from the variations their ancestral variance. I like the example Gould uses of family Equidae, of which there is only one remaining extant genus, containing only a few remaining extant species. Especially if you compare such a family with a family that is still massively prolific, like Muridae, and you can see the often disparate effects of evolutionary pressures. In both cases, the environments in which those organisms lived pushed them toward better adaptation, and for one, the family flourished, while the other languished.

  75. Tethys says

    While I agree that pregnancy is very well described as a parasite/host relationship, I have to point out that embryos do not burrow aggressively (or at all), it is the embryonic tissue layer that forms the placenta which roots itself deep into the uterine tissue and blood supply.

    Everything you want to know about placental structure and classification can be discovered by clicking the link.

    In depth info on many species of animals at comparative placentation.

    Apparently most bats are hemochorial.

  76. Part-Time Insomniac, Zombie Porcupine Nox Arcana Fan says

    I’m going to need a new folder in Favorites for articles like this. This was some good stuff. Come to think of it, I never really thought of the abortion debate in terms of evolutionary struggle either. But how, and pardon if this already was asked, does that explain why some women are so anti-choice?

  77. bromion says

    It’s not that “good EP” doesn’t get published… it’s that the sensational work gets mass media attention. I have the same problem in my field, robotics — there’s tons of amazing research and development, but ask people about robotics and they can only really think of Roomba or Asimo.

  78. carlie says

    Mammals also have a maternal-child weaning conflict that can be explained in evolutionary terms; for a period of development the child is entirely capable of finding food on their own, freeing up the mother to reproduce again, but it’s easier for the child to still nurse from mom so they keep trying to do so while being fought off by the mother.

  79. Azkyroth says

    But how, and pardon if this already was asked, does that explain why some women are so anti-choice?

    It’s similar to the way toxoplasmosis makes mice lose their natural fear of cat urine?

  80. dvoracek says

    I really love evolution. PZ, once again you are one of the most effective teachers i’mve ever encountered.

  81. Luc says

    @34, that sounds very fine, but it’s not what actually happens, neither in humans nor in any species of apes. Among other factors than have an influence, mothers often choose the better father (tip: one that isn’t willing to work isn’t a good one); and a dedicated father is much more likely to see any son make it to reproductive age than a careless one. If fucking whatever is favored by evolution, we need to explain why apes don’t behave like that.

  82. C.C.Fuss says

    A minority of human beings have a high level of obnoxiousness combined with obsessiveness, which causes them to act in ways which, to the casual observer, may seem maladaptive. For example, derailing interesting discussions in order to go on and on about their pet topic. Clearly, we need an evolutionary-psychology explanation for how these kinds of traits can have persisted rather than being selected against.

    My (totally scientific) hypothesis is that some humans in the EEA, who had the Obnoxiousness-Obsessiveness mutation, continually engaged in behaviours like loudly interrupting tribal meetings to ask why everyone was praising Ogg for killing a bison but not praising him, Grogg, for throwing his spear with deadly accuracy at empty air.

    Mostly, the rest of the tribe would just ignore Grogg, or throw him out. However, every now and again, they would get together, draw straws, and assign someone to have sex with Grogg just to get him to shut the fuck up for five minutes. While this didn’t happen often, it happened just often enough that there are still a few people with the Obnoxiousness-Obsessiveness trait in present-day populations.

  83. Brother Ogvorbis, OM: Reading Comprehension Fail Warning! says

    Ogg for killing a bison

    Me like Ogg. Ogg great^68 grandfather. And Og have bison on sleeve patch.

    (So is this obnoxious? or obsessive?)

  84. says

    I checked GSS data to see if there was any difference in support for abortion depending on the number of children a woman “has”. (They do not have a variable that I can see which more specifically askes how many she’s given birth to, so this may count some adoptions and stepchildren.)

    There is an anti-choice effect. But it doesn’t become large until a woman has 2 children. Having 1 or 0 children is almost the same, a difference of only 2 percentage points.

    Here’s the graph.

    If you want to play with it, here’s what I did:

    row: abany
    column: childs(r: 0;1;2;3;4;5-8)
    control: sex
    selection filter: year(2010)

  85. lmccarty says

    I’ve always used as the proof in UNintelligent design the following formula:

    38 years of fertility * 12 mos/year = 456 periods.

    456 FUCKING periods to knock out 2.1 kids.

    Yeah, REAL intelligent!

  86. says

    Thanks for all the explanations everyone. Very helpful!

    I guess the only bit I’m still a bit weirded out by is how it’s basically all about the reproduction in the end. Like that if a certain trait is more successfully reproduced in a population than a contradictory trait, then it eventually becomes common to that population and the less-successful one dies out. So all the adaptations in terms of survival and stuff are really only in service of the organism being able to successfully reproduce (and in many species, raise the young to the point that they’re self-sufficient). So any trait that would privilege survival of an individual over the ability to reproduce (and thereby pass the trait on) could never really be selected for. That’s sort of where my confusion was coming from.

    But I think I have the tools to work this out from here. Thanks!

  87. David Marjanović says

    I suppose we could blame The Curse on The Fall, but then this phylogeny would suggest that Adam and Eve were part of a population of squirrel-like proto-primates living in the early Paleocene.

    Or Eocene. Depends on whether tarsiers or only monkeys (Anthropoidea) menstruate.

    the lining of the uterus is also a sensor for fetal quality, detecting chromosomal abnormalities

    How does that work???

    Science, schimence. The correct explanation of the “phylogenetic” tree, is that since primates are mostly frugivores we were the only species to eat the apple and get kicked out of the garden. Also Glossophaga bats must have pollinated the tree of knowledge and once the apples fell to the forest floor, Elephant shrews must have scavenged the fruit.

    Win-win-win!!!

    Importantly, your hypothesis is testable: it predicts that tarsiers, pure insectivores/carnivores that they are, don’t menstruate.

    It also predicts that Callithrix marmosets have immaculate conception. Don’t ask me how to test that, but it’s noteworthy that they feed on gum and sap more than on fruit.

    Science! Fuck yeah. :-)

    …Oh. Desmodus rotundus, which menstruates, is a vampire bat.

    I didn’t know there was a shrew that menstruated, either.

    Elephant-shrews aren’t shrews. As the tree almost shows*, they’re more closely related to elephants** than to shrews.

    Of theological importance is the fact that they’re omnivores, and have been at least since the… Eocene.

    * Shrews aren’t present in the tree; their closest shown relative is the hedgehog Erinaceus europaeus.
    ** African savanna elephant Loxodonta africana.

    Wait…wait… hold up. Presumably, a mother surviving to have several subsequent pregnancies is better for the species as a whole, so why would an aggressive embryo/fetus be selected for in the first place? I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species. ????

    There is no such thing as a species. (At least not for the purposes of this topic.) If I implant successfully, the genes of which I am a carrier become more common in the population. Period.

    Here is something interesting that our culture doesn’t usually take into account. If the mother TOTALLY breastfeeds for six to eight months (no solids, no supplements, no pacifier), the menstrual cycle is suppressed, thus keeping the mother from conceiving again while the baby most needs her resources.

    Interestingly, according to its Wikipedia article, lactation doesn’t work as contraception in Callithrix.

    hypotheosize [...] hypotheoses [...] hypotheoses

    Hypothesize, hypotheses. Theses, not gods.

    Contrast apotheosis “the process of becoming a god”.

    No IUD for me though, since I don’t have kids, no doc will give me one.

    …What the literal fuck. Emigrate.

    And then, from a safe distance, sue the ass off every one of those doctors. Sue their titles off, even.

    This is the heart of the evolutionary controversy over group selection. And so far, the only cases where we know for sure that group selection works, are special exceptions wherein one can clearly see the group functioning as, and taking on the characteristics of, an individual. (Think of the social insects as one giant individual in multiple parts, with the individuals acting like the cells of a multicellular organism. The queen is the gonad/germ line/stem cell, and the workers are the specialized, differentiated soma cells. The colony is the organism/organ.)

    I’ve never before seen kin selection counted as group selection at all.

  88. David Marjanović says

    @34, that sounds very fine, but it’s not what actually happens, neither in humans nor in any species of apes. Among other factors than have an influence, mothers often choose the better father (tip: one that isn’t willing to work isn’t a good one); and a dedicated father is much more likely to see any son make it to reproductive age than a careless one. If fucking whatever is favored by evolution, we need to explain why apes don’t behave like that.

    It often happens that two completely opposite strategies both work even within the same species. If you’re a stereotypical oriental monarch who begets hundreds of children with his well-equipped harem, some of them will survive to reproductive age by pure chance.

    The descendants of the guy who founded what eventually became China’s last imperial dynasty in the 17th century number in the millions today.

    if a certain trait is more successfully reproduced in a population than a contradictory trait, then it eventually becomes common to that population and the less-successful one dies out

    Bingo. That’s all there is to it.

  89. carlie says

    But how, and pardon if this already was asked, does that explain why some women are so anti-choice?

    Culture.

  90. Heliantus says

    @ Luc

    If fucking whatever is favored by evolution, we need to explain why apes don’t behave like that.

    To start with (disclaimer, I’m not specialist in evolution), I don’t think that “fucking whatever” is either favored or non-favored. It’s just a strategy among others.

    I was also under the impression that apes do fuck whatever (at least, the Bonobo chimps do). And they do it because it’s fun, not because they are looking for children.

    What we are talking here is the quantity vs quality approaches.
    Many marine animals or insects like flies are just carpet-bombing eggs, in a true fire and forget approach; big animals like elephants tend to go the other way, with only one pregnancy per decade, but the whole herd watch over the few offspring.
    In an extreme case, I remember reading an article on some species of birds, whose mating habits were function of the geographical location: at some place, males and females form lasting couples; a few dozens clicks away, a male rapes the female before flying away in search for the next female.

    With these birds, in the first case, you have quality – the 2 parents invest time and resources in the survival of their offspring; in the later case, you have quantity – sure, most pregnancies/children will not be viable (because the female will be alone raising the chick, because another male will show up right after the first and it’s his gametes which are going to reach the ovula first…), but enough of them will.
    As long as the strategy keeps the species alive…

  91. changeable moniker says

    melissameverden:

    While it’s intriguing to have these suggested answers, it’s also downright creepy to consider, particularly as someone who’s interested in incubating a fetus in the next few years. The final notes about the mother vs. fetus biological conflict made me think about the terrible new Twilight movie that apparently depicts the fetus destroying its mother from the inside…ugh!

    Having seen two of my children nearly kill my wife, I’d say you’re wise to be worried.

  92. Amphiox says

    It often happens that two completely opposite strategies both work even within the same species.

    Consider the “humble” cuttlefish. The big, flamboyant alpha males battle to control harems of female. The small, sneaker gamma males impersonate female coloration to sneak matings right under the noses of the big alpha dudes.

    I’ve never before seen kin selection counted as group selection at all.

    Selection acting on whole social insect colonies competing against each other would count as group selection, (As I understand it, whether such group selection works at all remains up for debate) while selection acting on related individuals within the same colony would count as kin selection, which isn’t controversial and is well recognized to exist.

    Or at least that’s how I always thought it went….

  93. Amphiox says

    and a dedicated father is much more likely to see any son make it to reproductive age than a careless one

    Not necessarily. As previously mentioned, some powerful men historically have had literally hundreds of first generation descendents, a few of which had literally hundreds of their own. Consider the examples of Ramses II, Genghis Khan, etc.

    One also has to factor in a certain level of random bad luck into the equation. A dedicated father might spend all his resources grooming one or two sons into finely honed specimens of reproductive manhood, and hand the world on a platter at their feet to inherit, only for a random flood or tornado or accident or war to snuff them both, while a careless Casanova might father a hundred sons, none of which he even knows exists, and 95 of which end up destitute, dead, or otherwise reproductively doomed, but maybe 5 by luck or individual skill (or a very dedicated, smart, loving, supportive single mother – never count that factor out!) make it big.

    And thus Casanova wins that evolutionary round.

  94. Amphiox says

    I can’t see how whatever benefit that holds for the individual would outweigh the long term detriment to the species.

    Natural selection can only “look ahead” one generation. If a trait benefits individuals now, but may have longterm detriment to the species 100 generations later, it will expand inexorably until that tipping point 100 generations later.

    And then the population will crash.

    You might think that this will eliminate the trait, leading longterm to the removal of such traits, and this might well happen sometimes, but this not necessarily always so. Thanks to genetic drift, and tiny population of survivors could make it through the crash, and which allow the trait to expand again (and crash again, of course).

    And for comparatively simple traits there’s always the small possibility that it could arise again via new mutations, too.

    Not to mention the environmental conditions that cause the trait to be detrimental can (and do) change over the intervening 100 generations too.

  95. says

    Since roughly age 12/13–about the age it occurred to me that there really should be a lemon law for the female reproductive system–I’ve considered menstruation to be among the best evidence available to support evolution. I mean, what sort of intelligent, benevolent creator would have purposely designed THIS as the most effective method to ensure the continuation of his/her/its creation? Never bought the ‘Eve’s curse’ thing either, because seriously, who would be such a dick? Now, nearly three decades later, I’m jealous of my friends who have had hysterectomies and can’t wait to join their club (‘the way for me is uterus free!). I have friends who actually celebrate the anniversary of their hysterectomy rather than their birthday.

  96. Amphiox says

    So any trait that would privilege survival of an individual over the ability to reproduce (and thereby pass the trait on) could never really be selected for.

    Yep. Promotion of those kinds of traits requires intelligent design (ie human culture, and maybe non-human cultures too, on this planet or elsewhere).

    One of the reasons cultural evolution can, and does, counter and over-rule biological evolution guided by natural selection is in part because it works so much faster than natural selection. Culture can produce new traits and behaviours so quickly that even if those traits are detrimental to reproduction, and there is heritable variation available that counteracts the cultural trait for natural selection to promote, natural selection does not usually act fast enough to counter the spread of the cultural trait in question.

  97. Amphiox says

    I would like to point out that as far as I know, PZ has never actually criticized the so-called “good” EP.

    His critiques of EP have only been targeted at bad EP, and most of them have been directed at specific examples thereof, so anyone interested can evaluate the validity of PZ’s criticisms against the original claims for themselves.

  98. says

    @nataliebaldwin

    I guess the only bit I’m still a bit weirded out by is how it’s basically all about the reproduction in the end. Like that if a certain trait is more successfully reproduced in a population than a contradictory trait, then it eventually becomes common to that population and the less-successful one dies out. So all the adaptations in terms of survival and stuff are really only in service of the organism being able to successfully reproduce (and in many species, raise the young to the point that they’re self-sufficient). So any trait that would privilege survival of an individual over the ability to reproduce (and thereby pass the trait on) could never really be selected for. That’s sort of where my confusion was coming from.

    Welcome to the study of biology. =)

    Increasing just survivability while not increasing reproductive viability (i.e. some mutation that increases longevity but produces sterility) doesn’t play a role in evolution at all, because from a trait transmission perspective, such a mutation is deleterious. When we look at the concept of evolutionary fitness, we’re looking at the capacity of an individual organism to produce viable offspring. And that’s why the battle between mother and fetus is so vital to human survival – it ensures the best possible chance for beneficial gene transmission, while minimizing the chance for complete removal of a set of viable traits from the population.

  99. says

    @love moderately, amusing, but always remember that correlation is not causation. It may be that their beliefs led to their larger family size, or that an underlying factor (religious affiliation?) causes both.

  100. Ichthyic says

    And that’s why the battle between mother and fetus is so vital to human survival – it ensures the best possible chance for beneficial gene transmission

    beneficial implies utilitarian. sex competition and parent/offspring conflict are not utilitarian at all.

    it has nothing to do with human survival; it has to do with the frequency distribution of traits in a population. that one might be more successful than another at fixing itself in a given population does not at all reflect that it is or is not beneficial to the population as a whole.

    here, in a sentence:

    differential reproductive success is NOT comprehensive population reproductive success.

  101. Ichthyic says

    It’s an oversimplified viewpoint of how selection really operates within populations, but Dawkin’s Selfish Gene is probably still one of the better books that gets to the crux of the issue involved with selection.

    I would recommend it still to anyone confused on the issue of selection somehow favoring the survival or fitness of a species, or just an entire population for that matter.

  102. Ichthyic says

    Not necessarily. As previously mentioned, some powerful men historically have had literally hundreds of first generation descendents, a few of which had literally hundreds of their own. Consider the examples of Ramses II, Genghis Khan, etc.

    well, it’s interesting that you mention this.

    all of those examples are men who had nearly unlimited power and control, and had little to fear that their offspring would ever be harmed.

    IOW, they had no need for direct parental investment to increase their personal fitness.

    OTOH, in situations where there are either direct (predators, both inter and intraspecific) and indirect (insufficient resources for one parent to successfully get offspring to reproductive age) then parental investment becomes more important to increase personal fitness.

    I’m betting this matches up historically pretty well, if we apply these same rules to human individuals reproducing under the varying circumstances noted.

    Humans have pretty flexible mating systems.

    so do a lot of fish, coincidentally. and often for similar reasons.

  103. says

    @Ichthyic

    beneficial implies utilitarian. sex competition and parent/offspring conflict are not utilitarian at all.

    I don’t understand what you’re trying to say here – it seems fairly apparent that a “utilitarian” model doesn’t fit here. There is no “happiness” or “utility” associated with reproductive models – in this case, “beneficial” simply means “confers reproductive advantage”.

    Which is true – a trait’s ability to fix itself in a population is directly related to the reproductive advantage it confers. That’s why the variance curves in traits shift in response to selection, because new versions of the traits confer some benefit that allows the individuals bearing the new versions a better chance to reproduce successfully.

  104. says

    @love moderately, amusing, but always remember that correlation is not causation. It may be that their beliefs led to their larger family size, or that an underlying factor (religious affiliation?) causes both.

    Yep. The main thing I wanted to check was whether there was any hint of a biological effect; in that case I would expect a significant drop off between 0 and 1. None found.

  105. Ichthyic says

    I don’t understand what you’re trying to say here – it seems fairly apparent that a “utilitarian” model doesn’t fit here.

    by utilitarian, i mean “best for all” (the pragmatic form of its usage), which is directed at what you said here:

    “And that’s why the battle between mother and fetus is so vital to human survival

    and again, I stress selection has NOTHING to do with human survival, but to differential reproductive success only, and you seem clear enough on that at least when you say here:

    because new versions of the traits confer some benefit that allows the individuals bearing the new versions a better chance to reproduce successfully.

    yes, it’s INDIVIDUAL reproductive success that is the issue here, that does not translate to species, or even population, survival. sexual selection rarely produces individuals that have higher fecundity, for example, or even higher survival rates for that matter.

    There are many people who visit this blog who learn from what we write here, and it should be as accurate as possible.

    since it appears you know better, you might refrain from utilizing species level selection arguments, even as dramatic license.

    sorry, this isn’t just a pet peeve, it’s simply the way selection works, and it’s not right to confuse people about the issue.

  106. says

    @Ichthyic

    Thank you for the clarification – I understand your point now. However, I think you’re missing the forest for the trees. Yes, I’ll grant that selection occurs at the level of the individual organism (do people actually think differently? I wasn’t trying to confuse anyone – it really never occurred to me that I had to make that distinction), but selection for reproductively advantageous traits in individuals ultimately drives adaptation and evolution, as those traits pass into a larger and larger segment of the population. And evolution and adaptation are population-level processes. Separating evolution from trait selection is artificial, at best – the importance of each is best expressed as a function of its relationship to the other.

  107. Ichthyic says

    but selection for reproductively advantageous traits in individuals ultimately drives adaptation and evolution,

    …which are uncorrelated to the survival of the populations involved, or the species they represent.

    because a trait gets fixated in a population does NOT mean that population then has a better chance at survival as a whole, or a higher reproductive output than even a neighboring population.

    this is why I gave the example of the Irish Elk.

    selection only favors relative increased fitness of individuals within a population, whether the result would then actually increase the survival of the population as a whole is random.

    Separating evolution from trait selection is artificial, at best

    huh? that’s my point. evolution is nothing more than changing gene frequencies within populations.

    that’s all it is.

    selection acting on individuals is one mechanism that affects gene frequencies, drift is another.

    adaptation is just another way of phrasing selection, so again, we look at individuals within a population, not the population as a whole.

    you seem to have some basic ideas, or at least terminology, confused here.

    I would recommend reading Futuymaa’s “Evolutionary Biology” textbook if you haven’t yet. It’s an excellent summary coverage both of terminology and usage, with great examples, and his writing style isn’t too dry either.

    you don’t have to even grab the latest version; the second edition is available cheap, or at most public libraries even.

  108. says

    @Ichthyic

    Ah, it was with my comment on population survival that you took issue. As I read your comments, that is clear (now) – apologies for being dense.

    I still think you’re over-simplifying the issue, though – you can’t just ignore the dynamics of a population in the process of adaptation. If the shift in trait frequencies in a population forces that population out of its niche in an environment, even partially, the evolutionary pressures on that population change drastically. So, it’s a combination of the reproductive benefit conferred on organisms with well-adapted traits and the pressure exerted by the limited resources of an environment (ubiquitous to all populations, even those occupying their “normal” niche), as well as the pressure exerted specifically on those organisms that seek to compete outside the niche.

    So, population survival absolutely does depend upon individual adaptations staying within the confines of their niche OR conferring such a benefit that the organisms that evolve out of their role in an environment are reproductively superior to those populations they displace. And yes, I understand that selection doesn’t automatically lead to a more prolific population (which is why I gave the comparative example of Equidae and Muridae earlier), but it’s not possible to have population proliferation without selection to support it.

  109. says

    @love moderately

    Interesting. Thank you for that link – we’re working on a 7-12 curriculum map at school, and I can hopefully use that article to help us better create a sound evolutionary approach to biology.

  110. Ichthyic says

    please tell me you don’t actually try to teach evolutionary biology at the secondary or college level?

    I have nothing else to say to you but that you really should read a good standard textbook on the subject before you expound in threads like this.

    you’re getting way to hung up in jargon, and I think you are quite confused on a lot of the terminology and usage.

  111. says

    @Ichthyic

    Actually, I’m quite successful at teaching biology (not specifically evolutionary biology – I teach general biology classes – but I can’t conceive trying to teach biology without an evolutionary framework on which to build the ideas) at both the secondary and the college level. =)

    I will absolutely check out the textbook you recommended, though. Thank you for that – I’m always looking for more information.

  112. otis says

    Wouldn’t a simpler explanation to this be that both menstruation and non-menstruation exist because neither one offers a clear selectable advantage over the other i.e. selection pressure is low. When selection pressure is low evolution may respond with cycles of both convergence and divergence, which may explain why distant relatives such as humans and elephant shrew would menstruate. And if menstruation leaves an animal more susceptible to predation, the shrew may have adapted by becoming a nasty little bugger.

  113. wbenson says

    So miscarriage is eugenic. How much of the human mutational load is eliminated by this process?

  114. Ms. Daisy Cutter says

    Amphiox:

    As previously mentioned, some powerful men historically have had literally hundreds of first generation descendents, a few of which had literally hundreds of their own. Consider the examples of Ramses II, Genghis Khan, etc.

    Aside from what Ichthyic said, such men also had the wealth to make sure their scores of children were well cared for by the standards of the era. Joe Average Peasant, not so much.

  115. LightningRose says

    I’ve long thought that menstruation was the single best argument in favor of the Abrahamic vision of a misogynist god.

  116. muirmaid says

    Will insurance pay for the removal of a parasitic infection of the uterus? Could be a useful description…

  117. bubblewrap74 says

    If menstruation is a response to aggressive embryos, and all of the mammalian species that menstruate are also hemochorial, then wouldn’t the contrapositive also be true? If you read the link posted above (http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/structure.html) on placental structure and classification, it says that rodents as a group have the same type of placentation. However, according to PZ, only a few species menstruate.

    Or maybe it’s not reasonable to expect that it would be true in all cases? Maybe rodent mothers have some other type of protection against their embryos.