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Bacteria typically rely on secreted metabolites, potentially shareable at the community level, to scavenge resources from the environment. The evolution of diffusible, shareable metabolites is, however, difficult to explain because molecules can get lost, or be exploited by cheating mutants. A key question is whether natural selection can act on molecule structure to control loss and shareability. We tested this possibility by collating information on diffusivity properties of 189 secreted iron-scavenging siderophores and the natural habitats occupied by the siderophore-producing species. In line with evolutionary theory, we found that highly diffusible siderophores have preferentially evolved in species living in structured habitats, such as soil and hosts, because structuring can keep producers and their shareable goods together. Poorly diffusible siderophores, meanwhile, have preferentially evolved in species living in unstructured habitats, such as seawater, indicating that these metabolites are less shareable and more likely provide direct benefits to the producers. [ABSTRACT FROM AUTHOR]