Genetic drift is the random change in frequencies of alleles (forms of a gene, like the A, B, and O alleles of the Landsteiner blood-group gene) due to random assortment of genes during meiosis and the fact that populations are limited in size. It is one of only a handful of evolutionary “forces” that can cause evolution—if you conceive of “evolution,” as many of us do, as “changes in allele frequencies over time” (“allele frequencies” are sometimes called “gene frequencies”). Other forces that can cause evolutionary change are natural selection and meiotic drive.
Genetic drift certainly operates in populations, for it must given that populations are finite and alleles assort randomly when sperm (or pollen) and eggs are formed. The question that evolutionists have been most concerned with is this: “How important is genetic drift in evolution?” We know that, if populations are sufficiently small, for instance, drift can actually counteract natural selection, leading to high frequencies of maladaptive genes. This is what has happened in small human isolates, such as religious communities like the Amish and Dunkers. It’s not clear, though, that this has happened with any appreciable frequency in other species.
Drift was once implicated by Sewall Wright, a famous evolutionist, in his well-known “shifting balance theory of evolution“, which maintained that drift was essential in producing many adaptations in nature. That theory was once influential, but has now fallen out of favor, and I take credit for some of that (see my collaborative critiques here and here).
Related to this are various theories that see genetic drift and its maladaptive effects as crucial in forming new species (e.g., the “founder-flush” theory of speciation). In my book with Allen Orr, Speciation, we analyze these ideas in chapter 11 and conclude that drift has been of minimal importance in speciation compared to natural selection.
Finally, genetic drift was an important part of Steve Gould’s theory of punctuated equilibrium, for it was the force that allowed isolated populations to undergo random phenotypic change, tumbling them from one face of “Galton’s polyhedron” to another. This was one of the explanations for why change in the fossil record was jerky. Well, the fossil record may well be punctuated, but Gould’s theoretical explanation was pretty soundly dismantled by population geneticists, including several of my Chicago colleagues (see this important critique).
While one can cite examples of genetic drift operating in nature, like the expected loss of genetic variation in very small populations, in my view it hasn’t been of much importance in speciation, morphological and physiological evolution, or in facilitating adaptive evolution by pushing populations through “adaptive valleys.” Even the view that it has made species vulnerable to extinction by reducing the pool of genetic variation needed to adapt to environmental change has been exaggerated. I know of no extinctions caused by genetic drift, though I haven’t checked on the cheetah example lately (they were said to be highly inbred because of small populations, but I’m not sure that this is what makes them vulnerable to extinction). In fact, for conservation purposes, I believe the importance of loss of genetic variation through drift has been much less than the importance of reduced population size itself that makes populations vulnerable to extinction because individuals can’t find mates or overgraze their environment, or simply because if you’re a small population, random fluctuations in numbers are more likely to make you go extinct. This is demographic rather than genetically based extinction.
But drift has been important in molecular evolution, causing a turnover of gene variants over long periods of time. If those variants are “neutral”—that is, they are equivalent in their response to natural selection—then they will turn over at a roughly linear rate with time, and the changes can be used as a sort of “molecular clock” to estimate divergence times between species. This kind of molecular divergence has been used to construct family trees of species as well as to estimate the times when species diverged. This is a fairly new usage, for such molecular tools and estimates have been available only since the 1960s.
On to the New Scientist bit about drift in its latest issue, a special on evolution.
The 13-point section about how new findings will expand our understanding of evolution includes section 9 about drift, called “Survival of the luckiest.” It first recounts, accurately, how drift operates, but then exaggerates its importance by mentioning two studies of urban populations of animals, populations that in principle should show more drift than wild populations because populations living in cities are small and fragmented. The section says nothing about any of the things I just told you, which is what evolutionists have really been concerned about with respect to genetic drift.
Here’s the entirety of how New Scientist says drift is revising our view of evolution (the author of this section is Colin Barass):
Biologists have known about genetic drift for a century, but in recent years they realised that it could be especially common in urban settings where roads and buildings tend to isolate organisms into small populations. A 2016 study of the white-footed mouse, Peromyscus leucopus, in New York supported the idea. Jason Munshi-South at Fordham University, New York, and his colleagues discovered that urban populations have lost as much as half of their genetic diversity compared with rural populations.
Last year, Lindsay Miles at the University of Toronto Mississauga, Canada, and her colleagues published a review of evidence from about 160 studies of evolution in urban environments, in organisms ranging from mammals and birds to insects and plants. Almost two-thirds of the studies reported reduced genetic diversity compared with rural counterparts, leading the researchers to conclude that genetic drift must have played a role. “Genetic drift can definitely be a significant driver of evolution,” says Miles.
These findings have big implications, because populations lose their ability to adapt and thrive if they lack genetic diversity for natural selection to work on. Of course, genetic drift isn’t confined to urban settings, but given how much urbanisation is expected to grow, the extra threat it poses to wildlife is concerning. It highlights the need to create green corridors so that animals and plants don’t become isolated into ever-smaller populations.
I don’t think those findings do have “big implications”, because the important of reduced genetic variation in urban environments is unclear, particularly when the genes assayed have no clear connection with natural selection. And the import of losing half of your genetic diversity is also questionable: after all, a single fertilize female contains half of the “heritability” of an entire population. Everything rests on whether evolution by natural selection depends on very low-frequency genetic variants, present only in big populations, and we don’t really know if this is the case. And the above study is in white-footed mice, only one species among millions, and only for populations in urban environments. That’s not to denigrate it, just to point out that its relevance to nonurban nature is unclear and its relevance to evolution is equally unclear.
You can read the Miles et al. study at the link (here), and having read it, I wasn’t impressed, since the authors themselves don’t come to nearly as strong a conclusion as does New Scientist. Here’s from the paper’s conclusions:
Although our review of the literature with quantitative analyses of published urban population genetic data sets demonstrates trends towards increased genetic drift and reduced gene flow, these patterns were not significant and were not universally seen across taxa. In fact, over a third of published studies show no negative effects of urbanization on genetic diversity and differentiation, including studies supporting urban facilitation models at a much higher proportion than previously realized. How populations and species respond to urbanization clearly depends on the natural history of the taxa investigated, the number and location of cities being sampled, and the molecular techniques used to characterize population genetic structure.
In other words, although two-thirds of the studies showed reduced variation or increased inter-population differentiation, these patterns were not significantly different from non-urban populations. And if those differences were not significant, you needn’t start speculating about genetic drift. The authors conclude simply that different species show different genetic patterns when living in urban environments.
Miles’s statement that “genetic drift can definitely be a significant driver of evolution” is ambiguous, because she doesn’t say what she means by “significant” or by “evolution” (is she talking just about patterns of molecular evolution, like genetic diversity, or other types of evolution?)
New Scientist, in other words, fails to make the case that genetic drift has changed our view of how evolution operates, much less that it’s modified the modern synthetic theory of evolution. We already knew that small populations lose genetic variation because of genetic drift, and that’s been standard lore for decades. The real novel claims about drift—that it facilitates adaptive evolution, that it’s an important driver of speciation, and that it explains punctuated patterns in the fossil record—have disappeared because of the absence of both data and theory supporting those claims.
I am weary of going after New Scientist, and this may be my last critique of that issue. But be aware that virtually every one of the other nine points is exaggerated as well. Move along folks—nothing to see here.
Sub
I remember hearing that all living cheetahs were so close genetically that skin taken from from any one and grafted on any other would not be rejected by the recipient. If true, that’s pretty amazing. I think they went on to speculate that the world’s cheetah population had passed through a recent bottleneck of only 100 individuals.
I read the Miles et al. meta analysis and I thought it was pretty good. Genetic drift and gene flow are not opposite sides of the same coin, and they have different effects on population biology (here I’m just summarizing for other readers, not telling Jerry what he knows better than I do). Miles et al. did try to distinguish those effects, which is something that lots of papers in this area of landscape genetics fail to do (they tend to be totally focused on gene flow, not on effective population size).
The problem is that, in the papers Miles et al. are summarizing, the population genetic model in most cases doesn’t include two separate parameters for gene flow and for effective population size. In most of the papers in that database, there is just one population model parameter and it combines the effects of both gene flow (or isolation) and effective population size (and genetic drift). This is the Fst parameter or something akin to it, for any of the biologists here who might remember that.
Miles et al. try to get around this by analyzing several different summary statistics from each of the studies, and arguing that some of these summary statistics are more likely to be affected by drift or by gene flow, but none of them directly separate the effects of genetic drift vs. gene flow. There are other population genetic models (including good ones by Jody Hey’s lab group, whom Jerry gave a shout-out to here a few days ago), but those models were not used in the papers analyzed by Miles et al.
So I thought the Miles paper was pretty good, but the New Scientist article totally over-hyped its conclusions in a way that Miles et al. did not over-hype themselves.
Also wanted to say thanks to our host for putting up these posts. They’re super interesting, but I can imagine it’s wearying to find all this hype instead of substance in the reporting. I am holding out hope for one more post on niche construction!
What’s annoying about this is that there are so many of these topics that, I think, would be interesting to readers even if they just got it right.
PCC you said something about the social sciences a long while back which has stuck with me, and which I think is generally applicable to this situation and the hard sciences as well as many other situations: every new academic (read here: popular science journal) is striving to come up with a finding that is both surprising and true. The problem is, most of the surprising findings aren’t true, while most of the true findings aren’t surprising.
I think they run afoul of the whole “man bites dog” problem, wherein only things that SEEM or PRETEND to be radical and unexpected catch people’s attention. Click-bait seems to be only the latest form of this. Thus, people become terrified of flying in ridiculously safe airlines while tens of thousands of motorists die and many more are injured constantly, but people find the latter information simply boring. There ends up being a kind of natural selection of articles, even in scientific journals.
People are weird.
I must express my appreciation for PCC(E)’s hard work in dissecting and exposing the very thin anatomy of New Scientist’s thesis.
I do hope that incensed readers bombard the mag with multiple copies of the last few posts. Can even NS carry on ignoring the evidence?
I will have to reread this post to check for understanding but I guess one thing I’m hung up on is the issue of urban environments. Would they be that much different than any other ecosystem with populations that get isolated, like an island or a region undergoing desertification, or any number of geological alterations to the landscape? Limited or isolated populations with little to no new influx of individuals and an environment that is less amiable to those stuck there and reduced or different inter- and intraspecific competition, so what makes urban areas special here? Or maybe I’m just stuck on the wrong details. As always, I reserve the right to be wrong or to have completely missed the point.
I especially appreciate the 7 paragraph summary of genetic drift to set the stage for the dissection of the claims in the New Scientist. I find that background and historical information to be most helpful.
I have come to be fairly impressed with drift in how it may influence things like the large amounts of junk DNA in eukaryote genomes, and possibly the unnecessary complexity of cellular machinery. That being described as “constructive neutral evolution”.
But I guess they missed completely the really sexy bits of neutral evolution, and focused on … inbreeding in urban mice.
My elementary understanding of neutral drift is that it acts on the same sort of mutations or variations that natural selection would act on. The difference is that a neutral allele doesn’t get selected for or against, but drift can result in the frequency of that allele going up or down just by chance. It is not an alternate source of variation, which is how they seem to be describing it.
Am I wrong to think that any variant will be selected for or against if it makes a difference, and will drift if it is nearly neutral?
Sounds to me like more “writing to the theses”. That is they found an idea that makes a good headline, then played wordsmith long enough to come up with a plausible sounding justification.
That’s a kind of yellow (science) journalism.
I got the magazine and read the article. I learned a lot more here. Each item is described in gushing prose, but in many cases they finish by sheepishly admitting a lack of evidence or that the claim is disputed. Basically, it is a print version of clickbait. I would like to hear what you think about natural induction and evolvability.
Thanks for writing about this article.
I enjoyed all your 4 analyses of the New Scientist issue. I wish I could write like that.
One comment:
Meiotic drive seems a type of natural selection, operating at the level of genes/alleles; and stage specific, operating at that stage in the life cycle of a sexual species when gametes are formed.
Natural selection operates in diverse ways at all stages of a life cycle, for example survivorship/viability selection driven by differential mortality between birth and reproduction; or sexual selection at the stage when females & males come together to make babies in sexual species; or selection because of gametic compatibilty; or fecundity selection.
There is a tendency to equate natural selection with viability/survivorship selection as an evolutionary force, but then one would have to recognize all these other stage-specific types of selection that I list above also as separate evolutionary forces. Not sure we are ready to do that, and it seems simpler to recognize all these stage-specific types of selection as special cases of selection subsumed under natural selection.
Kudos to Jerry, but I don’t think that one needs to argue the unimportance of genetic drift in evolution to debunk the New Scientist article. If some click-seeking
magazine site says something like “Latest news: there is genetic drift, and population genetics has to take it into account!” that should set off a chorus of yawns. Not such hot news — genetic drift was assimilated into population genetics starting 100 years ago, by Sewall Wright and company. If people find its presence to be a revelation, then that is an indictment of the teaching of high school biology, and there is no reason to expect an evolutionary biologist to be interested at all. It reminds me of the old headline from a Mad magazine parody of Ridley’s Believe It or Not: “Contrary to popular opinion, bats are nearly blind!”
Oops, make that Ripley’s Believe It or Not.
Jerry, thanks for another informative science post!
The links to the Miles study and the 2016 P. leucopus study direct to a U of C login page.
I would like to hear your opinion on the work of Michael Lynch (an evolutionary biologist from ASU). His work really emphasises the role of drift in evolution. For example the drift-barrier hypothesis (https://www.nature.com/articles/nrg.2016.104) which I find quite compelling states “that natural selection primarily operates to improve replication fidelity, with the ultimate limits to what can be achieved set by the power of random genetic drift”. In that paper Lynch et. al argue that the mutation rate will scale negatively with the effective population size, because small populations are more prone to accumulate deleterious mutations than large ones.
The mutational hazard hypothesis is also an interesting one (https://science.sciencemag.org/content/302/5649/1401.abstract), so is the idea of a passive emergence of biological complexity (https://www.pnas.org/content/pnas/104/suppl_1/8597.full.pdf).
I think these ideas show that genetic drift is very important force and sadly an underrated one by most biologist.
I’d also appreciate your (Jerry’s) response to Larry Moran’s commentary on this post at https://sandwalk.blogspot.com/2020/10/on-importance-of-random-genetic-drift.html