Insect herbivores and the drivers of plant speciation

This is a guest post by Christina Baer, a Ph.D. candidate in Ecology, Evolution, and Systematics at the University of Missouri-St. Louis

Fifty-two years ago, two biologists published a huge paper proposing that caterpillars and their food plants evolve in response to each other (Ehrlich and Raven 1964). They presented quite a bit of evidence for this: closely related caterpillars usually eat closely related plants and can tolerate the same defensive chemicals produced by plants. And in the last fifty years, biologists have found many examples of plants changing their chemical defenses in response to insects, or insects evolving new ways to get around plant defenses. But Ehrlich and Raven went one step further and proposed that insect herbivores could cause new plant species to evolve. Unfortunately, they didn’t suggest any step-by-step scenarios for how insects could cause plant speciation, which made it difficult to test the hypothesis. So far, no one has.

Several years ago, the Marquis lab decided that we would try to come up with ways that herbivorous insects could cause new plant species to evolve. To do this, we had to think of ways that insect herbivory could affect different groups within a plant species differently. If the entire species responds the same way, then the species’ characteristics will change, but it will still be one species. So we brainstormed all the different things that could cause individuals to respond differently (location? soil? timing?) and looked for previous research that illustrated each step.

The simplest scenario we developed would apply to plants that use the same chemicals to defend their leaves and attract pollinators. In both cases, simple chemical changes can have dramatic effects on how the animals respond. If some plants evolve new defensive chemicals in response to herbivores, then those same chemicals could attract different pollinators to the flowers. If the original pollinator is not attracted, then the plants with the new chemicals would be reproductively isolated from the rest of the population. Given time, the two groups would separate into different species. To see the other scenarios we came up with, you can check out our new article, “Ode to Ehrlich and Raven or how herbivorous insects might drive plant speciation”.

ecy1534-fig-0001

Scenarios by which herbivores may coevolve with host plants: a) the reciprocal arms race model, and b) the speciation arms race model. From Marquis et al. 2016.

Writing this article was interesting for me because it was very different from writing up an experiment. With an experiment, you usually start in the middle of the article (the methods and results) and work out to explain why you did the experiment (the introduction) and what it means (the discussion). It’s fairly clear from the beginning what needs to be in the article and how it should be organized. With a conceptual article, it’s much less obvious what needs to be included and it takes even more work than usual to make your ideas clear and organized. I think the article got pulled apart and put together in a completely different order two or three different times while we writing it. At one point, I made a list of each paragraph’s main ideas so that I could arrange them in different orders and see what made the most sense. In the end, it took about as long for us to write the article as it did to come up with the possible scenarios.

Conceptual articles are important for scientists because they rearrange what we know and help us see how we can test ideas in new ways. In this case, we’ve outlined ways of testing for herbivore-driven speciation and pointed out a couple of systems that already have evidence for several steps. We’re hoping that other ecologists will use these ideas to finally test Ehrlich and Raven’s hypothesis. Since most of the scenarios we came up with are complicated, it will be interesting to see how common herbivore-driven speciation is.

Cited papers:

Marquis, R. J., D. Salazar, C. Baer, J. Reinhardt, G. Priest, and K. Barnett. 2016. Ode to Ehrlich and raven or how herbivorous insects might drive plant speciation. Ecology 97:2939-2951.

Ehrlich, P. R. and P. H. Raven. 1964. Butterflies and plants: a study in coevolution. Evolution 18:586-608.

About the author:

cbaersnapshot

Christina Baer is a PhD candidate in the Marquis Lab at the University of Missouri-St. Louis. Her research interests include plant-insect interactions, natural history, and community ecology, so she’s doing her dissertation research on how tropical caterpillars build shelters to protect themselves from predators and parasitic insects. She wants to be a professor when she grows up.

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