How disturbed are most cheaters, really?
Yesterday, my wife asked, "why are there so many theoretical papers in evolutionary biology?" I suggested one reason may be that evolutionary theory is better developed, in the sense of making accurate predictions, than theory in much of biology. This week's paper, comparing results from an evolution experiment to predictions of a mathematical model, is a good example.
The paper is about the evolution of cooperation. This is a hot topic and also my own area of research. Humans enforce cooperation, to varying extents. For example, we often punish cheaters, those who try to benefit from cooperative activities of others without contributing anything themselves. Human cheaters are mostly pretty stupid -- don't even think about plagiarizing this blog for a term paper! -- but what about cheaters with no brains at all?
Cooperation among cells is essential to multicellular life. Adult plants and animals are made up of billions of cells, which cooperate in various ways. Only sex cells (sperm, eggs, pollen, etc.) have direct descendants in future generations. If one of your lung cells starts reproducing on its own, we call it cancer. The evolution of multicellular life must have required, at some point, that most cells in a group give up individual reproduction. But why would natural selection ever favor anything giving up reproduction? Remember that natural selection can't look ahead and see long-term benefits, any more than a river can flow uphill towards the ocean.
The evolution of multicellularity is an example of what John Maynard Smith and Eors Szathmary called "The Major Transitions in Evolution." Another example is free-living algae giving up independent reproduction to become chloroplasts. These transitions are actually much harder to explain than the origin of "irreducibly complex" structures (which always turn out to be achievable through a series of steps), because individual selection tends to undermine group benefit. But we are starting to understand how such transitions can happen.
This week's paper is "Cooperation peaks at intermediate disturbance" by Michael Brockhurst, Angus Buckling, and Andy Gardner, from Liverpool, Oxford, and Edinburgh, published in Current Biology (vol. 17 p. 761-765). They developed a mathematical model of the effects of disturbance on the evolution of cooperation in bacteria, then did an experiment to test the model's predictions.
I will explain the experimental system first, to make the discussion less abstract. They worked with bacteria already shown to evolve the ability to make goop that sticks cells together into a "biofilm." Biofilms are important in disease and may resemble an early stage in the evolution of multicellular life. In liquid culture, bacteria benefit from being in a biofilm, because they float on the surface, where there is more oxygen. (The Exploratorium, in San Francisco, used to have a great demonstration of this. A flask of light-emitting bacteria glowed mainly at the surface. Pushing a button mixed oxygen into the flask, so the glow spread all the way down.) Mutants that don't produce goop are "cheaters"; they benefit from being in the biofilm, without paying the cost of making goop. A flask full of cheaters doesn't grow well -- most cells don't get enough oxygen -- but a few cheaters in a biofilm will tend to out-compete cooperators, because they use all their resources for their own reproduction, instead of for making goop. In the long run, who will win, cheaters or cooperators?
"It depends on disturbance", suggested the authors. Their model included two different effects of disturbance on cooperation. Each disturbance kills most of the cells in the flask. After a disturbance, the cells in the flask tend to be more related to each other than before. (Imagine a nuclear war that destroys every US state except West Virginia.) So each group is mostly cooperators or mostly cheaters. Cooperating groups grow faster than cheating groups, so disturbance should favor the evolution of cooperation. This is an example of "kin selection": a gene for cooperation can spread if it causes behavior increasing the survival of other copies of the gene, usually in close relatives.
But wait! Frequent disturbance kills so many cells that there may not be enough to form a biofilm. (Ten people who never cheat on their taxes may not have as good a school as ten thousand people, even if some of them cheat.) Combining these two effects of disturbance, the model predicted that cooperation would be most common at medium rates of disturbance.
When they did the experiment, that's exactly what they found.
The paper includes a nice discussion of their assumptions and of other possible interpretations of their results. For example, the least-disturbed cultures tended to run out of nutrients. This hurt all the bacteria, but maybe especially the cooperators, due to the cost of making goop. Cooperators might have done a little better, in the least-disturbed system, if nutrients were replaced more often. It might be possible to improve this experiment, but the authors have a better suggestion: try similar experiments with different kinds of bacterial cooperation. For example, Buckling has previously studied bacteria that cooperate by releasing molecules that help all bacteria (even cheaters that don't make the molecule) to take up iron. I look forward to more papers on this topic.