Field of Science

Tragedy of the Commons in Evolution

Based partly on the feedback from last column, I'd like to probe a bit deeper into the connection between altruism, evolution, and space. Not outer space, mind you, but space here on planet Earth.

We all know that in Darwinian evolution, the species that survive and reproduce best in their environment are the ones that persist and evolve. We know from observing nature that this system tends to produce sustainable ecosystems in which every species seems to play a useful role, even if they are also competing for survival. In particular, no level of the food chain eats so much of the level below as to cause it to go extinct.

Now suppose that in a grassland ecosystem, some animal speices got really good at eating grass. So good, in fact, that it could devour an entire field, roots and all, in a season, and use all that energy to reproduce faster much than its competitors. It would seem that this species has an evolutionary advantage over its slower peers. Of course, this advantage would be very short-term; the grass couldn't grow back the next season, so all species, including this super-eater, would starve.

This situation might be called a Tragedy of the Commons, a phrase popularized by a 1968 Science article by Garret Hardin. This phrase refers to a general situation where there is a shared resource everyone depends on. Without some check on everyone's behavior, some individuals may be tempted to take more than their share, and if this happens too often, the resource is depleted and everyone suffers. (The current depletion of the global edible fish population is one of many real-life examples.)

The question is, why hasn't this tragedy wiped out life on earth by this point? What's to stop a super-eater from spontaneously evolving somewhere, multiplying rapidly, spreading throughout the planet, and destroying all life everywhere?

Several studies (May and Nowak, Werfel and Bar-Yam, Austin et. al.), each taking different approaches, point to a common answer: space. If a selfish overeater evolves somewhere, it will exhaust the resources around it, but then it will die off while other species in other ecosystems live sustainably. As long as there is sufficient space in the world, an overzealous species will cause its own destruction before it can spread very far. In this way, evolution on a sufficiently large planet actually favors organisms that live in harmony with their environment.

Now, if there was some species that could not only suck its environment dry, but also move fast enough to outrace the devastation it was causing, we'd have a real problem on our hands. Fortunately, it seems this has never happened.

Or has it???

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