All I Really Need to Know, I Learned from Aphids

In our lab, we throw around the terms “eco-evolutionary dynamics” or “eco-evolutionary feedbacks” pretty loosely to describe any interactions between ecological and evolutionary processes (two more terms that are defined pretty abstractly). But we can do a better job of defining eco-evolutionary feedbacks, and a recent paper by Martin Turcotte, Dave Reznick, and Daniel Hare reminded me of that. This is another paper from the Eco-Evo special feature in The American Naturalist from a couple months ago mentioned previously on this blog. And Reznick’s lab has been at the forefront of this eco-evo stuff (note recent write-up in Science about The Great Guppy Experiment), so this is a good place to turn to for a definition or an example. According to their paper, Eco-evo feedbacks occur “when ecological and evolutionary dynamics reciprocally influence each other on contemporary and commensurate timescales”.

            Ok, so that’s a little dense and pretty jargon-y. So maybe an example (like, oh say, the experiment in their paper) will be more illustrative. They use populations of green peach aphids to manipulate ecological and evolutionary factors. Aphids are a nice model system because all the ecological and evolutionary processes go really fast in these populations. In other words, you can do an experiment in a few weeks or months, rather than centuries. Anyway, they manipulated the ecology of the population by changing the starting densities of the aphid populations. They manipulated the ability of populations to evolve by changing the number of genotypes in the population. When there is only one genotype, there is no evolution (assuming that mutation is slow or unimportant). When multiple genotypes are present, evolution can change the relative frequencies of those genotypes. So they’ve set up a two factor experiment that manipulates ecology (density) and evolution (number of genotypes).

Turcotte figure

What they found is a nice eco-evolutionary feedback. As populations evolve, the faster growing genotypes start to take over the population. So evolution (change in genotype frequencies) has occurred. Consequently, the population grows faster and densities get higher. From the density manipulations, they know that densities affect how populations evolve. At low densities, one clone grows faster than another, but at high densities, the second clone grows faster than the first. So as the population grows denser, the clone that was driving population growth up becomes less favored, which should favor evolution towards the other clone. So evolution has a big effect on the ecology of the community (population growth and density), but changes in the ecology of the community have a big effect on how the population evolves. Eco-evolutionary feedback…Bam!

You can imagine that it might be pretty tough to demonstrate these sorts of dynamics in a lot of populations. It would either take a long time for these dynamics to play out, or it would be impossible to manipulate ecological or evolutionary factors in an experiment. So this experiment is novel in that it’s one of the first to really show mechanistically how these eco-evolutionary feedbacks can play out. For me, the exact mechanism by which this occurs was really interesting. Just because evolution affects an ecological process, it doesn’t mean a feedback will occur. Rather, the ecological process that’s affected by evolution must also be the same process that drives selection and evolution. Here, density is a key factor related to both ecological and evolutionary processes, so the feedback works. But imagine a different scenario. Say herbivores drive the evolution of chemical defense, which affects ecological interactions with herbivores (i.e. an eco-evo effect). A feedback will occur if interactions with herbivores continue to drive further evolutionary change. But it’s easy to imagine that if interactions decrease, then herbivores will impose less selection, so the strength of the feedback might weaken over time. These are really cool questions that hopefully will continue to be explored with more experimental work in these types of systems.

Turcotte, MM, Reznick, DN, and Hare, JD. 2013. Experimental test of an eco-evolutionary dynamic feedback loop between evolution and population density in the green peach aphid. The American Naturalist 181: S46-S57.

 

Casey terHorst

About Casey terHorst

Casey is a community ecologist interested in how species interactions are affected by genetic variation within species and evolution that occurs on ecological time scales. He is currently a post-doc in Jen Lau's lab at the Kellogg Biological Station. In the Fall, he will be starting as an Assistant Professor at California State University, Northridge.
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