"Plus ca Change" and the Importance of Going Extinct

Derek Turner writes...

According to the usual story, environmental change drives evolutionary change in populations. Any biological population will contain heritable variation. When environmental conditions change, any individuals in the population with traits that confer even a slight edge in the new conditions will tend to have more offspring. Over time, the frequency of the advantageous traits will increase. That’s how natural selection works.

On the other hand, suppose that the environment remains stable for a long period. What would happen then? You might expect natural selection to “push” the population towards something like an optimum. Or using a common metaphor, you might say that natural selection will push the population up towards a peak in the fitness landscape. Once the population reaches the peak, any further changes will tend to decrease fitness, because there’s nowhere to go but downslope. Therefore, when the environment is stable, selection will tend to keep the population hovering in the neighborhood of the optimum.

It all sounds so simple. Environmental change means evolutionary change. Environmental stability is like resting at the end of an uphill journey in the fitness landscape. Of course this picture involves a lot of idealization: natural selection isn’t the only thing that can affect a population. For example, you also have to consider random drift, and you have to take development into account. And many organisms can move around. So for example, as ocean temperatures increase, lobster populations on the east coast of the U.S. are shifting northward rather than hanging around and adapting to warmer waters, which is good news for lobster trappers in Maine but bad news for those in Connecticut and Rhode Island.

Paleontology messes up this simple picture even more. For example, we know that there are some lineages—Darwin called them “living fossils”—that have persisted for many millions of years with little or no morphological change. And we know that environmental conditions have changed dramatically during the “lifespans” of these lineages.

In the 1990s, an Australian park ranger who was exploring the backcountry in Wollemi National Park, not too far from Sydney, collected some leaves from a tree that he found in a secluded ravine. It was one that he had never seen before. Later he showed the sample to a scientist friend, who noticed that they looked exactly like fossilized leaves from the mid-Cretaceous, some 90 million years ago, and is very similar to much older fossils from the Jurassic. It was the botanical equivalent of stumbling upon a pack of dinosaurs hanging out in some remote spot. Now you can purchase a Wollemi pine tree for your garden. (Actually, it’s not technically a pine at all, but a member of the Araucariaceae family.)

From the point of view of evolutionary theory, the really interesting thing about the Wollemi pine is that it has persisted for 90-100 million years without much visible change. But during that time, the world has seen a mass extinction event (66 million years ago), an almost complete turnover in the types of herbivores that might, for example, like to munch on Wollemi saplings, and huge temperature swings—the planet was much, much warmer during the Paleocene-eocene thermal maximum, 55 million years ago. So you have a case of evolutionary stability across massive changes in environmental conditions. What’s going on?

In the 1990s, a British paleontologist named Peter Sheldon developed a counterintuitive model for explaining evolutionary stasis.[i] I don’t know if his model gets things right, or even all that close to right, but it might, and it reminds us to try to stay open-minded about how evolution might work. Sheldon’s “plus ça change” model turns the traditional story about evolution upside down. Remember that in the traditional story, environmental change means evolutionary change. But what if, the more the environment changes, the more things stay the same? In other words, what if lots of environmental change makes for evolutionary stability? How could that be?

The trick to seeing how Sheldon’s model works is to shift the level of analysis a bit. Instead of thinking about how natural selection causes change within populations, think about differential extinction. (Paleontologists love to shift the level of analysis in this way.) When there is a lot of environmental turmoil, that could be good news for some species and bad news for others. Back in the nineteenth century, the American paleontologist Edward Drinker Cope (most famous for his bitter feud with his dinosaur-collecting nemesis, the Yale paleontologist O.C. Marsh), advanced a so-called “law of the unspecialized.” Cope thought that the ecological specialists of a later era are typically descended from the ecological generalists of earlier times. No one today would call this a “law,” but it might be an interesting pattern. Part of Cope’s thinking may have been that when there is some shock to the system, the ecological generalists are more likely to survive. Ecological specialization comes with a higher extinction risk. This is an old idea, but conservation biologists still rely on it in their efforts to assess threats to different species.[ii] And it is also the key to Sheldon’s “plus ça change” model: lots of environmental change means higher extinction rates for specialists, but generalists will tend to do better. So what you’d expect to see is a bunch of long-lived generalist species without much morphological change. The more things change, the more they stay the same.

I still have a lot of questions about Sheldon’s model. To start with, is it true that most long-lived, stable lineages, like the Wollemi pine, are generalists? Nor is it the only model that evolutionary biologists have for explaining stasis, so we also have to consider how well it stacks up against the alternatives. And in biology, everything is messy. Sheldon’s model might be the whole story about a few cases, a small part of the story about some other cases, and totally irrelevant to many other cases. Or maybe it’s not the whole story about any cases, but something of a factor in virtually every case. Who knows?

But the model affords a great illustration of how paleontologists often think about evolution. Paleontologists are not always completely satisfied with the textbook stories about how evolution works. Understanding evolutionary history may involve a shift of focus to the species level, where the question that really matters is: who goes extinct?

[i] Sheldon, P.R. (1996), “Plus ça change—a model for stasis and evolution in different environments,” Palaeogeography, Palaeoclimatology, Palaeoecology 127: 229-227.

[ii] Gallagher, A.J., Hammerschlag, N., Cooke, S.J., Costa, D.P., Irschik, D.J. (2015), “Evolutionary theory as a tool for predicting extinction risk,” Trends in Ecology and Evolution 30(2): 61-65.