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This BBC article claims that:

The regular use of Caesarean sections is having an impact on human evolution, say scientists.

And that the evidence comes from a study looking at data over a 50-60 year time span:

The researchers estimated that the global rate of cases where the baby could not fit through the maternal birth canal was 3%, or 30 in 1,000 births.

Over the past 50 or 60 years, this rate has increased to about 3.3-3.6%, so up to 36 in 1,000 births.

That is about a 10-20% increase of the original rate, due to the evolutionary effect.

This is only a couple of generations, which I do not think is enough for evolutionary effects to present themselves.

James
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  • I heard something along the same line on radio, it might have been BBC news and they did quote some study, but might not be notable enough if it's only based on one study... – ventsyv Dec 07 '16 at 17:37
  • Let's avoid unsubstantiated pseudo answers in comments, thanks. – Sklivvz Dec 07 '16 at 21:46
  • Do you have access to the paper? I feel that when you ask a question about a statement based on a single study, you should definitely read the full study and boil down your question to one about the methodology used, or the interpretation of the results: https://doi.org/10.1073/pnas.1612410113 – bers Dec 07 '16 at 23:30
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    @bers: That would be nice but it assumes an ability to critically read scientific papers that many of our readers don't have experience at. – Oddthinking Dec 07 '16 at 23:36
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    I might be misunderstanding but how could a C-section affect evolution? Whether the child is delivered through the birth canal or body cavity, its genes are the same. The method of delivery isn't altering anything genetically. – syntonicC Dec 08 '16 at 02:58
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    @syntonicC - possibly because before 50-60 years C-sections were less common leading to higher mortality "where the baby could not fit through the maternal birth canal" - as C-sections become more common, survival rates increase; in turn, increasing the likelihood that the genes are passed on. –  Dec 08 '16 at 03:34
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    @HorusKol Alternatively, the mothers with smaller birth canals survive to get pregnant again, not only adding to the number of c-sections, but spreading their genes along. (Are you not referring to survival rates of the child?) – Double AA Dec 08 '16 at 04:30
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    @DoubleAA actually, this could be a parallel effect - the mother survives and thereby increases likelihood of having more children, and the child survives increases likelihood of propagation into the next generation. –  Dec 08 '16 at 04:48
  • @bers The PNAS site was returning an error when I tried to access it on several occasions – James Dec 08 '16 at 08:30
  • There's far less c-sections done in non-western countries. Most likely, western doctors have gotten in the habit of pushing c-sections. –  Dec 08 '16 at 14:05

2 Answers2

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The BBC article is based on this recent paper: Cliff-edge model of obstetric selection in humans that was published in October 26, 2016.

It is a bit too soon to see what post-publication peer-review will say about this, but noted Skeptic and evolutionary biologist, Professor P.Z. Myers has posted at his Pharyngula blog:

the paper doesn’t show a causal relationship.

[...]

Actually, they do do what they say: they present a mathematical model of how a disparity between head size and pelvic canal size could hypothetically lead to a selection effect, given a particular frequency of disproportion. They don’t actually measure or observe anything, though. They pull together a number of factors, like the heritability of pelvic and head size, and estimates of the frequency of serious birthing difficulties, etc., all of which show a wide range of reported values, and then put together an abstract series of calculations to show that hey, this could potentially have an effect. That’s it. Don’t panic. We’re not looking at an imminent future of bulbous-headed babies and pencil-hipped women because we’ve removed an important constraint on selection.

I concur with his assessment that this study is not an experiment that actually measured the heads of babies born over decades, found that it was increasing, and said that was because of caesareans. Instead, it is a theoretical mathematical model that predicts that there should be an increase:

We predict that this weak directional selection has led to a 10 to 20% increase in the rate of fetopelvic disproportion since the regular use of Caesarean sections.

Unrelated to the paper, there have been some comments on the loose use of terminology in the BBC article and in the question. "Affecting evolution" is a vague term. Natural selection can have a measurable effect on population distribution within one generation.

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Oddthinking
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  • It seems there has been a large leap from their results to the interpretation. There doesn't even seem to be any identification or measurement of relevant gene expression. Social changes, such as women having fewer births seem more likely to lead to the changes in figures presented. I've changed the question title to make it align with the article better. I suppose extinction events would lead to rapid changes, but are there any examples of this kind? – James Dec 08 '16 at 08:55
  • I am confused. Which results? How do they relate to fewer births? – Oddthinking Dec 08 '16 at 09:54
  • I love how this answer has exactly the same main arguments as mine (no causal effect on biology; merely a selection effect; a comment on the "evolution" terminology; and even an (unsubstantiated) statement that a single generation can be enough for an evolutionary effect to show) and is upvoted four times, while mine was downvoted, deleted within an hour of posting, saying that it doesn't address the question, and later undeleted. What a way to run a "skeptics" SE. – bers Dec 08 '16 at 12:12
  • @bers: Have a closer look and you may see the differences. This answer includes references for the all the quotes, so people can check that they are accurate and taken in context. It doesn't rely on my personal (and entirely valueless) opinion as an answer, but instead refers to the opinion of a well-known third-party known expert, who has actually read the paper. – Oddthinking Dec 08 '16 at 13:54
  • @bers this answer quotes "no causal relationship", not "no causal effect on biology"; they're not the same. A causal effect on population statistics would be a causal relationship. Yours contrasts statistical population changes with what you feel people misunderstand "evolutionary effects" to mean. This one assumes off the bat that we're talking about statistical population changes, and contrasts observed changes with theoretical changes. – Dan Getz Dec 08 '16 at 15:13
  • @Oddthinking The results being the increase in births where the baby didn't fit through the birth canal. The fewer births overall relates to a lessening of advantages of having a wider birth canal. – James Dec 08 '16 at 21:15
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    @James: I know it is seems pedantic, but that wasn't the result. The result was a mathematical model, of the [spherical cows in a vacuum](https://en.wikipedia.org/wiki/Spherical_cow) kind, that *predicted* there would be an evolutionary pressure towards an increase. – Oddthinking Dec 08 '16 at 23:43
  • @Oddthinking I appreciate that their work was a mathematical model, but the claim they made from it, as reported by the BBC, was the increase in births that didn't fit through the birth canal was caused by evolution. – James Dec 13 '16 at 10:23
  • @James: If the scientists made that claim, I missed it. Can you quote where they say that in the paper? – Oddthinking Dec 13 '16 at 12:34
  • @Oddthinking `For the time range that Caesarean sections have been regularly and safely conducted in industrialized countries (since the 1950s and 1960s, i.e., roughly two generations), our model predicts a 20% increase of the incidence of fetopelvic disproportion for model I and a 9% increase for model II. Note that these are predictions about the actual disproportion rate, not Caesarean section rate, which has increased much more rapidly for other reasons; the obstetric literature typically considers the actual disproportion rate constant.` – James Dec 13 '16 at 13:20
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I got hung up on your phrase "This is only a couple of generations, which I do not think is enough for evolutionary effects to present themselves." You seem to think that an evolutionary effect involves a change in biology, which I don't think is the case.

Disclaimer: I have not read the full paper, but I do want to make the following point:

A single generation can be well enough for an evolutionary effect to present itself.

By means of a very simple example, assume there are no men. Every woman will be pregnant exactly once (holy spirit!); due to some gene, 1% of all babies are too large to deliver safely; the babies are all girls; they all inherit the gene of having too large babies; half of them die at birth. Consider this an equilibrium state. Now you introduce CS at time x, saving all babies. Only a single generation later, the rate of too large babies will be approximately 2% (in fact, 200/101) -- close to 100% more!

As a kind of explanation, note that an evolutionary effect does not necessarily mean that babies grow differently; it may simply be possible that the selection of babies of different sizes changes over time. In fact, the paper suggests exactly that:

We demonstrated that due to these three properties weak directional selection favoring large neonates relative to the maternal pelvic dimensions is sufficient to account for the high incidence of fetopelvic disproportion in human populations.

Which, in my opinion, says exactly what I started with: there is not necessarily a change in biology; the observed size change can be explained without assuming any change in biology.

Finally, I feel the wording of your question title reflects that misunderstanding: CS are not affecting evolution (your question title); the selection through CS has an "evolutionary effect" (BBC article) in the sense that it is evolution.

bers
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  • You should like to the source of the quote; I thought it was from the BBC article, not the original paper. – Oddthinking Dec 08 '16 at 00:35
  • The wording "an impact on evolution" might not reflect a misunderstanding: "____ evolution" often refers to the results; it doesn't always refer to the process. – Dan Getz Dec 08 '16 at 15:13
  • I changed the question title to align better with the article. Outside extinction events, my understanding was that evolutionary processes are slow, many thousands of generations, but I accept there are many facets to it. I'm not sure I follow your supernatural example, isn't it even slower for species with asexual reproduction? – James Dec 08 '16 at 18:11
  • @James I think his point is that a selection effect can alter the proportion of different characteristics in a population after a single generation. In his example (which is only a thought experiment) in the first generation 1% of babies are born with a hereditary gene, and in the second, 2% are. – Dan Getz Dec 08 '16 at 21:19