Can Invaders Become Darwinian Demons?

There are two things I think are really cool and drive most of what I think about (well, I’ll pretend there are only two). The first is trade-offs. They’re pervasive in ecological and evolutionary thinking. The basic idea is that everybody is good at doing something, but also bad at doing something else. This comes into play in ecology because each species has its own little niche in the world, where it’s really good at eating a particular food, or avoiding a predator, or tolerating cold temperatures, or one of countless other tasks that it takes to survive in a tough world. The reason that any one species doesn’t take over the planet is because it’s also bad at doing something else. A good competitor might be easy prey for a predator, for example. For ecologists these trade-offs are important to think about because, for the most part, they are what allow so many species to coexist with one another.

Trade-offs are really important in evolutionary biology too because they are a constraint to how traits can evolve. A species might be able to evolve spines to protect itself from a predator, but making spines takes energy, and it means you have less energy to devote to some other important function. The Darwinian Demon is a hypothetical creature that would evolve if there were no constrains on evolution; it would reproduce infinitely, avoid being eaten, eat everything else, and tolerate all environments. (Feel free to Google Image search “Darwinian Demon” for some interesting/terrifying representations of one. My favorite is this guy). Obviously, there are no Darwinian Demons in real life; that’s because every species experiences some constraints on its evolutionary potential.

So that brings me to the second thing that I think is cool—rapid evolution. By that, I mean evolution that occurs fast enough to affect ecological dynamics….let’s say on the order of days to centuries (much shorter than the scale of millions of years over which we traditionally think about species evolving). There are lots of examples of rapid evolution, but just think about bacteria evolving anti-biotic resistance over really short time periods: it’s fast and it’s a problem for the species that bacteria interact with. In the last 20 years or so, there’s been a lot of thought about how invasive species might evolve rapidly. They are introduced into a new environment, probably experience strong selection pressure from that environment, and might be forced to evolve quickly or die. In fact, with respect to evolution of herbivore defense: when you play the Game of Thorns, you win or you die.

In fact, some have posed that the reason some species are invasive at all is that they are able to rapidly evolve in response to their new environment. One reason why species might evolve rapidly in a new environment is that they’re no longer exposed to their natural enemies (herbivores, predators, parasites, etc.). A plant that doesn’t have to contend with a particularly nasty herbivore anymore is going to have higher fitness (grow bigger, produce more/bigger seeds, etc.). But that freedom also frees up that species to evolve in different ways. Anti-herbivore defenses are important and costly, so if you don’t have to invest energy in them anymore, you have more energy to invest in other important things…like competition with other species. Now a species can invest more energy into competition, without the cost of less herbivore resistance. In other words, they can escape that evolutionary trade-off. This hypothesis is called the Evolution of Increased Competitive Ability (EICA) and might explain why some species become such successful invaders. And while this sucks for everything else in nature (because invasive species generally do some pretty bad stuff), it’s pretty cool to think about as an evolutionary biologist. Come on, if you saw a Darwinian Demon walking down the street, wouldn’t you want to take a quick peek before fleeing in terror?
EICA pic

This brings me to the point about a cool paper by Sabrina Kumschick (and her colleagues Ruth Hufbauer, Christina Alba, and Dana Blumenthal), published last month in Journal of Ecology. They are interested in how common mullein (Verbascum thapsus) is successful in its introduced range, the continental United States (yeah, I hadn’t heard of this plant either and had to Google it….here’s an image).

They collected plants from the invasive range and from the native range (Europe) of this plant (27 populations from the invasive range and 23 populations from the native range….an impressive sample size!) and tested which ones were better at certain tasks: competitive ability, herbivore resistance, and tolerance of low water and low nitrogen stress. They found that the plants from the invasive range were better at pretty much everything. They survived better, grew bigger despite whether competitors were present, and were eaten less by a generalist herbivore. In fact, the herbivore effect was huge. The amount of tissue consumed by herbivores on plants from the native range was about 5 times more than on plants from the invasive range.

One cool thing about these results is that they show that traits in the native and invasive range populations have evolutionarily diverged over the last few centuries (this plant was introduced by early European settlers to the U.S.). Plants in the invasive range grow more quickly and are more tolerant of low nitrogen conditions. That’s pretty cool, but not hugely surprising because several others have found the same thing. What I think is even cooler is that there doesn’t seem to be a cost to evolving these traits. Plants in the invasive range were also more resistant to herbivores and they were better competitors. In terms of testing the EICA hypothesis, these plants evolved increased competitive ability, which matches the EICA predictions. However, it’s supposed to occur because they don’t have to invest in herbivore defense. Yet, these plants do a pretty good job of resisting herbivores too, which presumably must be expensive. For me, I just think that’s a really interesting evolutionary pattern in that this species seems to be evolving to be a Darwinian Demon. It’s escaped the trade-offs! It’s good at everything! Before you pack your bags for another planet though, it’s probably bad at something, but the authors just haven’t measured that yet. Here’s hoping they’ve planned a follow-up experiment.

The Evolution of Demonism (I’m copyrighting that phrase, as well as the previous Game of Thorns quote, so you must acknowledge me and donate one dollar to my research fund every time you use them) illustrates why invasive species are such a problem. Think about the famous examples: garlic mustard, kudzu, zebra mussels. They tend to be good at a lot of things and it’s hard to find a way to kill them effectively because they tolerate most things that are bad for the species around them. That’s probably driven by the fact that these species are able to evolve without the constraints that other native species have to contend with.

Sabrina Kumschick, Ruth A. Hufbauer, Christina Alba, and Dana Blumenthal. 2013. Evolution of fast-growing and more resistant phenotypes in introduced common mullein (Verbascum thapsus). Journal of Ecology 101: 378-387.

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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