Stabilizing selection and a stable environment

koquin is away, so I’ve been asked to keep the blog going this week. I’m also an evolutionary biologist who likes to read and has cats, so hopefully it will be a smooth transition.

I just got back from a symposium celebrating the work and mentorship of Dr. Warren Abrahamson. Abe spent much of his career studying goldenrods and flies that live inside their stems. His work is one of the best examples of stabilizing selection.

Let me start by explaining the basics of natural selection. (If you’re familiar with this, skip on to the next paragraph.) Picture a population of individuals (lets say birds) in a given environment (an area with seeds for food) with a trait important for survival in that environment (beak size). If there is variation in beak size, and not all birds can survive, the birds with the beak size best suited for eating the seeds will be the ones to produce the next generation. If beak size is a heritable trait (birds with large beaks produce offspring with large beaks), then the next generation should have beak sizes to match the surviving birds: evolution by natural selection. Most of the time, we have several environmental factors that can all affect a trait at the same time. Sometimes the factors favor the same version of the trait (a large beak) but many times, they favor different versions (large beak for some, small beak for others).

Photo of a goldenrod gall

Now, back to Abe’s research. One species of fly, Eurosta solidaginis, lays an egg on the stem of goldenrod. The egg hatches and the larva eats its way into the stem. As the larva feeds, it causes the plant to deposit tissue around it, forming a gall. Conveniently, that tissue serves as both food for the larva, and protection from predation and parasitism.

Opened gall revealing larva inside.

It turns out protection from parasitoids requires different traits than protection from predators (birds). Parasitoids are wasps that lay eggs in the developing larva. They must insert their ovipositor (a part of the wasp that looks like a stinger) into the gall and be able to reach the larva to lay their eggs. So, larger galls are better at protecting the larva. Birds, on the other hand, favor large galls because they are easier to see. So, larvae that produce medium-sized galls have the highest fitness (the highest survival and more offspring). The mechanism that controls the size of the galls is heritable, therefore the offspring also tend to make medium sized galls.

Voila! Evolution via stabilizing selection.

Cool examples of evolution are wonderful, but I learned a lot more than science at this symposium. The most impressive feature of the conference was the respect and friendship Abe’s current and former students have for him. Unfortunately, some scientists can be very competitive with each other, even within the same lab group. Earning a PhD can be a difficult experience, especially if an adviser either neglects or shows animosity towards his or her students. It became very apparent that Abe is not one of those people. Students and post-docs in Abe’s lab continually build upon the past research in the lab. In fact, several of the speakers had published papers together, but had never actually met one another. It was inspiring to see such a collaborative group of people, all of which have gone on to establish their own scientific careers. I could tell that they took Abe’s lessons to heart and modeled their labs after his. I wish all graduate students and post-docs could experience such a stable environment.


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