Mammalian evolution and the Great American Interchange

Marshall et al. Science 215(4538): 1351-1357. Mammalian evolution and the Great American Interchange

Another kind of great American interchange - the ill-fated I-10 Papago Freeway's hellicoil that never was.  Taken from the US Federal Highway Administration.

Another kind of great American interchange – the ill-fated I-10 Papago Freeway’s hellicoil that never was. Taken from the US Federal Highway Administration.

Lynsey McInnes

Lynsey McInnes

Not quite sure where to start this week. What a great story! And a great textbook example of so many biogeographic and macroevolutionary phenomena. Marshall and Co. outline the evidence for the Great American Biotic Interchange (GABI) framed in terms of MacArthur and Wilson’s equilibrium theory of island biogeography and it sounds so neat, so tidy, almost unreal! We’ve got two different dynamic equilibria, we have different sized source faunas, we’ve got tropical and temperate fauna, we’ve got replicated patterns at different taxonomic patterns, we’ve even got an emergent uber trait for the North American fauna that enables them to infiltrate the niches of South American mammals (gotta have something inexplicable, right?).

The above sounds like a highlights summary of any recent macroecology journal. And this paper was published 32 years ago. Ouf. Sure the stats have moved on, but the patterns and conclusions. alongside the inexplicable bits really haven’t, have they?

I had two main thoughts when reading this paper. Number one was that we haven’t moved on much in the insights we are having on broad scale patterns in biodiversity. Which, in some ways, is totally fine, its not like the patterns have changed. We’ve just reinvented the wheel, sliced the cake thinner and topped it with fancy stats.

The second thought was, for me, more frustrating! Over the past couple of years, my intuitive belief in dynamic equilibria and carrying capacities and ecological limits to diversity has been thoroughly shaken. I was coming to the conclusion that these were patterns we were finding in our data because they are neat and tidy and clever and merge beautifully ecological and evolutionary time scales. My opinion was shifting that these patterns might emerge by chance and that regions are not closed enough to reach equilibrium, that biotic interactions mattered more than we gave them credit for and (meta)populations not biogeographic ranges were the appropriate units to look for the processes underlying biogeographic scale patterns, that niche construction mattered, that we were missing tons of important features in our quest to understand biodiversity patterns.

And then a paper like this comes along, and, in theory, it should do nothing to my emerging mindset as all of the above arguments could, more or less, be applied to Marshall and Co’s dataset and reasoning. But somehow the simplicity of the analyses, the merging of masses of paleo data with the explicit linking to M&W’s equilibrial theory has set me back to square one (maybe square two) in buying into equilibria in macroevolution. Wah.

Where to go from here? What data would complement what was already compiled here? What contemporary data could be added? What experiments could be conducted? What stats would we like to apply? What was the elusive North American trait?

Will Pearse

Will Pearse

I was really struck by how few papers tell as a coherent a story as this. It really is almost like a story, with remarkably little in the way of obfuscated statistics and graphs to obscure the general take-home message. In re-reading this, I think I’ve even detected a beginning, middle, and end!

I’m no expert in this field, but I was somewhat surprised how well these methods have stood the test of time. Yes, there are some who would claim rarefaction methods are outdated, but I would hope the signal in these data would overpower methodological quibbles. Equally, the equilibrium models employed here are simplistic – more sophisticated models of carrying capacities I’m sure could now be fit – but their only purpose is to demonstrate that something else is going on, and they do that very well.

What is it about North American species that makes them speciate so much more? The authors seem to come down on the side of some meta-trait, some family-level inheritance of speciation ability. I’m not someone who believes strongly in the concept of taxonomic units as real biological things (although see this, and Aelys does some great work on higher evolutionary units), but it seems there is something about these clades. Perhaps it’s related to how the South American groups have already equilibrated to a particular level of diversity – maybe there’s some kind of genetic inertia and traits associated with dispersal into a new continent give recently-moved species a radiating edge. I’m sure, somewhere, Ricklefs is screaming “parasite load” at the top of his lungs, and maybe there’s something to that. Perhaps it’s because competition is only as fierce as your competitors; the North American lineages had to fight particularly hard (more diversity crammed in? Evolutionary fluke?) and so they won out.

One thing I am certain about is that we need more stories in biogeography and evolution in general. Evolution is an inherently historical science – that doesn’t mean we can’t do comparative analyses, but it does mean that we should be a little more understanding that regressions aren’t everything. Species distributions are not stationary; different processes are operating across different spatial and  temporal scales, and there are far too many idiosyncratic events like the Great American Interchange for us to simply sweep them under the rug. More stories, fewer stats, please. I never thought I’d write that!


About will.pearse
Ecology / evolutionary biologist

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