Plus ça change — a model for stasis and evolution in different environments

Peter Sheldon. Palaeogeography, Palaeoclimatology, Palaeoecology 127: 209-227. Plus ça change — a model for stasis and evolution in different environments

Storm of the Bastille - plus ça change? From Wikimedia (unknown artist)

Storm of the Bastille – plus ça change? From Wikimedia (unknown artist)


Lynsey McInnes

Lynsey McInnes

Continuing our choosing-classics strand of PEGE, I chose this paper after reading it years ago and remembering it now as impressively daring. I’ve got a soft spot for discursive papers, where the authors are not scared to be a bit radical and talk their way through an argument, throwing caution and data to the winds.

Rereading the paper this week, I knew I was on to a good thing as Sheldon starts with a quote from Levin about scale:  ‘the problem of relating phenomena across scales
is the central problem in biology.’ And a consideration of scale is one of the issues that has popped up in many PEGE posts this year. Since this paper, there has been tons of literature produced for and against punctuated equilibrium, see the great piece by Pennell et al just published in TREE sorting the whole jumble out, but Sheldon, here, provides, to my mind, a very even handed treatment of what you can, and cannot, hope to ascertain from the extremely patchy fossil record stretching from biases in perception, the links between micro- and macroevolution to emergent macroecological patterns (and much inbetween).

Temporal scale…stating the obvious, we might think patterns are a mix of punctuated bursts and stasis from our contemporary view, but they are actually pretty damn gradual.

Spatial scale…let’s think about the environments lineages are persisting through when deciding whether there is stasis, gradual or bursts of evolution.

I’m realising more and more that I am most grounded in macroecology – however much I ‘want’ to be an evolutionary biologist or a population geneticist. So, I really appreciated Sheldon cutting to the chase on the processes that might generate high tropical diversity (specialist species, easier to speciate, gain some ecological distance and persist as a ‘good’ species, rather than generalists populating (on land) temperate areas, where the generalist ancestral phenotype works best, swallowing up precocious young species trying to match themselves to every last environmental fluctuation (excuse the gross anthropomorphisms). He just states as obvious the expected broad-scale effects of abiotic factors and briefly mentions higher expected impacts of biotic interactions among specialist species and other factors that have been discussed to death in the ensuing two decades of macroecological research. He touches on my pet topic intraspecific variation, although he goes on to suggest (I think) that locally adapted populations responding to broad-scale environmental change could lead to punctuated bursts of evolution (or at least the signal of such), something I’m not too sure about.

I also wonder how his thoughts on the effect of contemporary climate change and evoluationary responses to it were taken at the time of publication. We are so used to these ideas now, but were they radical then? Not sure. I loved that he matter of factedly states that predicting species’ responses is going to be exceedingly difficult.

I’ve written this post in a rush and I’ve realised it’s pretty thin on the ground in terms of actual commentary – my lasting impression of this paper is being awed by Sheldon’s ability to cut to the chase across a range of fields from biases in the fossil record to drivers of species’ diversity. If I had more time, I’d like to go through his conjectures with a fine-toothed comb to see which have stood the test of time and the ravages of ‘proper’ analysis. My hunch is quite a few. Not least the idea that geological timescales are just really long versions of ecological timescales, this can be interpreted in various ways – at the most basic – generalists do better – across timescales – in fluctuating environments.

In short, this paper is well worth a read, if for no other reason that the multitude of brilliant metaphors…pullovers, human rebellion, loud sneezes.


Will Pearse

Will Pearse

There are a number of really cool ideas in here that really spoke to me, and it’s been quite interesting to imagine the impact this paper had on a younger Lynsey! I’m afraid I’m not going to focus on the main thrust of the paper, not because I don’t like it, but because I got wildly over-excited about one aspect of the paper.

A racemose phylogeny (look here if you’re not a plant person) is a  phylogeny with lots of bristly, transient off-shoots that die out quite quickly (it’s attributed to Williams), and it immediately brings to my mind that first phylogeny Darwin drew. People get very excited at the idea that particular sub-populations of a species can act so differently; if we all talked about raceme phylogenies and how our definition of species is somewhat arbitrary a little more explicitly (and not just when we’re leading that Biology 101 class), I think we wouldn’t be so surprised. Species are collections of populations, always budding off one-another and then re-joining the main body. This got me thinking: what would our expectations of trait evolution look like if we accepted a raceme where species are constantly being born and die, and each separate raceme spike has a slightly different trait? Remember that these tiny, off-shoot branches are probably never truly lost, and maybe they just act as repositories of genetic diversity that get pulled back into the main population.

I have never been sure what an evolutionary response over geological time looks like. I think of evolution as the outcome of lots of ecology over lots of time, and as such I have always found it hard to imagine the outcome of evolution as anything more than the emergent property of ecology. But when coordinated with the raceme ideas above, I think I finally see it. Geological time is like the mother of all ecological storage effects – perhaps species and traits that are (maybe only slightly) mal-adaptive now can survive over longer periods of time (perhaps in the tips of these racemes…) until they are useful later, and then those traits come to dominate. Thus the species survives through these stored pools of variation, in a constant state of flux, and yet somehow appearing the same. Plus ça change.

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About will.pearse
Ecology / evolutionary biologist

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