You may have seen a small flurry of reports this week about a science paper showing that “Darwin was wrong.” The paper wasn’t a creationist or ID screed, however—it was a paper in a good science journal (Biology Letters) by a crack team of paleontologists from the UK and Canada (Sarda Sahney, Michael Benton, and Paul Ferry). What did the paper say? Did it really show that Darwin was wrong? I’m here to answer your questions.
What did the paper say? It reported a correlation in the fossil record between the number of tetrapods (four-legged vertebrates) existing at different times and the number of ecological ” modes of life” those species adopted, all over the 400-million-year period since vertebrates colonized the land. To be exact, it divided up that time period into 66 sub-periods, and in each sub-period the authors totted up the number of tetrapod families that were represented by fossils and the number of modes of life they adopted. Here’s the time plot showing that, as the families diversified exponentially over this period, so did the number of modes of life adopted, with the changes almost in lockstep:
What do they mean by a “mode of life”? The authors defined a “mode of life” as the ecology of a species that fit into one of three body size categories (length less than 15 cm, between 15 and 150 cm, and greater than 150 cm), one of 16 diet categories (browser, nectar, molluscs, carrion, and so on), and one of 6 habitat categories (marine, arboreal, subterranean, and so on). This gave 288 potential modes of life (3 x 16 x 6), only 75 of which were actually seen.
If you simply plot, among all 66 time periods, a graph of the number of families existing at a given time with the number of modes of life they occupied during that period, you see a very tight correlation:
What does this correlation mean? The authors interpret the tight fit between biodiversity (number of families) and ecological diversity (number of modes of life seen among those families) to mean that what drove tetrapod diversity over this period was open niche space: ecological opportunities that had not yet been realized. They oppose this to another explanation that, they say, their data did not support: diversification was driven not by the availability of ecological space, but by competitiion. The competition theory would, say the authors, predict that as organisms began to lose elbow room, they would simply subdivide their already-occupied “modes of life” into finer ones. If competition drove diversification, then, you wouldn’t see such a tight correlation between modes of life and diversity.
Is this interpretation correct? I’m not so sure. The problem is that it might not be possible to separate the “force” of competition from the selective pressure to occupy new niches. After all, animals may be driven to adopt new modes of life by competition itself. The occupation of the land by ancestral fish may, for example, have been the result of selection to reduce competition for prey by finding a nice new place with lots of prey (insects) and fewer competititors. I don’t think that showing a correlation between “modes of life” and “number of families” tells us that competition did not play a big role in driving that diversity. In other words, I am not convinced that, at least from the fossil data, you can separate competition from ecological opportunity.
Also, it’s possible that some of the correlation is an artifact. It may be—and I’m not sure of this because I’m not a paleobiologist—that different taxonomic families are partially recognized by large differences in characters like body size and adaptations to diet or habitat. In that case you only get a new family when there’s a sufficiently large difference in what we’d recognize as a “mode of life.” This would be true regardless of the evolutionary force driving the difference. And to the extent that this is the case, it devalues the correlation as a way to recognize process.
This doesn’t mean the paper is bad. Far from it—it’s a very good (and laborious!) correlation between ecology and diversity, and the correlations between them do demand explanation. I’m just not sure if the authors’ answer is the right one.
Where did the “Darwin was wrong” stuff come from? It comes from the notion that Darwin saw competition as a major cause of ecological diversity. There is some justification for this: in The Origin, for example, the only figure (the famous “tree of life”) shows an increase in diversity over time, with Darwin attributing this to competition between species for niche space. His “principle of divergence” maintained that organisms inhabiting a small area would always be competing with each other, and would benefit by seizing on slightly different niches to reduce that competition. A grass, for example, might inhabit soils of different moisture content to avoid competing for space with other grasses.
We shouldn’t think, though, that Darwin saw competition as the overweening force in promoting biological diversity. In The Origin he adduces other factors, including simple adaptation to physical factors (“a plant on the edge of a desert is said to struggle for life against the drought”), and to biotic features like predators and parasites. I think that Darwin may have seen biotic factors as a whole (which include but are not limited to competition), as important drivers of diversity, and emphasized them so that his readers would see that natural selection didn’t solely promote responses to the physical, non-biotic environment.
So did the paper show that Darwin was wrong? Hardly. As I said above, I don’t think the authors ruled out competition as an important force in ecological diversity. Indeed, you could almost construe the data as supporting Darwin, who emphasized throughout The Origin that the more different species were, the better their chances of leaving descendants. (See pp. 111-125 of the first edition of The Origin for this view.) He emphasizes, for instance, that plants have a better chance of invading a new area if they were already quite different from the species that were already there.
But of course Darwin was wrong about many things. Nobody pretends that the man was a god, or omniscient. He was dead wrong, for instance, about genetics. We know a lot more about biology now than we did in Darwin’s time, and we can see that his ideas were often incomplete or incorrect. So it’s bizarre to see every modern discovery through a lens of either supporting or refuting his ideas. If we did that, every paper in genetics could be sold to science journalists as showing that Darwin was wrong about inheritance! We’ve moved on. It’s amazing how right Darwin was—that’s one of the reasons he’s a hero to many of us—but, like all scientists, his ideas get supplanted and revised.
Why did the press sell the paper this way? Hype, pure and simple. A paper on taxonomic diversity over time gets a lot more interest if it can be said to disprove Darwin. I suppose there’s some residual anti-evolution or anti-Darwin sentiment in all this. Here are some of the headlines about the Sahney et al. paper:
And, of course, the good old HuffPo:
Who’s responsible for this hype? I’d like to think it was just the press, for they always love a controversy. But I’m curious how all these different journalists managed to hit on the same Darwin-was-wrong hook. Are a lot of science journalists really conversant enough with Darwin to immediately and independently see a Biology Letters paper as refuting his ideas? Well, maybe one of them did it and was copied by the others. But I wonder whether the authors, or the authors’ universities, or even the journal, issued a press release that sold the paper as a “Darwin refuter.” I posted a query to this effect on Sahney’s website (she’s the first author), but my query hasn’t show up.
Sahney, S., M. J. Benton, and P. A. Ferry. 2010. Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters 6:544-547. doi:10.1098/rsbl.2009.1024
h/t: David Reznick, for Darwin discussion