Last week, David Attenborough told The Guardian that humans "have put a halt to natural selection," and with it our genetic and physical evolution. In this expertly crafted rebuttal, University of Rhode Island anthropologist Holly Dunsworth explains why he's wrong.
Top photo by Ricardo Moraes, via Reuters.
It should come as no surprise that I didn't actually speak to David Attenborough* recently. I'm just here writing about what I read about what he said about human evolution...
"We stopped natural selection as soon as we started being able to rear 90-95% of our babies that are born. We are the only species to have put a halt to natural selection, of its own free will, as it were," he tells this week's Radio Times. [source: The Guardian]
I think a lot of these conversations come about because people like to ask people like David Attenborough (and even me!) what will happen to us in the future. And instead of saying that it's impossible to predict the future of evolution (because it depends on so many probabilistic, seemingly random, and maybe truly random events, big and small), many public intellectuals please the crowd by offering some kind of speculation. Often the questions are about whether we'll grow tails or larger brains; whether our brains will shrink because of computers; whether our wisdom teeth, pinky toes or appendix will completely disappear. And often the experts play along with tongue in cheek, or sometimes seriously, building future scenarios.
The problem with Attenborough's answer — which might be aimed at avoiding this sort of speculation about the future — is that natural selection cannot possibly actually stop, not even at the hands, the hearts, the minds of humans. We're pretty amazing, but not that amazing.
To start, natural selection doesn't just enable the most obvious, visible, awesome changes to our bodies over time. To be sure, natural selection explains adaptations of all sorts — from fur to feather, prehensile tail to flagellum, macro to micro. (Ahem... as long as we're sure whether what we're covered with, dangling from or whipping around is actually an adaptation.) [Veni Vidi Vici via]
But natural selection also explains how we are here, alive and working well enough to be here, alive. In that sense, in its purifying sense, natural selection's at work, if you will, on practically everything about us and on practically everything about everything else that's alive now or that's ever been alive. Natural selection is always happening. Always. Culture or no culture... and because of culture!
Furthermore, even if we could consider all environments (and our cultural abilities to adapt to them) to be equal and constant, there will always be both new combinations of old genes and new mutations in each and every person (100+ brand new base changes to each person’s code compared to mom and dad) — some of which will cause human lives to end before they've passed on those combinations and those new mutations.
That each of us is unique, demonstrates that evolution is always occurring even though scientists prefer to think of it as change over time at the population level. Regardless, and this is very important, natural selection allows for most of this perpetual change in lineages and in populations, which is why so much life on the planet is humming and thrumming away, and has for the last 4 billion years.
Image source: Evolution P.S.A.
However, any genetic-based infertility, any genetic condition that directly or indirectly inhibits procreation, or any genetic disorder or disease that ends a person's life before they pass it on will disappear due to natural selection, along with their entire genome, including everything that had little to do with early death or infertility. So regardless of medicine and birth control, there will always be lineages that are more prolific than others (i.e. differential reproductive success) and there will always be lineages that disappear — both due to constant natural selection. The same is true about differential reproduction due to constant genetic drift — that is, chance change in a gene or trait’s frequency over time or differences in a gene or trait’s frequency between populations due to chance differential reproduction and other evolutionary processes occurring differently in those populations. Like selection, drift is always occurring, but it can escalate in intensity, for example, after a tsunami.
So, it may be true that, generally speaking, humans today have more egalitarian reproductive "success" compared to our ancestors who were arguably more vulnerable to nature red in tooth and claw. [Teddy Roosevelt conquering a moosevelt. Via @HistoryInPics]
And it may be true that the odds are greater for any one human to produce offspring that go on to bear their own offspring than for an individual in another, and maybe many many other, species — those that are, arguably, more vulnerable to nature red in tooth and claw.
And it may be true that because we birth relatively few offspring — compared to, for example, octopuses that bear thousands and thousands at once — that there's not a whole lot of difference between Alex's fitness, having three kids and three grandkids, versus Alice's having three kids and two grandkids.
And furthermore, it may be true that compared to innumerable species, we have more lineage continuance and less lineage extinction due to fewer juvenile deaths per birth thanks to opposable thumbs, throwing ability, weapons, medicine, extended family, extended love, extended memory, food storage, food production, cooking, sanitation, and so much more that contributes to or falls under the “culture” umbrella.
Classic examples of recent and potentially currently occurring natural selection that people call on in discussions like this are lactose tolerance and malaria resistance. Both are usually used to argue that natural selection has not stopped. But leaning on them can give the impression that we know of only two ways humans are presently adapting. There are others, like amylase and immune system genes, which are potentially powerful too, but again, examples of human biological adaptation in present or near-present times are not exactly overwhelming us. No matter! Because there are, unfortunately, myriad mutations (individually rare, but not so rare in total), both de novo and inherited, that are selected against every day, all around us, too numerous to list. This is natural selection.
So it's just wrong to say that natural selection has stopped unless to "stop" is a relative term. And even then, saying natural selection has relatively stopped or has effectively stopped in humans is making an assumption about what's good for humanity (that will sadly never evolve) and what's bad (that we're so busy propagating). It's also making an assumption about the relative strength of selection for forming adaptations out there in the "wild" — one that isn't grounded in firm understanding or consensus among many thoughtful scientists.
If we're going to consider natural selection to be so strong an evolutionary mechanism, then we have to consider mutation and genetic drift to have the potential to be just as strong — depending on the snapshot in time/space/organism and there have been many of these snapshots, to put it mildly. Further, to ignore the potential for gene flow to spread and to create new combinations of genes with potential to create new phenotypes that fail or flourish (i.e. under natural selection and/or genetic drift) in different circumstances is to lack an imagination — and a guttery one at that. [Image source: Evolution P.S.A.]
And these assumptions about selection's great strength relative to other mechanisms of evolution illustrate our biases and fashions, as well as the limits of our observations, our methods, and Science. These major issues in evolutionary biology are real and they're certainly not respected by statements like " We stopped natural selection." For a public that often mistakenly equates evolution with natural selection, it’s just reinforcement. And, no, evolutionary understanding isn’t just about being academically correct; many of us strongly believe that how we understand or misunderstand evolution affects how we think about social inequality, race, other organisms, caring for the environment, etc and, therefore, how we behave regarding those issues.
I’ve written a few posts that speak to the importance of evolutionary understanding, like:
- Evolution is the only natural explanation. And it’s all we need.
- The F-words of Evolution
- Another F-word of Evolution
- Evolution’s got a P.R. problem
- Taking our (lovely lady) lumps
- Luck of the draw
- Please make us teach creationism
And, granted, Attenborough admits he’s privileged so this isn’t a comment aimed at him, but… Isn’t it just a bit soon to be including the entire human species in such a healthy, well-nourished, low youth mortality, long life expectancy state of nature that has “stopped natural selection” on ourselves? I think we have a long way to go before such a definition of humanity can be granted to all of us. And even if or when we, as a whole, achieve such a comfy state, we still won’t be able to say that natural selection has stopped for the reasons I discussed above, and for others that I didn't.
What’s more, if we did achieve the kind of wizardrous skills that deem us impervious to natural selection, it couldn’t have occurred until recently. So it’s probably a bit premature, based on our limited observation, to conclude that’s where our species is.
I'm right there with Attenborough in sharing his astonishment at how we affect our own evolution, and that of other species, in ways that might not have unfolded if our cognition and culture had not evolved first. In fact, I think it's so fascinating that I'm writing a book about it right now. But if we claim that human cognition and culture have ended natural selection, we're denying our place in a universe that we cannot completely control. We are not immune to effects of the world around us, no matter how masterful we are at manipulating it. Climate, weather, geological processes, disasters, infectious diseases, parasites, symbionts (we're full of 'em and covered in 'em!)… these are just some of the things that affect our evolution through natural selection (and other means) as well as all the evolution of the species we depend on for food and all the species besides us that our food depends on. Evolution that's occurring just as constantly in everything around us and on us and inside us as it is in our own genomes will directly and indirectly affect our future evolution. There are few blanket rules in biology but here’s two: Evolution of an organism is always happening and always will. Evolution in one organism is affected by evolution in another.
So, sure! I can speculate about our future just like anyone else. Here’s how natural selection will affect human evolution at some point in the future. It's not so much a tale of our interconnectedness with other species as a tale of our interconnectedness with the planet. Everyone knows that heat is bad for sperm production. It’s possible that the earth will eventually get so hot (thanks to our cognition and culture???) that sperm production ceases in many men living in the hottest parts of Earth and that it only persists in the men who live in cooler regions and in the men with mutations for overcoming the obstacle. As a result, we’ll lose lineages evolving in the tropics and maybe all lineages. And if the warming occurs quickly and uniformly, and if there are no mutants already alive who can make sperm in the heat, global warming will certainly cause human extinction. There. See how easy it is to speculate about future human evolution? You can’t prove me wrong.
And, like our speculation about the evolutionary future, many of our hypotheses for the evolutionary past are nearly as unpwnable as natural selection and evolution are.
Thanks to Barbara J. King for sparking me to think about these things. Her reflection, including some of my input, is up on her blog at NPR.
*I don't know him, but like many of you, I have admiration and maybe even a little (or a lot of) affection for David Attenborough. So this isn't about reacting to a popularizer of science, as a person. These are just my thoughts about something he said about how evolution works. This isn't about Attenborough, it's about us.