Are humans still evolving?

October 24, 2009 • 10:07 am

Whenever I give a talk on evolution for the public, I always get this question:  “Are we still evolving?”  People want to know if our species is changing.  Are we getting smarter or better-looking?  Or are we degenerating as medicine keeps alive many of us who would have been ruthlessly culled on the savanna?

My answer usually goes like this:  “Yes, many of the genes that would have been eliminated now persist.  I am myopic, and wouldn’t have been a good hunter. Genes for nearsightedness are undoubtedly accumulating in our species. So in some respects we are genetically degenerating. But so long as there are sources of mortality — and the genetic variation to resist them — we will continue to evolve.  In Africa, for instance, thousands of children die yearly from infectious diseases. It’s highly likely that genes for resistance to those diseases are becoming more numerous.”

Now this answer is speculative, based on evolutionary theory.  There’s not a lot of evidence about whether and how fast the human species is changing.  But a new paper in The Proceedings of the National Academy of Sciences goes one step further: it not only demonstrates genetic variation for sources of mortality, but shows that that variation is correlated with reproductive output.  If both of these hold, then we must be evolving.  The paper goes on to predict how much genetic change for several traits that Homo sapiens will experience over the next ten generations.

The paper is by Byers et al. (see reference below), and should be online this week. (Thanks to Steve Stearns for letting me see a galley proof.)  There has already been a flurry of publicity — one example is here — and there will be more.

The sample population included about 5,000 women from the famous “Framingham heart study,” a survey begun in 1948, with the individuals sampled every four years for a variety of physical and traits (weight, height, age at first reproduction, blood pressure, serum cholesterol, etc.).  Since the individuals measured are now on the senior side, the original sample population (now past reproduction) has produced their allotment of children.

This gives us a basis for detecting natural selection.  The underlying rationale is that we look for a correlation between an individual’s traits and the number of children she produced.  If we find one, and there is genetic variation for the trait, we can reasonably assume that natural selection is acting on it, and we can see what type of natural selection by seeing how variation in the trait is associated with variation in offspring number.  If, for example, higher cholesterol is associated with lower reproductive output (as it might be if cholesterol is associated with heart disease that strikes before reproduction is finished), then we can assume that there is natural selection against higher cholesterol. If we know how much variation in people’s cholesterol is based on variation in their genes (a proportion called heritability, which ranges between zero and one), we can then calculate how we expect the trait to change in the future.  That is, we can see if we’re evolving, and predict how much.

The underlying method, developed by quantitative geneticists such as my old mate Russell Lande, involves looking at several traits, any or all of which may be subject to selection.  These traits may have some “genetic correlation,” that is, genes affecting one trait might affect another. (For example, genes for higher weight might raise blood pressure as a byproduct).   These correlations can be measured using data on the observed variation in the traits and the degree to which this variation is passed from one generation to the next (the heritability of the traits).

If you calculate all these correlations and then multiply them by the amount of selection that appears to act on each trait (this has to be discounted using the correlations), you can estimate the degree of genetically based change you expect each generation: that is, the amount that each trait will change over one generation by natural selection.

What they found.

Several traits did indeed appear to be undergoing selection.  From the amount of this selection, we can predict the percentage change in the trait that we expect to see after ten more generation of reproduction (roughly 300 years from now).

Total cholesterol: going down.  Projected to drop 3.6% in ten generations

Weight:  going up a tad, projected to increase 1.4% in ten generations

Height:  we’re getting shorter projecting a drop of 1.3% (2.1 cm) in ten generations.

Systolic blood pressure:  Going down, as predicted. Projected to drop 1.9% in ten generations.

Age at menopause:  Going up; projected to rise 1.6% (0.8 years) in ten generations.

Age at first reproduction: Going down. Projected to drop 1.7% (from 26.18 to 25.74 years).

So women, at least, are getting shorter,  stouter, and reproducing earlier and over a longer period of time.   This is evolutionary change.  Based on this study, we can tentatively say, with more assurance than I used to, that yes, our species is still evolving.

But there are two important caveats to this study, both of which were recognized by the authors.

1.  The “inheritance” of the trait includes not just genetic inheritance, but cultural inheritance.  Humans pass not only their genes to their offspring, but aspects of culture that may mimic a genetic inheritance.  For example, parents who eat a lot may induce their kids to eat a lot, and some of the correlation of weight between parents and their kids may be due not to shared genes, but to shared food.  Parents who for cultural reasons have their kids early may induce their own kids to produce grandchildren early.  As the authors say, “We are not able to differentiate the effects of genes and culture with these data.”

This is a bit of a problem, because the evolutionary projections are based on assuming that all of inheritance is genetic.   It’s hard to get around, since distinguishing genetic from cultural inheritance involves difficult work using data from adopted children or twins raised together versus apart.  Nevertheless, we can probably assume that some selection is acting on these traits, discounted by the degree to which parent/offspring resemblance reflects cultural similarity.

2.  The predictions may be hard to verify, because they assume that the environments of our species — that is, the environments that are relevant to each trait — will remain constant over the next few generations. Cholesterol is predicted to drop from 224 to 216 mg/100 ml blood over the next ten generations, but this assumes that diet is constant.  If people stop eating fatty foods, the drop may be even steeper because of this dietary change.  Conversely, if people flock even more frequently to fast-food joints,  the predicted drop may be negated by an increased intake of burgers and fries.

These problems mean that one has to be a bit careful about not only predicting the degree of selection, but also testing those predictions in the future. Yesterday Stearns was on NPR’s Science Friday, discussing how the authors deal with the conflation of culture/environment and genes, and why the prediction of increasingly plump women may not be what it seems.

But regardless of the caveats, the study of Byars et al. is based on a good idea, and makes a reasonable case that selection is indeed acting in our species.  One would, of course, like confirmation from other datasets, but since the Framingham study is unique, this may be hard to do.


h/t: Steve Stearns (who reviewed this post for accuracy).

Byars, S. G., D. Ewbank, D. R. Govindaraju, and S. C. Stearns. 2009.  Natural selection in a contemporary human population.  Proc. Nat. Acad. Sci. USA: in press.

26 thoughts on “Are humans still evolving?

  1. One thing that I’m curious about though is whether (and if so, how) sexual selection continues to drive our evolution. Are our evolving cultural, sexual and family tastes influencing who has more children?

    1. It seems to me that religious submissive women and religious dominating men are more likely to produce many children– thus passing on both “genetic susceptibility” to religiosity (genes that are involved in trust, intelligence,forming social hierarchy, etc.) into vectors (newborn babies) that are ripe for indoctrination of religious memes (go forth and multiply… contraception and abortion are wrong…god doesn’t give you problems bigger than you can handle… etc.) This makes religion a very virulent meme in regards to how humans are evolving.

      As an uppity atheist woman, I doubt I could have survived to produce children in eons passed. And my education seems to play a role in my revulsion to producing a “quiverfull” of spawn for some megalomaniac man.

      I think that once a young woman has started on that path, she develops a NEED to convince herself that it is part of some higher plan.

      Evolution doesn’t select for the brightest or most environmentally aware– just the humans who have the most grandchildren (and those such as priests and polygamist clans who can convince others to do so.)

      It sounds somewhat abysmal, but we live in interesting times. And I suspect the internet might help spread information– a remedy of sorts to halt the virus. I do think it’s in our own best interest to take charge of population control and the education of our species as a whole, because “natural selection” can be rather cruel.

      1. “religion a very virulent meme”

        Don’t you mean a “very successful” meme?

        Evolution does not care about your happiness, your ideology, your atheism or your revulsion at large families. Evolution cares about reproductive success.

        And the fact is, relgious people reproduce in relatively large numbers. Secular people, not so much.

        So I ask, which society would be superior (in a Darwinian sense), a society of atheists or a society of theists. If birth rates are any indication, theists win hands down (see The Economist magazine:

        “Religious people have more babies than non-believers…The rapid rise of Mormons in America, growing by an average of 40% every decade in the 20th century, is mainly due to their large families…Ultra-Orthodox Jews, however, do have plenty of offspring. This fact is changing the face of Israel, where such families have three times more children than other Israelis. As a result, at least a quarter of Israel’s population of under-17s is expected to be ultra-Orthodox by 2025…Conservative Protestant denominations as a whole grew much faster than liberal ones in 20th-century America, and it has been estimated that three-quarters of this growth is due simply to higher birth rates….”

        One can only conclude that theists are the superior “species”. It would appear that atheism is a meme that renders its victims impotent/infertile – and therefore inferior in a Darwinian sense.

        Ironic, no?

      2. No, I meant virulent. Viruses don’t care and success for a virus is often detrimental to it’s vector.

        And, one does not need to conclude that religious people are “superior” unless one’s definition of “superior” means “successful at passing on one’s DNA”.

        Also atheism does not render one impotent/infertile though there is a correlation between intelligence, number of offspring, and atheism. (Higher I.Q’s are associated with fewer offspring AND decreased religiosity.)

        Other than that, I think I pretty much said what you said, but I think I said it better.

  2. I am skeptical of these results. My gut feeling is that this is too small a sample spanning too small of a time span. I also am skeptical of the representation of Homo Sapiens from that sampling of people, not being diverse enough.

  3. For some interesting speculation on sexual selection in humans you might find the artcle referenced below interesting.

    Google the title and a pdf posted on the author’s site should come second.

    Aarssen, L.W. 2007. Some bold evolutionary predictions for the future of mating in humans. Oikos 116: 1768-1778.


  4. Dr. Coyne:

    Your perspective concerning “Are humans still evolving” seems not to have extended to your concept of “the modern theory of evolution” and “throwing off many new and diverse species” as stated in your book on “Why….”

    As a matter of curiosity, are you implying that the current state of evolution of humans can be considered in a different light than evolution as defined in your book, and as addressed in Chapter 8?

    Or do the findings of Byers et al. constitute a current “trait” that is in the “small step” process of changing into a “new” trait?

    Do the Byers’ findings signal a small-step trait change toward a new species?

    If not, then their findings convey no plausible implications concerning human species evolution.

  5. Myopia can be adaptive, in a society where close-in vision at a later age is important. In other words, forget hunting, and make the stuff that hunters with good vision trade for.

    Surely someone, somewhere has studied this possibility and produced some numbers.

    Overall, on the theme of the post, of course we’re still evolving. I don’t place any stock in the predictions listed, however. I don’t think there’s any statistical significance given the sample sizes, quite apart from the fact that some of the variables are almost certainly affected more by culture than genetics (most notably weight).

    1. Myopia only develops in cultures where children do near-sighted work. Seems like the gene which switches off eye growth when light falls focussed on the retina is a bit more sensitive in in some people, meaning that it registers the unfocused peripheral areas of the visual field and tells the eye to continue growing.

      When no close work (esp. reading) is done in childhood eye growth is normal. (See Nesse & Williams’ book on evolutionary medicine.)

  6. I was quite satisfied with John Hawks et al studies, where selective sweeps have been occurring for as long as their methods goes back to as long as their methods reach the present, with an increase of efficiency of selection 2 orders of magnitude as the population increases 6 orders of magnitude. And this incorporates advances such as cooking et et cetera.

    I wouldn’t expect any change due to medicine, but it’s good to test the above yet again.

    Genes for nearsightedness are undoubtedly accumulating in our species. So in some respects we are genetically degenerating.

    Surely this is beneficial variation in the presence of eye correction techniques? 😀

    1. D’oh! I meant beneficial _accumulation_ of variation. The variation can be near neutral, I guess.

  7. If our current civilization lasts long enough, then perhaps these sorts of questions can be answered with the help of very large medical databases. Of course there is the challenge of getting the relevant information without revealing other information which should be confidential.

  8. I have to say this article is very bad.

    1. As one responder already pointed out the sample size cannot be used to determine anything about homo s. sapiens as a species. At most could tall about one small population of human beings.

    2. All of the factors that were supposedly found to be be evolving vary hugely with diet. Effects of dietary change would completely swamp any relatively small evolutionary change over two or three generations (your two caveats address this but don’t seem to recognize the scale of the problem). It especially silly to talk about wight in this context. The genetic factors by which fat is stored more efficiently (typically in desert dwellers like Apaches) are well known. That is an adaptation. But it doesn’t produce an increase in weight until a western diet is adopted.

    3. If these traits were being selected for, it would mean a differential in the number of offspring produced. By what mechanism, for instance, could height decreasing by less than 2% increase the proportion of shorter people’s children going into the gene pool of a population?

  9. the low-carbohydrate and zero-carbohydrate advocates claim that myopia is NOT purely genetic, that it represents an imbalance of modern dietary intake from that which we evolved in. they claim that 14,000 years of agriculture is as nothing compared to 5 million years of foraging/gathering/hunting.

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