One of the nice things about summer is catching up on everything that got put to the side during the semester, right? It seems like reading literature is always one of the first things to go, so I’ve been spending some time reading a number of papers that I had set aside to be read “sometime” since they’ll definitely feature in a background section in my future. I’m quite interested in the evolution of cooperation, and one type of cooperative “game” is the production and use of public goods. A public good is a product that is useful to an organism, but is for some reason physically outside of the organism’s control and so must be shared with surrounding organisms. In the simplest systems, a “tragedy of the commons” scenario can occur in which organisms that don’t contribute to the public good still benefit from it. Eventually it isn’t advantageous for anyone to contribute and everyone loses out on the potential benefits. So without further ado, here are four papers from the last decade or so (I’ve not been waiting that long to read them, I just found them recently!) that deal with the evolutionary dynamics of cooperation through public goods.
- Evolutionary games and population dynamics: maintenance of cooperation in public goods games (Hauert et al.) – Hauert et al. tackle mathematically modeling a public goods game with a new addition: ecological dynamics. As I said above, the first question with any cooperative system is how it is able to persist in the face of cheaters that gain a benefit without paying the same cost as cooperators. Hauert et al. found that by allowing population density to change during iterative games, cooperators could get a larger benefit in small populations and thereby recover from the emergence of cheaters in large populations. This leads to a stable midpoint where cheaters and cooperators are able to co-exist and any deviation above or below that point increases pressure to return to that point. It’s great to see ecology being added to evolutionary models and very interesting that such a simple measure has such a stabilizing effect.
- Availability of public goods shapes the evolution of competing metabolic strategies (Bachmann et al.) – Bachmann et al. found another way to combat cheaters in a public goods game. They performed a wet-lab experiment, in which they used water-in-oil emulsion to better separate organisms and thereby prevent cheaters from being able to take advantage of the public good produced by cooperators elsewhere in the population. Their goal was actually to find a selection regime that would select for increased biomass yield of the entire population (i.e. more offspring per individual) instead of increased growth rate of individuals. The oil droplets kept the growth-limiting nutrient from diffusing to cheaters who could grow more quickly by not producing the public good themselves.
- Resource supply and the evolution of public-goods cooperation in bacteria (Brockhurst et al.) – Brockhurst et al. were interested in how increased resources could effect the evolution and stability of a public goods strategy. Their logic is that there is only so much you can invest into growth before the diminishing returns makes it not very useful. Therefore, if there are so many resources that you’ve reached a point where you aren’t getting much return from investing those resources into growth, there isn’t as much of a cost to diverting those resources to public goods cooperation instead. Their experiments showed that cooperative bacteria do indeed increase as resource supply increases due to the decreased cost of investing those resources.
- Improved use of a public good selects for the evolution of undifferentiated multicellularity (Koschwanez et al.) – This paper is pretty long, but a very exciting read that I definitely recommend. Koschwanez et al. reasoned that the problem with relying on public-goods cooperation is that the valuable resource can diffuse away and the cooperator will only get a fraction of its investment. However, if the cooperator is actually part of a multicellular clump of undifferentiated cells, neighbors can use the public good produced from others in the clump, much like in a structured population. Koschwanez et al. engineered three strains, one of which forms multicellular clumps, to verify that this strategy does increase fitness. They then also experimentally evolved budding yeast (Saccharomyces cerevisiae) and found that 11 of the 12 successful lines evolved to use multicellular clumps among other strategies such as increased expression of invertase and hexose transporters. On top of all that, they also identified the causal mutations of the evolved phenotypes.
So if you found your summer lacking in discussion of cooperative strategies, I hope I’ve remedied that problem for you. What papers have caught your eye recently?