11 October 2017 by pencechp • permalink
I was struck the other day by a theme that Richard Lewontin pulls out in a long discussion of the concept of “adaptation” in Scientific American.1 Two common ways of talking about evolution, he argues, wind up embroiling us in a fallacious way of talking about the environment.
The first is a common way of talking about adaptations. Take the classic case of the peppered moths’ having adapted, twice, to color changes on tree trunks, first to a darker variant as pollution colored the tree bark, and then to a lighter variant as local industry cleaned up its act. One way we often talk about cases like this describes this change in terms of design problems. The environment set a “problem” that the moths needed to “solve” – increased predation on tree trunks as a result of a color mismatch. Those organisms that “solved” the problem better than their compatriots – in this case, by happening to vary in the direction of darker pigmentation – would increase their representation in future generations.
Despite its intuitive appeal, Lewontin argues that there’s a flaw here. Talking about the environment in this way seems to presuppose that there’s some fixed list of “design problems,” already out there in the world, with organisms running down a tick sheet doing their best to solve them. Put differently, in the language of contemporary ecology, there’s some fixed set of niches in the world, and organisms struggle to fill them. This, however, makes (at least) one crucial mistake. The design problems or niches would have to, somehow, exist ahead of time, out there in the environment, waiting on the right sorts of organisms to come along. And this, in turn, belies the fact that niches are dynamic objects, created by the interaction of organisms and environmental factors. Think again of the moths. It’s not as though “color mismatch” was some a priori essential problem that a moth would have to solve. Only in the context of (1) predators that worked on the basis of color vision, (2) pollution causing a change in the background color of tree trunks, and (3) the existence of enough genetic variability in the moth population to successfully respond to the selection pressure did we actually wind up with a “problem” and its “solution.”
The same may well be true, Lewontin argues, for some interpretations of the Red Queen Hypothesis. The RQH was offered by Leigh Van Valen as an explanation for what appears to be the constant probability of extinction of genera over time. This is a rather strange empirical result, Van Valen noted, because if organisms really are becoming better adapted as a result of evolutionary change, we should expect the probability of extinction to drop over time, as organisms steadily solve more and more of the design problems posed by their environments. The RQH explains away this apparent oddity by noting that the environment – both in a biotic and an abiotic sense – is constantly changing as well. Organisms, therefore, have to evolve just to be able to keep up with the constant pace of environmental change. Eventually they will no longer be able to keep up (say, by running out of available genetic variation), and will go extinct. (The name derives, of course, from Alice Through the Looking Glass, in which the Red Queen tells Alice that “it takes all the running you can do, to keep in the same place.”) But the RQH, as well, seems to consider the environment as an “external” factor – forever hovering just outside the reach of the organisms running to keep up with it.
In both of these cases, nothing prevents one from carefully modifying the theoretical concepts at issue to bring us back to a more dynamic, integrated picture of an organism and its environment. Some of the work needed to do this for the “design problem” picture has been done by Bouchard and Rosenberg, and the Red Queen Hypothesis just needs a more cautious presentation that depends less on the contentious idea of a “fitness landscape.”2 These aren’t, that is, fatal mistakes.
But this discussion of the environment as a dynamic structure reminded me of another project on which I’m working (with lab alumnus Daniel Swaim, now a PhD student at Penn), tracing the concept of the economy of nature (EoN) from Linnaeus to the present. For Linnaeus, precisely the function of the EoN is to describe the preexisting structure of the environment – laid down, in this instance, as part of the divine blueprint for creation. Interestingly, despite removing all of the theology, Darwin makes a surprising amount of use of the EoN in the Origin. Much of Darwin’s thinking about diversification derives from his view of the EoN as a background structure against which evolution takes place, with organisms competing for “places” in the EoN, and evolutionary novelty and innovation acting to “subdivide” places into ever finer gradations, permitting successively fiercer competition. For example, in his early Sketch of natural selection, written in 1842, he notes:
I need hardly observe that the slow and gradual appearance of new forms follows from our theory, for to form a new species, an old one must not only be plastic in its organization, becoming so probably from changes in the conditions of its existence, but a place in the natural economy of the district must
be made,come to exist, for the selection of some new modification of its structure, better fitted to the surrounding conditions than are the other individuals of the same or other species.3
Darwin even footnotes this sentence in his manuscript to add, “Better begin with this.”
The story gets more complex as we move forward in time – by the Origin, his use of the EoN has become more sophisticated, and we see the role of environmental structure taken over by genetic structure as we advance into the Modern Synthesis. Regardless, however, it seems as though the question of how to handle ecological structure in evolutionary theory is as old as the theory itself, and touches areas as disparate as adaptations and the Red Queen Hypothesis.
(For more on this, check out a preprint of our article, currently under review.)