Fig. S1.

Illustration of the origin of stochastically induced drift along the CM. The gray dashed line shows the form of the deterministic CM, which intersects the x axis at a higher value than the y axis (phenotype X has a higher carrying capacity due to the production of the public good). The orange ellipse illustrates the form of the Gaussian noise centered on the point ${\mathit{x}}^{\left(0\right)}$ on the CM. Fluctuations in the population are equally likely to increase or decrease the frequency of the Y phenotype to the points ${\mathit{x}}^{\left(1\right)}.$ Away from the CM, the deterministic pressure to the CM becomes prominent, forcing the system along quasi-deterministic trajectories back to the CM, at the points ${\mathit{x}}^{\left(2\right)}.$ The resulting distribution of ${\mathit{x}}^{\left(2\right)}$ does not have a mean centered on ${\mathit{x}}^{\left(0\right)}.$ Rather, the distribution is shifted, inducing a drift in favor of the producing X phenotype.