It takes multiple mechanisms to drive evolution

I went through high school thinking that evolution was pretty neat. I read all the popular treatments — those Time/Life books, National Geographic, science magazines, etc. — and they made it sound like Darwin had got everything right in 1859. I could go along with that.

But then, in 1975, I went off to college and had access to a university library and started reading the real hardcore stuff. I was just mainlining PNAS & Science & Nature & Evolution & the Journal of Embryology and Experimental Morphology and soaking in all this exciting stuff, and that’s when I discovered the concept of genetic drift (specifically, thanks to Lewontin). My mind was blown. Suddenly, questions I had never even thought to ask were answered with clarity. Certainly nothing in biology makes sense except in the light of evolution, but likewise nothing in evolution makes sense except in the light of neutral genetic drift.

I guess Holly Dunsworth had the same revelation, and also noticed the sad gap in public education.

In the USA, K-12 evolution education standards are missing GENETIC DRIFT as well as the word NEUTRAL.

(see here:

I don’t know how to understand life’s perpetual change without those fundamentals.

If students know about meiosis (which *is* in the standards), then they know about genetic drift. It’s just a matter of linking meiosis to evolution.

Genetic drift is a very simple concept and makes natural selection make a whole lot more sense!

I shout about this on Twitter and I get “just be happy anyone teaches evolution in K-12 at all” and “teachers don’t know about drift” in response. But evolution without genetic drift is not evolution, and if teachers knew about drift, then they’d be more comfortable teaching evolution. I guarantee it!

Without drift, it’s too easy to just replace God with natural selection. And that habit of narrating evolution by giving agency to natural selection is super-duper weird for non-believers let alone for believers! (This a no hatred of religion, faith, or creationism zone.)

Genetic drift paves the way for thinking about and then narrating evolution as the constant change that life/nature/biology is. Everyone gets that time is just constant change and they will get that nature/biology/life is too. They embody it themselves, being different from their parents.

Plus, continuing to teach evolution as only being natural selection (which is what the standards are doing), is also dangerous. That selection-obsessed mindset is tied to racism, sexism, essentialism… all the stuff that we have to remove from our species’ shared origin story. Darwin only had selection (not drift, etc) to work with and look what his imagination did with evolution: racism, sexism, essentialism

Oh wow, yes. I’ve got to stand up and cheer at that. Also specifically the bit about meiosis, because I’m teaching genetics right now and am constantly emphasizing the role of chance in inheritance. We’re built on a platform of coin flips and die rolls and long shots in heredity, yet somehow in our public education, that all vanishes and is replaced with something close to fixed destiny (in higher ed, that isn’t true, fortunately. Usually.)

Also, drift nicely bridges all those gaps in comprehension the creationists love to inflict on us.


  1. DanDare says

    It drives me nuts that people don’t know about or understand drift. You can’t have selection without it.
    There’s also an external factor, change in environment over time. There iis competition and changes in competitors but also neutral environment drift such as temperature, salinity, atmospheric gas mix etc etc

  2. says

    Three cheers! It would be four cheers if you add migration too. And of course there are lots of other, lesser, phenomena as well. Ones which people often use to claim that the current theory of evolution is Wrong and that they should be given credit for establishing a new theory.

    Incidentally anyone wanting to do their own simulations of the four processes at a single locus with two alleles can use the program PopG, a Java program written in my lab, which can be downloaded here:

  3. jenorafeuer says

    I remember you giving a talk on the role of chance in inheritance at Eschaton in Ottawa back in 2012. (This actually came up in my mind recently because a friend of mine and I were talking just last weekend about the related fact that, after sufficient time, any organism from many generations back pretty much ends up being the ancestor of everybody or nobody.)

  4. stwriley says

    Argh, the “Next Generation Science Standards”! These are the supposed standards that science teachers largely ignore because plenty of us have our own state’s standards to follow and many of these are more general and leave it up to us how we will teach the specifics of critical ideas like evolution. As a high school biology and environmental science teacher, while I can’t speak to what everyone does, I can assure everyone here that most of us do, in fact, teach genetic drift as part of our teaching of evolution and very much stress the neutral nature of it. We also cover mutation (and the idea that it can be anywhere in the range between beneficial to neutral to harmful), sexual selection, migration (gene flow) and, of course, natural selection. It’s not that hard for high school students to grasp these concepts. Whether they claim to remember them when they get to a college genetics course is another matter, but we do what we can.

  5. Pierce R. Butler says

    My little MacOS dictionary (purportedly based on the New Oxford American Dictionary) gives a standard biological definition for meiosis, then adds

    2. another term for litotes.

    which I have never seen or heard of in this context before, though the etymological reference dates it from the “mid 16th century” (via Latin from Greek) while the scientific sense only goes back to the early 20th.

  6. billseymour says

    Oh, yeah, it’s a plurality of mechanisms. Not just one, or three, even.

    A bit meta perhaps, but I think I remember Condoleezza Rice once saying that Bush really hated it when she said, “It’s complicated, Mr. President.”  Could that be a conservative problem generally?

  7. says

    Dear PZ: Our HS textbooks (“BIOLOGY by Miller and Levine”) have ALWAYS included Genetic Drift as a major topic. I checked each edition, dating back to 2004 (I can check the older ones if you like). I completely agree that evolution doesn’t make without it, but rest assured that the #1 textbook in the country does its best to make students aware of genetic drift and has done so for years.

  8. chrislawson says


    That’s great about current and previous textbook editions, but this is about the proposed new curriculum.

    If you have a look at the link in the OP, the new curriculum is clearly intended to limit the understanding of HS students to simplistic adaptationist models with a near-exclusive emphasis on competitive natural selection as the only important factor driving evolution. The only three “Disciplinary Core Concepts” listed are common ancestry/diversity, adaptation, and natural selection. And this is how they want students to understand evolution:

    Construct an explanation based on evidence that the process of evolution primarily results from four factors: (1) the potential for a species to increase in number, (2) the heritable genetic variation of individuals in a species due to mutation and sexual reproduction, (3) competition for limited resources, and (4) the proliferation of those organisms that are better able to survive and reproduce in the environment.

    Note that “primarily”, which in most cases would be a sign that they will acknowledge other factors, but in fact it is just a weasel word here because they don’t just ignore genetic drift and co-operative evolution in this proposed curriculum, they specifically exclude it from consideration.

    [Assessment Boundary: Assessment does not include other mechanisms of evolution, such as genetic drift, gene flow through migration, and co-evolution.]

    No drift, no migration, no co-operation, no co-evolution, no endosymbiosis, no epigenetics. And if it becomes the new curriculum, then the big textbooks will change to meet it.

  9. bcw bcw says

    I agree that limiting everything to selection just feeds into misconceptions that evolution has purpose or trends towards a prefect ideal case. This was what I was taught fifty years ago to the extent anything of evolution was taught. It is sad to move backwards. The question is whether this choice is just foolish or steered by race and supremacy ideas.

  10. Paul Ditz says

    Here in Australia, the NSW Science curricula only uses the term, “Evolution by Natural Selection”. Most frustrating, but given the opportunity I will always mention genetic drift and near neutral theory.

  11. says


    When the NGSS document was still in the draft stage I sent the drafting committee a critique of the Evolution standards criticizing them as “overly selectionist,” neglecting genetic drift and other factors that clearly play important roles in evolution. The text did change a bit in response to many such critiques, but not enough. Joe Levine and I figured that our best course of action would be to continue to include the very topics you mentioned in our widely used textbooks, including migration, coevolution, endosymbiosis, and most recently, epigenetics.
    I have to say, however, that nationally my biggest concerns focus on the districts and classrooms where even the NGSS standards on evolution are watered down or not taught at all. When the opportunity arises to update the national standards I’d urge you and anyone interested to provide as much feedback to the drafting groups as possible.