People evolving to be shorter & stouter

i-b619b51be3c6f3d337b1d074b3864447-hobbit.pngOne of the things about evolution you sometimes hear is that it has "stopped" for humans. Steve Jones, a British geneticist, is one of the more prominent public expositors of this viewpoint today. The key fact that most people latch on to is that infant and child mortality is very low, so the vast majority of humans reach the age of potential reproduction. Random genetic drift aside, evolution via natural selection does not necessarily need differential mortality as a necessary precondition (though this is obviously an efficacious mechanism from the viewpoint of evolution). All that needs to occur is for reproductive fitness to track variation in a heritable trait. Imagine for example that a woman was born with a mutation which prevented her from ever reaching menopause. Assuming that the allele does not go extinct, this is a variant which might actually spread in the modern world, as many women are delaying childbearing until the latter half of their fertility curve. A woman who might have one child at the age of 40 might very well wish to continue having more children before she retires (this is already happening).

But this is theory. What does the data say? As it happens a few months ago a paper came out which looked at this question. They used the Framingham Heart Study study sample, which goes back to 1948, and compared two generations, both with N's somewhat in excess of 5,000. Natural selection in a contemporary human population:

Our aims were to demonstrate that natural selection is operating on contemporary humans, predict future evolutionary change for specific traits with medical significance, and show that for some traits we can make short-term predictions about our future evolution. To do so, we measured the strength of selection, estimated genetic variation and covariation, and predicted the response to selection for women in the Framingham Heart Study, a project of the National Heart, Lung, and Blood Institute and Boston University that began in 1948. We found that natural selection is acting to cause slow, gradual evolutionary change. The descendants of these women are predicted to be on average slightly shorter and stouter, to have lower total cholesterol levels and systolic blood pressure, to have their first child earlier, and to reach menopause later than they would in the absence of evolution. Selection is tending to lengthen the reproductive period at both ends. To better understand and predict such changes, the design of planned large, long-term, multicohort studies should include input from evolutionary biologists.

Several of these are medical traits, exactly the sort of factors which are presumably greatly palliated by modern medicine. Below are the traits with their heritabilities:

Height (HT) = 0.84
Total Cholesterol (TC) = 0.61
Systolic Blood Pressure (SBP) = 0.53
Weight (WT) = 0.52
Diastolic Blood Pressure (DBP) = 0.49
Age at menopause = 0.47
Glucose level = 0.34
Age at first birth = 0.09

Remember that heritability is simply the proportion of phenotypic variation which can be explained by genetic variation. So for height ~84% of the variation in height is due to variation in genes. By contrast, only 9% of the variation in age at first birth is due to variation in genes. This is important because the higher the heritability, the easier it is for differences in reproductive outcome to translate into natural selection. The response to selection is directly proportional to the product of the heritability and the strength of selection. In other words, tall people tend to have tall offspring. Women who were unusually young when they first gave birth did not usually have offspring who were also unusually young when they first gave birth. This makes some sense since height is a straightforward biological trait, while age at first birth is presumably strongly conditional upon social circumstances and random environmental events.

Here are the selection gradients for the traits above:


N = sample size
μ = mean value of the trait
SD = standard deviation of the trait
β = unstandardized selection gradient
βμ = standardized selection gradient
P = p-value (Poisson model)

The direction and the magnitude of the gradients are rather clear. Women who are heavier, shorter, and have lower cholesterol have higher lifetime reproductive values. Note that they controlled for level of education, foreign or native birth, and whether the individual smoked, in their regression model.

i-d439625aded8711414774836ae413ff0-selecttable5.pngHere's table 5 reformatted a bit for clarity. You can see how using the &beta values calculated above they've projected the change in some of these traits over time. Within the text they do hedge a bit about the validity of these projections; the genetic architecture of these traits are not necessarily well elucidated and selection itself can exhaust the variation which it relies upon to change trait value. Or changes in the genetic architecture might lead to a non-linear response at some point in the future. But it seems that the point here isn't to make a projection about future humanity which none of the researchers will be able to examine. After all, it is unlikely that the Framingham Heart Study will be active 225 years from now. Rather, this study shows that there are diferences in reproductive outcomes which track heritable traits in humans; in other words,evolution has not stopped. You can measure its movement.

Citation: Sean G. Byars, Douglas Ewbank, Diddahally R. Govindaraju, and Stephen C. Stearns, Natural selection in a contemporary human population, October 26, 2009, doi:10.1073/pnas.0906199106

More like this

From a Yale press release: Yale University researchers have detected the effects of natural selection among two generations of contemporary women and predict their descendents will be slightly shorter and chubbier, have lower cholesterol and blood pressure and have their first children earlier in…
Update: The author of the paper clears up confusions. Update: Here's the paper. End Update The British media is abuzz with another paper from Satoshi Kanazawa, the evolutionary psychologist who has great marketing savvy. I can't find the study online anyway, so here is the Times Online: In a study…
I was chatting with a friend about a few quantitative genetic "back-of-the-envelopes." Specifically, about the expectation of the heights of the offspring of any given couple in the United States. I say the United States because it is a nation where most people get enough to eat; that means that…
When someone tells you that height is 80% heritable, does that mean: a) 80% of the reason you are the height you are is due to genesb) 80% of the variation within the population on the trait of height is due to variation of the genes The answer is of course b. Unfortunately in the 5 years I've…

"Infant and child mortality is very low, so the vast majority of humans reach the age of potential reproduction."
Isn't this an incredibly "developed-world-centric" point of view? The majority of births today do not occur in developed countries, but in parts of the world where child (and early adult) mortality is still a factor. For example, if there were a mutation that provided defence against AIDS, it could easily spread in sub-Saharan Africa.

Even in the developed world, there are factors that determine whether someone has offspring. What determines whether someone decides to have children? What causes someone to neglect contraception or to eschew abortion? The population who has children is different in many respects from the population who doesn't.

I read Jones regularly in the Telegraph. It has never seemed likely to me that his socio-political views are science-based; the reverse is likelier to be true, I'd guess.

By bioIgnoramus (not verified) on 18 Jan 2010 #permalink

Age at first birth has a heritability of only 0.09? There goes my theory of a round heel gene. ;-)

What do you think of the notion that our technology influences evolution? Like the idea that more people with, say, poor vision reproduce than in the past because poor vision is now more easily corrected by glasses and surgery. Does this make for measurable change? Has anybody ever tried?

I don't get evolutionists like Jones, there are many things that can drive human evolution in modern times. The influence of culture, food and parasites are three phenomena that drive human evolution, he ignores them completely.

Sure, technology, medicine and health care keep more people alive than ever before, but all those people that are kept alive today, that would have died 200 years ago still haven't found ways to overcome the evolutionary pressures mentioned above.

And what about sexual selection? Modern women are still attracted to tall men with relatively high status, strength, wisdom and good looks; men still love sweet, young beautiful women. If that isn't selection, Jones should ask the men and women deficient in such qualities if they've got an opinion about evolution.

@Maciano: Men prefer certain looks in a woman, and vice versa. Still, it's not a priori clear to me that those traits produce a net reproductive advantage at the moment. For instance, it may be that good looks influence social status, and that higher social status reduces the average number of children for cultural and financial reasons. I don't know that this is so, but it's not implausible on its face. In the raw numbers game the battle may not go to the pretty.

But of course your main point is entirely correct.

Steve Jones seems to give this same interview every few months. No matter how often someone points that differential fertility can drive evolution, even when there is not much differential mortality, he never changes his tune. I wonder why.

He's carved out a nice niche for himself as go-to guy for an incorrect view people don't mind hearing. I don't think he even has to do any research to back up his position.

I read Jones regularly in the Telegraph. It has never seemed likely to me that his socio-political views are science-based; the reverse is likelier to be true, I'd guess.

Framingham is a middle class white community in New England. It was never held up as a "typical American community", but was probably subjectively considered to be approximately so in 1948, when the Framingham Heart Study was initiated. It certainly does not remotely resemble a random sample of the US population by 2010 standards.

Nevertheless, these results are of mild interest.

There is probably a reverse u-shaped curve relationship between childhood mortality and population growth rates.

When mortality is very high, it is hard for the population to increase.

In the opposite extreme, when children are very likely to survive, family planning based on the implicit goal of ideal outcome for each individual child becomes the norm, and small families are common. In this type of circumstance, poorer people tend to have more surviving children. Hypothetically, people who identify with a lower class cultural role may be less optimistic about outcomes for each individual child, and less concerned to put off early pregnancy or limit future pregnancies, in the interests of a "few children with ideal outcomes" strategy.

(In intermediate circumstances - children have a high risk of dying, but overall plenty survive to reproduce - large families tend to be the norm, at all socio-economic levels. In these circumstances, with many caveats, richer people tend to be able to support larger families and help their children survive better. (A paradoxical caveat of this is that improving the health of children has been, historically, a very effective means of population control.) This seems to be a common pattern in developing nations, as mentioned above, and was also seen in pre-industrial times in what are now developed nations, for example, the colonial period of US history.)

Charles Darwin himself pointed out that by the mid-nineteenth century, poorer people tended to have more surviving children.

Although population growth of the US has been largely due to immigration, since the mid-nineteenth century, poorer women have tended to get pregnant younger and more often, and have more surviving offspring.

If there are any highly genetic traits associated with poverty - not traits that "cause" poverty, as we can all agree that poverty is largely the result of complex, interacting environmental and historical factors, but traits that are largely genetic and associated with poverty - it is patently obvious that they are being selected for in contemporary US society.

Interesting?! So I can safely say to my shorter friends that "she is a modern human". But seriously, natural selection will take hold on this one, say, the warmer climate may require more surface area!