How cooperation evolves between species is much debated. William Harcombe, currently at Harvard University in Cambridge, Massachusetts, used bacteria to observe this evolution in the lab.
He plated out Petri dishes with an Escherichia coli mutant unable to produce an essential amino acid, and a Salmonella species that consumes waste from E. coli and excretes small amounts of the amino acid. In two out of ten dishes, Salmonella mutants arose that made large amounts of the amino acid.
When grown with E. coli and normal Salmonella, the cooperative mutants rapidly increased from 1% of the Salmonella population to more than 80%. When the bacteria were grown on different media such that the Salmonella no longer relied on the E. coli for food, cooperative mutant numbers crashed. This also happened when the bacteria were grown in flasks, suggesting that spatial structure of the bacterial colonies is needed for cooperation to evolve.
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Evolution: Creating cooperation. Nature 464, 145 (2010). https://doi.org/10.1038/464145d
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DOI: https://doi.org/10.1038/464145d
