30 August 2010

On competition, ecological opportunity, and Sahney et al.

ResearchBlogging.orgThere's already been a lot of blogospheric discussion of the BBC's recent declaration that "Darwin may have been wrong" based on a recently-published paleontology paper. I hadn't paid it much attention, because while sloppy science journalism irritates me, it's not quite in my wheelhouse, expertise-wise. Then I actually got around to reading the paper, and it turns out that it's directly related to some of my own work—and the conclusion that led to the sensationalistic sub-headline doesn't make any sense.

Coauthors Sahney, Benton, and Ferry analyze the fossil record of four-limbed vertebrates—tetrapods—to show that in general, as more species evolve, they also evolve to fill a wider variety of ecological roles [$a]. Ecological roles are here defined by combinations of body size, diet, and habitat. (Sahney et al estimate there are 207 such combinations possible, though only 75 are "occupied.") That's a straightforward and mostly unsurprising result—the number of tetrapod species increases as tetrapods evolve new ways to make a living. But then we get to the conclusions of the paper, and things get weird.

Sahney et al. conclude that because diversification is associated with finding unoccupied ecological roles, competition is mostly unimportant in the diversification of tetrapods: "Given the unrestricted access tetrapods have to ecospace, perhaps there is little need for competitive interactions to shape diversification." In other words, if diversification happens by finding ways to make a living that aren't already occupied, competition isn't important.

Except that the very reason species diversify following an ecological opportunity like the development of a new ecological role is the lack of competition the new role provides. As my coauthors and I documented in a recently published literature review, competition shapes the kind of diversification documented by Sahney et al. in two ways: first, by its absence following the evolution of a new lifestyle; then in spurring an adaptive radiation as new species evolve to partition up the newly-available "ecospace."

What makes this doubly odd is that Sahney et al. refer to another kind of ecological opportunity, the extinction of competitors, as a good example of competition-driven diversification. But a central insight of the literature on ecological opportunity is that diversifying because a whole bunch of ecological roles have just opened up is not fundamentally different from diversifying after a new mutation makes a never-before-seen ecological role possible. Think of it like starting a new business: to avoid competition, you could either sell an existing product in a place where no one else sells that product, or you can invent a product no one else offers. Both approaches give you a market all to yourself, and both are defined by competition.

It's hard for me to understand why Sahney et al. don't make this conceptual connection—which, for what it's worth, has its roots in The Origin of Species.


Sahney, S., Benton, M., & Ferry, P. (2010). Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters, 6 (4), 544-7 DOI: 10.1098/rsbl.2009.1024

Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J., Schenk, J.J., Spear, S.F., & Harmon, L.J. (2010). Ecological opportunity and the origin of adaptive radiations Journal of Evolutionary Biology, 23 (8), 1581-96 DOI: 10.1111/j.1420-9101.2010.02029.x

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