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.