Figure 4: Conceptual representation of the mechanisms linking evolutionary history and the stability of cooperation.

We observe that defector inhibition is the main predictor of defector spread. As phylogenetic distance between competing species increase, the investment into cooperation remains stable, but the efficiency of defector inhibition decreases. Based on the mathematical prediction and the experimental data, this reduced inhibition favours the spread of defectors: in closely related communities (a), cooperators not only produce public goods but also toxins efficiently inhibiting defectors. Rare defectors (for example, emerging by mutation) face high toxicity cancelling the advantage of defection and cannot invade communities of cooperators. In contrast, in loosely related communities (b), rare defector profit from the high investment into public goods by the cooperators. Defector fitness is negatively frequency dependent, and rare defectors may invade. Functions for both public goods and defector inhibition are quadratic, reflecting that per capita investment by cooperators is density-dependent (quorum-sensing-regulated).