Fig. 1.

Spatial structure promotes the evolution of active metabolite excretion. a A mutualistic interaction between two mutualistic partners. Partner 1 (green cell) excretes metabolite A (green circle) that promotes the growth of partner 2. Partner 2 (red cell) excretes metabolite B (red circle) that promotes growth of partner 1. b In a well-mixed environment, a mutant green cell that actively excretes more metabolites (light green cell) should decrease in frequency. This is because the mutant cell would pay all of the cost for increased metabolite excretion but would receive the same benefit as all the other green cells. c In a spatially structured environment, a mutant green cell that actively excretes more metabolites (light green cell) could potentially increase in frequency. This is because spatial structure might cause the mutant cell to receive a disproportionate amount of the benefits from increased metabolite excretion. These benefits originate from the positive effects of increased excretion of metabolite A on partner 2, thus resulting in increased excretion of metabolite B