A Mixture of “Cheats” and “Co-Operators” Can Enable Maximal Group Benefit.
-- MacLean RC, Fuentes-Hernandez A, Greig D, Hurst LD, Gudelj I (2010) PLoS Biol 8(9): e1000486. doi:10.1371/journal.pbio.1000486

The classic result that co-operators are best for society arises from models such as the snowdrift game, where individuals are stuck in snow and can either co-operate by shoveling it or cheat by just using the cleared paths. In yeast, making invertase is the equivalent of shoveling snow and providing glucose is the equivalent of clearing paths.
While the snowdrift game has been used to show that yeast populations are most fit when everyone co-operates, the researchers point out that this model makes assumptions that are not true for yeast. The snowdrift game assumes that the benefit of production remains constant, whereas yeast uses glucose less efficiently when it's abundant. The game also assumes that the total cost is fixed because there is only so much snow to be shoveled. However, the cost of producing invertase is not fixed because yeast keep making this enzyme even after all the sucrose is gone.