Policing occurs in complex insect, animal and human societies, where it is used as a conditional strategy to prevent cheating and enforce cooperation. Recently, it has been suggested that policing might also be relevant in enforcing cooperation in much simpler organisms such as bacteria. Here, we used individual-based modelling to develop an evolutionary concept for policing in bacteria, and identify the conditions under which it can be adaptive. We modelled interactions between cooperators, producing a beneficial public good, cheaters exploiting the public good without contributing to it, and public-good-producing policers that secrete a toxin to selectively target cheaters. We found that toxin-mediated policing is favored when (i) toxins are potent, (ii) cheap to produce, (iii) cell and public good diffusion is intermediate, and (iv) toxins diffuse farther than the public good. Overall, we show that toxin-mediated policing can enforce cooperation, but the parameter space where it does so is quite narrow. Moreover, we found that policing decays when the genetic linkage between public good and toxin production breaks. This is because policing is itself a public good, offering protection to toxin-resistant mutants that still produce public goods, yet no longer invest in toxins. Our work suggests that very specific environmental conditions are required for genetically fixed policing mechanisms to evolve in bacteria, and offers empirically testable predictions for their evolutionary stability.
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