The Black Queen Hypothesis (BQH) proposes that gene loss in free-living organisms can lead to dependency on cooccurring microbes for lost metabolic functions. This theory suggests that gene loss provides a selective advantage by conserving resources, especially when the gene's function is dispensable. The BQH explains how natural selection drives such dependencies, as organisms lose costly, leaky functions that produce public goods. These functions are essential for the community but not for individual organisms, leading to a balance where the loss of the function is selectively favored until the production of public goods is sufficient to support the community. The BQH is analogous to the Red Queen Hypothesis, which describes evolutionary races between species. However, the BQH focuses on the loss of functions that are costly but provide public goods. This concept is illustrated by the loss of the catalase-peroxidase gene (katG) in Prochlorococcus, which is essential for detoxifying hydrogen peroxide (HOOH). The absence of katG in Prochlorococcus is due to its loss in the common ancestor, leading to a reliance on other microbes for HOOH detoxification. The BQH also applies to other microbial functions, such as nitrogen fixation, inorganic nutrient acquisition, and biofilm matrix deposition. These functions are costly and leaky, making them targets for loss. The hypothesis suggests that such functions are maintained by a subset of the community, while others benefit from the public goods produced. The BQH provides a framework for understanding microbial evolution, including the persistence of rare but essential keystone species. It also explains the paradox of the plankton, where many species coexist despite limited resources. The hypothesis emphasizes that the loss of functions is driven by individual-level selection, leading to a division of labor in microbial communities. This process can result in mutualistic or commensal relationships, depending on the ecological context. The BQH highlights the importance of public goods in microbial evolution and offers a new perspective on classic problems in microbial ecology.The Black Queen Hypothesis (BQH) proposes that gene loss in free-living organisms can lead to dependency on cooccurring microbes for lost metabolic functions. This theory suggests that gene loss provides a selective advantage by conserving resources, especially when the gene's function is dispensable. The BQH explains how natural selection drives such dependencies, as organisms lose costly, leaky functions that produce public goods. These functions are essential for the community but not for individual organisms, leading to a balance where the loss of the function is selectively favored until the production of public goods is sufficient to support the community. The BQH is analogous to the Red Queen Hypothesis, which describes evolutionary races between species. However, the BQH focuses on the loss of functions that are costly but provide public goods. This concept is illustrated by the loss of the catalase-peroxidase gene (katG) in Prochlorococcus, which is essential for detoxifying hydrogen peroxide (HOOH). The absence of katG in Prochlorococcus is due to its loss in the common ancestor, leading to a reliance on other microbes for HOOH detoxification. The BQH also applies to other microbial functions, such as nitrogen fixation, inorganic nutrient acquisition, and biofilm matrix deposition. These functions are costly and leaky, making them targets for loss. The hypothesis suggests that such functions are maintained by a subset of the community, while others benefit from the public goods produced. The BQH provides a framework for understanding microbial evolution, including the persistence of rare but essential keystone species. It also explains the paradox of the plankton, where many species coexist despite limited resources. The hypothesis emphasizes that the loss of functions is driven by individual-level selection, leading to a division of labor in microbial communities. This process can result in mutualistic or commensal relationships, depending on the ecological context. The BQH highlights the importance of public goods in microbial evolution and offers a new perspective on classic problems in microbial ecology.