A caterpillar chomps on a leaf. The aphids nibble on your rosebushes. Wherever there are plants, you’ll find insects trying to eat them. But when you squash a Japanese beetle with a satisfying crunch, you might be interfering with plant evolution.
You probably heard some version of this simple evolution story – Bugs eat plants, plants with chemical defenses against bugs have an advantage, plants with gene for advantage become more common, leading to a slow motion arms race where insects develop adaptations to get around the plant defenses, so plants develop more defenses, and so forth and so on. Forever.
But this simple story raises a simple question. Why have some plants evolved much better defenses against insects than others?
The key to this evolutionary puzzle is actually ecology. Plants growing without predation pressures don’t need chemical defenses. This allows them to allocate their resources away from defense and to something else that will give them a competitive advantage. It’s an intuitive theory, but it’s been difficult to prove experimentally. However, in October, two research teams published independent reports in Science that demonstrate how insect predation can actually drive plant evolution, relatively quickly, in local ecosystems.
In a paper titled “Insect Herbivores Drive Real-Time Ecological and Evolutionary Change in Plant Populations,” Agrawal et al. studied how insect predation pressure would affect a pretty evening primrose named Oenothera biennis. They used natural populations of the plants that included other, competing species, and excluded insects from half of the experimental plots using an insecticide. Over five years, they watched how the O. biennis populations adapted to the different ecological forces — competition and predation. They measured competitive success by comparing the biomass of the O. biennis to the other species present in the plots. They found that the populations growing with predation stress developed higher resistance to the insects, including higher production of chemical defenses. When they compared the competitive ability of the O. biennis in each treatment group, they found that the populations without predation stress developed more competition tolerance and success than the population dealing with insects. In the report, the scientists write:
Our study provides definitive evidence for the rapid evolution of insect-mediated plant resistance traits in real time, supporting current interest in reciprocal feedbacks between evolutionary change and ecological dynamics…. Specifically, our finding that multiple herbivores attacking co-occurring host plants can alter competitive interactions is likely quite general in natural ecosystems, and the rapid evolutionary responses observed confirm predicted interactive effects of herbivory and competition on plant evolution
In the second report, Zust et al look at a set of genes controlling for a specific defense chemical in a small flowering plant named Arabadopsis thaliana. Genetic variation causes the plants to produce slightly different forms of the chemical. The plants grow across Europe, and the researchers wanted to see if there was a relationship between the type of the compound produced and insect predation. They chose to look at two aphid species with different geographic ranges that are affected by the chemical in question. The researchers found that the plants preyed upon by one species typically produced one form of the defense compound, while the plants preyed upon by the other species produced a different form of the defense compound.
As an experiment, they collected diverse plant populations from different locations, mixing up the natural geographic patterns, and exposed them to the two different types of aphids, as well as a third, control aphid who is unaffected by the defense compound in question. They found that within 5 generations of exposure to specific aphids, the experimental populations saw significant genetic change. The selective pressure from the insects resulted in evolution of the genes responsible for the defense chemical, so the experimental populations began to develop the naturally observed patterns. The researchers wrote:
Ecological theory has consistently emphasized the role of natural enemies in maintaining diversity both within and among species, but convincing empirical evidence has been lacking. Here, we demonstrate that even functionally similar herbivores such as different species of aphid have the potential to select for specific chemotypes and drive large-scale geographic patterns in plant defense profiles.
The idea that species evolve in their ecosystems is certainly not new, that’s why exotic species can wreak such havoc. What’s exciting is that both of these experiments manage to capture plant evolution in action. The simple stress of having some aphids sucking on their leaves gives certain plant traits such an advantage that they can quickly become common. Interestingly, these studies also show that when insect predation is absent, other traits that improve competitive advantage become more heavily selected for. Plant populations maintain a genetic balance and diversity of these traits so that the populations can respond to changes in their environments. So, keep the complex ecosystem in mind next time you want to squish some native insects. Exotic bugs, on the other hand — squish away!
This post was written as part of a contest sponsored by the National Evolutionary Synthesis Center. NESCent is a nonprofit science center hosted by Duke University, the University of North Carolina at Chapel Hill, and North Carolina State University, dedicated to interdisciplinary research on evolution. You can learn more about NESCent and find the other entries in the contest here.