Yesterday the top science journal Nature published a bomb-shell article, but my feeling is that biologists haven’t yet realized the explosive nature of the report. I’ll explain, but first we need to make a lengthy excurse into the history of the group selection idea.
Whether group selection is an important evolutionary force, or not, is a highly controversial question in evolutionary science. A substantial proportion of evolutionary scientists still think that it is not. The stakes are high because I and many other proponents of Cultural Evolution think that group selection (or, as we prefer to call it, multilevel selection—selection acting simultaneously on individuals and groups) provides the key to our understanding of the evolution of human ultrasociality—the capacity of human beings to cohere and cooperate in huge societies (millions and more of people).
Actually, the best theory that enables us to understand ultrasociality is “Cultural Multilevel Selection” (see my book Ultrasociety about how human societies evolved from small bands of hunter-gatherers of 10,000 years ago to the huge megasocieties of today by the process of multilevel selection acting on cultural traits). I tend to agree, to a certain degree, with the critics that genetic group selection is not a commonly encountered evolutionary mechanism in the field, although in the past it was clearly hugely influential. Because how would, otherwise, we get multicellular organisms? And genetic multilevel selection provides the best explanation, in my opinion, of other “major evolutionary transitions”, which include, in addition to multicellular organisms, such epochal events in biological evolution as the rise of the eukaryotic cell and social insect colonies.
The first proponent of genetic group selection was Charles Darwin himself, who wrote about it in his second major book, The Descent of Man and Selection in Relation to Sex, which was published 12 years after On the Origin of Species. Darwin’s ideas on group selection remained very influential for about a century. One of his well-known followers, the English biologist V. C. Wynn-Edwards, became an advocate of the idea that individual behaviors can evolve not just because they help individuals, but for the good of the species as a whole. In other words, Wynn-Edwards and other adherents of what later became known as “naïve group selectionism” thought that evolution could operate at the level of really large groups—whole species.
I attach the label “naïve” to these views because arguments of Wynn-Edward and others relied on simply pointing out the importance of a trait at the level of a group (or a species) without considering carefully how such traits affected individuals. Incidentally, this is why I prefer “multilevel”—because our current theories explain much better the evolution of behaviors that have opposite effects at different levels (e.g., favor the group but at the expense of individual fitness). It’s not a trivial question, and it requires non-trivial math to figure out when either the group-level or individual level force dominates.
In any case, by the 1970s the tide turned against group selection. The key thinker in this reversal was George C. Williams, and his ideas were popularized by Richard Dawkins in his wildly popular book The Selfish Gene. When I studied biology in graduate school in the early 1980s, nobody believed in group selection, apart from a few “heretics” like my good colleague and friend David Sloan Wilson.
In the last decade or so the tide started turning back. In particular, the hugely influential social biologist Edward O. Wilson (no relation to David) “flipped” from a critic to an adherent of group selection. But many, if not most, continue to reject it (for example, the geneticist Jerry Coyne or the evolutionary psychologist Steven Pinker).
Getting back to Darwin, it is not a coincidence that his second major book, in which he wrote about group selection, has the subtitle Selection in Relation to Sex. Here’s a fun example illustrating this connection, which I have recently used in a lecture in my cultural evolution class last week.
Many of my readers know that there was a species of deer in which males grew astoundingly huge antlers—the Irish Elk. It roamed Eurasia from Ireland to China during the Pleistocene, and went extinct around 8,000 years ago. Why did Irish Elk males sport such spectacular antlers?
The reason, as Darwin explained, is the sexual selection. Every fall male deer (including elk and moose) participate in jousts, with winners getting to mate with females. The larger is the weaponry that you bring to the contest, the better are your chances of winning it, which means mating and passing your genes to the next generation. So natural selection favors males with larger antlers (as they say, “size matters”).
Note “larger”, not huge. What’s important is not an absolute size, but a relative advantage. In the land of small-antlered, the medium-antlered elk is king. But then, in a few generations, everybody has medium-sized antlers, and so to get ahead of the crowd you need large ones. And so on. As the arms race continues, eventually only those elk with gigantic racks have any chance of reproducing. Growing huge antlers is energetically expensive, and a huge risk—whether getting tangled in the branches, or not being able to escape predators due to their heavy weight.
If only elk males could get together and agree to put a limit on the size of their antlers… Everybody would be much better off. Instead, each individual strives for advantage, resulting in a collectively suboptimal outcome. One could say that the Irish Elk went extinct as they literally collapsed under the weight of their antlers.
We don’t really know why the Irish Elk went extinct. Perhaps the reason was the run-away competition between males, resulting in unsustainably gigantic antlers. Or there could be another reason. And here’s why the Nature article, to which I referred in the beginning, is so interesting: High male sexual investment as a driver of extinction in fossil ostracods by Maria João Fernandes Martins and co-authors.
Ostracods are small crustaceans, shrimp-like creatures that protect themselves with bivalve carapaces (shells). These shells are well preserved as fossils, and so we know quite a lot about their evolution.
Fernandes Martins and co-authors analyzed the paleontological data on 93 species that lived in the area that is now Mississippi state between 84 and 66 million years ago. They assessed the strength of competition between males by how much males invested in reproduction, which you can tell by the shape of the shell (e.g., how long it is). Basically what they did was similar to estimating the strength of between-male competition in deer by looking at how large their antlers are. Then they did a statistical analysis on how the strength of sexual competition affected the probability of extinction of the species.
They found that the probability of extinction of the species in which males competed most intensely was ten times higher than in the species in which males did not compete very hard. This is a huge difference, and it lends credence to the idea that the Irish Elk went extinct because of intense between-male competition necessitating high investment by males into growing big antlers.
The implications of the study by Fernandes Martins and co-authors, thus, go far beyond an obscure group of shrimp-like organisms. What we have here is a clear example of multi-level selection. The individual level selection forces each male to invest into sexual competition as much as possible. But at the species level, those species in which between-male competition goes too far, has an order of magnitude higher chance of going extinct. As a result, most species find themselves at an intermediate level of male sexual investment.
It’s interesting that the authors of the Nature article do not even mention multi-level selection, nor do they talk about the broad implications of their study, as I have done in this blog. They are either not aware of them, which seems unlikely, or they simply didn’t want to enter the highly contentious debate about group selection. Equally interesting, Nature apparently did not deem this study to be important enough to devote a News-and-Views article to it. But I think that the implications of the study are explosive. If Wynne-Edwards were alive, he would feel vindicated.