Are we still evolving? Part 2

The perennial question I’m asked in public lectures is this:  Are we still evolving? (“We,” of course, means “humans.”)  A while back I highlighted a paper by Byars et al. that measured selection on various traits in a human population, showing that features like weight, blood pressure, and age of menopause were indeed under selection.  This shows that we’re undergoing selection, but not necessarily whether we’re evolving—for whether selection causes evolution depends on other factors like the amount of genetic variation that’s available to respond to selection, and whether there are correlated traits that may be selected in other directions.

But if you want a more comprehensive answer to the question of selection, check out the penultimate issue of Nature Reviews Genetics, where you’ll find a review paper by the same group of researchers summarizing all available studies in Homo sapiens. It’s a very good analysis, discussing the problems encountered in human studies, the methods used to estimate selection, and, of course, the data.

Those data come mostly from long-term studies in which individuals were followed over long spans of time.  Various traits of interest were measured (height, age at menopause,  blood pressure and so on) and were correlated with “fitness”, which the authors took as completed family size.  Differential fitness, which incorporates both survival and reproductive output, is the index of natural selection. For obvious reasons, this is more accurately measured in women than in men, so most measures of selection come from females.

The authors find three consistent results among the studies:

  • Both women and men are under selection for earlier age at first birth in all populations.  The authors see this as a result of lowered juvenile mortality resulting from improvements in medical care, nutrition, and sanitation.  If it costs you to reproduce early, but the benefits are increased because your kids no longer die so often, then selection will favor your reproducing at an earlier age.
  • “Women are under selection for later age at last birth in a pre-industrial population [Finland, 17th-19th century] and later age at menopause in two post-industrial populations [USA and Australia, 20th century]. The authors don’t explain the basis of this finding, but I suppose it’s because women now live longer, giving a selective advantage to females who can produce more offspring at ages that they wouldn’t have attained earlier.  Combined, these first two observations show the “temporal window of reproductive opportunity” is broadening in humans:  we reproduce both earlier and later than populations a few centuries ago.
  • “Women are under selection for increased height in one pre-industrial population [Gambia, 20th century] and for decreased height in three post-industrial populations [Great Britain and USA, 20th century].” The authors suggest that, for the same reasons that it’s adaptive to reproduce earlier in industrial populations, it’s also adaptive to mature at a smaller size and divert your effort to reproduction.  To explain the Gambian data, they suggest that this trade-off doesn’t exist in populations where infant mortality of shorter and younger mothers is higher.  This is, of course, special pleading, but that’s just speculation. The data are what is important here.

I don’t have the expertise to judge the authors’ evolutionary speculations, but these three conclusions represent the best answer to the question of “are we under selection?”.  Again, whether that selection produces evolutionary change depends on other factors that weren’t measured.  It will take a few centuries (if ever!) to determine if this selection really has caused evolutionary change.

Finally, the authors estimate selection on some behavioral traits, but take these estimates less seriously.  That’s because there are likely to be substantial cultural components to inheritance here, but also—though the authors don’t say this—because traits like IQ, income, and education are political minefields.  For what it’s worth, the authors conclude that “contemporary humans are under directional selection for less education, less income in women, and more income and wealth and higher rank in men, and decreased intelligence in both sexes.”  Reality shows and Tea Party members instantiate the last process.

Why is this important?  Given that these indices of selection, if real, aren’t all that strong, and will take centuries to produce any substantial evolutionary change, why does this matter?  I’m not sure, actually.  It does provide a solid answer to the question I’m asked most frequently, but remember that cultural changes can produce much more rapid and pronounced changes in human populations than can genetic evolution itself.

Since 1900, for example, the average height of Japanese has increased by more than 11 cm.—about 4.3 inches!  That’s only a couple of generations, and these changes are certainly due not to genetic change but to improved nutrition and medical care.  (Note that they’re also in the opposite direction to selection acting in other “post-industrial” populations.)  Even if selection is acting on us, the evolutionary change it causes is likely to be overwhelmed by changes due not to genes but to culture. And that’s something I always add when asked about whether we’re evolving now.

_________

Stearns, S. C., S. G. Byars, D. R. Govindaraju and D. Ewbank. 2010.  Measuring selection in contemporary human populations.  Nature Reviews Genetics 11:611-622.

26 Comments

  1. Torbjörn Larsson, OM
    Posted September 18, 2010 at 7:15 am | Permalink

    Thanks, interesting research as nearly always and humans are close to my hearth.

    It seems to this layman the measure of “evolving” is fixation. (Of functional, or at least discernible, traits, I take it.)

    Not a nitpick for once :-D, but an opinion: that is a bit idiosyncratic for my taste, I wouldn’t call an amplifier giving a static or noisy signal “non-amplifying”, the process is still acting. Oh well, potato, potatoe, and not my area.

    This is, of course, special pleading,

    Q: You know, I’m looking at this (the post mostly, the article is pay-walled) and I don’t see where the special pleading comes in!? It isn’t even ad hoc what I can see, since it is in line with their previous hypotheses, isn’t it? It may be handwaving for all I know (but again, pay-wall), which is often opposite of pleading an imagined solid ground: it means you are flying without safety net.

  2. Posted September 18, 2010 at 8:09 am | Permalink

    It seems obvious to me that evolution should still be going on in H. sapiens, if for no other reason than that our environment is rapidly changing. Surely the radical change in diet resulting in a massive outbreak of the metabolic condition, including in young children, will have some impact?

    I don’t expect natural selection to stop in humans until pre-natal genetic engineering becomes the norm. And, at that point, human evolution will accelerate to warp speed; it’s just that it’ll no longer be driven by random mutation and environmental selection for reproductive fitness.

    Cheers,

    b&

    • Matt D
      Posted September 18, 2010 at 8:29 pm | Permalink

      Hi Ben,
      You pose the question re diet change and its potential to act as a selective pressure.

      I just dont see it happening. Modern medicine is the greatest “artificial” selector of all time (dont get me wrong, modern medicine is great).

      The way i see it is medicine is selecting “for” maladaptive traits. People who would have otherwise not reached reproductive age or capability are living long, happy lives and leaving their maladaptive genes for futures generations.

      Again, I’m no nut-job eugenicist. I just feel that medicine “interferes” with natural selection. There is no reason for an adaptive gene to be selected “for”, when maladaptive genes aren’t being selected against.

  3. Kevin
    Posted September 18, 2010 at 8:10 am | Permalink

    Fascinating.

    I’m quite boggled by this statement:

    I don’t have the expertise to judge the authors’ evolutionary speculations.

    If not you, who?

    At the risk of going all HG Wells, it seems to me with regard to intelligence that “like attracts like”. As much as a pretty woman … well, you know, stirs my loins … I see little evidence that stable relationships develop between dis-equivalent intelligences. Instead, Bill Gates found Melissa, and trailer trash will shtup anything with the requisite body parts, and then home-school the output.

    If the overall intelligence of the post-modern civilizations is on the decline (sadly, all our anecdotal observations seem to confirm that), would not this at some point result in a cultural barrier between those with brains and those without? Leading (over centuries perhaps) to a genetic barrier?

    Did the authors note any bifurcation of intelligence? A camel’s hump instead of a bell-shaped curve? One suspects that they were only looking for the overall trend without looking for deviations from the median.

    I know, evolution as high school cliques. Eloi and Morlock. Pure speculation. Couldn’t happen…could it?

    • whyevolutionistrue
      Posted September 18, 2010 at 9:19 am | Permalink

      Life history theory is complicated and there are a lot of papers on it. For example, I recall that it’s “conventional wisdom” that high juvenile mortality selects for early reproduction, but in this paper the authors suggest that LOW juvenile mortality selects for early reproduction. I don’t know enough about the field to pass judgment on their speculations.

      • Kevin
        Posted September 18, 2010 at 9:29 am | Permalink

        I will have to go on an expedition…yes, the lower age of primiparity did surprise me as well.

      • Torbjörn Larsson, OM
        Posted September 18, 2010 at 12:08 pm | Permalink

        OK, so now I understand the basis, context actually, for the special pleading going on. Thanks!

    • Matt D
      Posted September 18, 2010 at 9:09 pm | Permalink

      Kevin, great pick up on the intelligence thing.

      I see no reason at all for “intelligence” to be selected “for” in a modern, industrialised society.

      Numerous studies (google it) show family size is a strong indicator of intelligence (of course not the only one). the correlation is negative.

      I agree with your “like attracts like” assertion. So if smart people are making fewer smart babies, and dopey people are making heaps of babies, clearly our modern societies select for stupidity.

      I too wonder if the cultural barrier could one day become a genetic barrier. But i dont see how it can happen – way too much mixing of genes across the population. As a species we are way too slutty.

  4. Posted September 18, 2010 at 8:32 am | Permalink

    The question that you’ve been asked in public lectures is “Are we still EVOLVING?”

    The question you tried to answer is, “Are we undergoing selection.”

    To me, it looks like you are equating the words “evolution” and “selection.” Is my impression correct?

    Random genetic drift is the dominant mechanism of evolution (by frequency) and it can’t ever be stopped as long as mutations occur. Thus, a correct understanding of evolution leads one inevitably to the correct answer to the first question. “Yes, humans are evolving just like all other species. Evolution can’t ever be prevented.”

    Here’s a few of my responses to the question. They try and take into account ALL mechanisms of evolution, not just natural selection. [Did biologists really think that human evolution stopped] [Have Humans Stopped Evolving?] [Are Humans Still Evolving?]

    • Neil
      Posted September 18, 2010 at 9:51 am | Permalink

      The question itself encapsulates the public’s misconception of the process of evolution. The question that is meant, presumably, is “is the process of evolution still changing the human phenotype in some interesting ways?”

      On that, I have read an interesting speculation that brain size (relative to BMI) is less in domesticated species (relative to their wild counterparts), and that there is evidence this has happened (is happening?) in humans (because we have “domesticated” ourselves.)

    • Adam M.
      Posted September 18, 2010 at 12:54 pm | Permalink

      “To me, it looks like you are equating the words ‘evolution’ and ‘selection.’ Is my impression correct?”

      No. He carefully distinguished between the two, and it was quite clear. For instance: “This shows that we’re undergoing selection, but not necessarily whether we’re evolving…” and “whether that selection produces evolutionary change depends on other factors that weren’t measured”.

  5. Sven DiMilo
    Posted September 18, 2010 at 8:39 am | Permalink

    Thanks for stressing the difference between (phenotypic) selection and (genetic) evolution-by-selection. That crucial distinction is too often glossed over when humans are the subject.

    Kevin, your comment is stupid enough to put you in the leftward camel’s hump if it existed.

    • Kevin
      Posted September 18, 2010 at 9:23 am | Permalink

      Yes, I know, pure speculation, as I said…

      And yet…I won’t go near a Tea Party person, no matter how “cute” they might be…nor a creationist. Why not? Because it would be a constant battle — to me, it’s an instant “turn off”. And I don’t need the aggravation because I can find suitable partners of equivalent intelligence.

      I suspect you segregate yourself from others by intelligence; you do not associate with those who you consider lower socio-economic status. Which functionally can be seen as a marker for intelligence to certain degree. (Yes, it can; let’s not pretend otherwise. Watch and episode of “Cops” and tell me there’s one person featured you would go out on a date with.)

      Choice of mate is a function of contact-opportunity. The lower the contact rate between one group and other, the lower the opportunity.

      If there is a genetic component to intelligence (and this is not only assumed in the paper above, but noted to be declining in certain groups), then there might come a time when the smart ones don’t wish to mate with the dumb ones, and vice versa.

      Separate the two groups by enough time, and eventually there will be not merely cultural barriers (“Date that brainiac! Ick!”) but genetic ones as well.

      That you don’t wish such an outcome or can’t envision it doesn’t mean that it can’t happen.

      Pure speculation, of course. HG Wells stuff, as I said before. But “dumb”? Well…nobody’s insulted my intelligence before, so I’ll have to ponder the implications.

      • Matt D
        Posted September 18, 2010 at 9:17 pm | Permalink

        I’m still with you Kevin, I completely understood the hypothetical nature if your enquiry.

        Nothing more than a mental exercise. Surely nobody could read it otherwise?

        And i still accept your “like attracts like” assertion. To claim otherwise is trying to to be too PC by half.

        Certainly not dumb, Kevin

      • Dominic
        Posted September 20, 2010 at 3:47 am | Permalink

        The procreative urge means that plenty of children are born out of one-off liasons, indeed in some sections of western society it is becoming the norm for a woman to have multiple children from multiple partners. There was historically more chance of a high status male having children across class barriers than of a low status male doing so. Perhaps that is changing. It will be interesting to see if paternity testing has an effect on the number of rich men who are unknowingly bring up the plumber’s child! My point is there will always be some cross-class mating & if you turn your nose up at the Tea Party girl, you are the one whose genes/memes will not get carried on! An interesting review of a reprint of the 1857 book by Sundt on illegitimacy in 19thc Norway is here in JSTOR (subscription required) here, – http://www.jstor.org/stable/3788912
        The reviewer say “In intraclass unions, a man from the owner class and a female from the cotter class were much more likely to be parents of illegitimate children than were daughters of owners and sons of cotters”.

  6. Neil
    Posted September 18, 2010 at 8:49 am | Permalink

    I would have thought that reduced juvenile mortality should have the opposite effect on the age of first birth. When juvenile mortality is high, you’d best reproduce early and often, despite the costs of doing so, in order to increase your chances. When juvenile mortality is low, you can afford to avoid these costs by (successfully) reproducing later.

    • whyevolutionistrue
      Posted September 18, 2010 at 9:20 am | Permalink

      Yes, see my comment above.

  7. NewEnglandBob
    Posted September 18, 2010 at 9:04 am | Permalink

    It seems that the answers to the questions require tiptoe-ing around until many more studies are done over long periods.

    • Torbjörn Larsson, OM
      Posted September 18, 2010 at 12:15 pm | Permalink

      There’s a lot of that going around in areas of actual knowledge, naturally. It strikes me as the opposite of “dwarfs on the shoulders of giants” (the original phrase, which suits better here).

  8. HP
    Posted September 18, 2010 at 11:17 am | Permalink

    As a layperson myself, I think that when people ask if humans are still evolving, they mean, are we turning into something else?

    But it seems to me (layman again) that since a) humans have an extremely cosmopolitan distribution, b) are capable of traveling enormous distances in a short time, and c) are constantly exchanging genetic material between subpopulations, it would be very unlikely for any truly novel traits to become fixed.

    • Adam M.
      Posted September 18, 2010 at 12:59 pm | Permalink

      Pardon my ignorance, but, setting aside the question of whether we’re “turning into something else”, wouldn’t our widespread gene exchange make it more likely for novel traits to become fixed (assuming they are of the right kind)?

      • HP
        Posted September 18, 2010 at 1:43 pm | Permalink

        Except that humans are distributed across all kinds of environments. The “right” kind of traits for living in suburban Chicago are not the right kind of traits for living in rural Gambia. But people keep traveling across the globe and having sex with each other, so that any specifically advantageous trait would be diluted out of the gene pool before it has a chance.

        This is of course, only my layman’s guess based on limited understanding. Happy to be corrected.

        • Matt D
          Posted September 18, 2010 at 9:36 pm | Permalink

          We are a kind of one-size-fits-all species.

          In the main the “right” traits for Chicage ARE the “right” traits for Gambia.

          However, the gene for sickle-cell disease is carried by as many as 40% of people in equatorial africa, as opposed to about 2% outside of africa.

          A single copy of the gene confers a significant survival advantge against malaria, while not afflicting the individual with sickle-cell disease.

          Certainly selection at work, but not evidence that we are “turning into something else”.

          I suspect you are right. too little isolation and too much sleeping around might mean we have reached a phenotypic stalemate.

          • Notagod
            Posted September 19, 2010 at 11:16 am | Permalink

            Yeah, the mormons are probably the only group that has a chance of becoming a sub-species.

  9. Kirth Gersen
    Posted September 19, 2010 at 12:51 pm | Permalink

    Can we state so confidently that there’s not a genetic (selection) to height increases? Find two guys of different heights but otherwise similar appearance, and ask almost any female which one is more attractive — in most cases, the answer is immediate and unequivocal in favor of the taller of the pair.

  10. Dominic
    Posted September 20, 2010 at 2:24 am | Permalink

    Steve Jones & PZ Myers discuss that in Focus Magazine here –

    http://preview.tinyurl.com/87rl27

    & Steve Jones did a UCL public lecture which you can find on i-Tunes UCL. Jones thinks that in the “Western World” it has stopped.


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