Figure 4.

The average steady-state fraction of unconverted cells, $\langle f\rangle$, at long times in (a) well-mixed populations cultured in a test tube (averaged over approximately six test tubes) and (b) two-dimensional range expansions (averaged over 10 colonies). The concentration for the range expansions was measured by sampling cells at the edge of a circular colony after 5 days of growth. The circles are the collected data points, which are used to make the interpolated color density plot in the background. The dashed lines are the theoretical predictions of the phase-transition lines (see section titled Theory and simulation). In (a), we expect that the transition occurs around $\mu \approx s$. In (b), we find a significantly different line shape, consistent with $\mu \approx A{s}^2$, with $A\approx 1.5$ as the single parameter fit to the data. To see this figure in color, go online.