Mutualisms, which are interactions between members of different species that benefit both parties, are found everywhere — from exchanges between pollinators and the plants they pollinate, to symbiotic interactions between us and our beneficial microbes.

Natural selection — the process whereby organisms better adapted to their environment tend to survive and produce more offspring — predicts, however, that mutualisms should fall apart. Individuals that gain from the cooperation of others but do not reciprocate (so-called cheaters) should arise and destabilize mutualisms. Yet to date, surprisingly little evidence of such cheating or destabilization exists.

A team of biologists at the University of California, Riverside, has now found strong evidence of this cheating.  Focusing on the interaction between nitrogen-fixing bacteria, or rhizobia, and their legume hosts spanning about 530 miles of California habitat, the researchers found that natural selection in their study populations favors cheating rhizobia.

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