Data from: The Black Queen Hypothesis: evolution of dependencies through adaptive gene loss

Reductive genomic evolution is common in endosymbiotic bacteria, where it
is driven by genetic drift. Genome reduction is less common in free-living
organisms, but it has occurred in the numerically dominant open-ocean
bacterioplankton Prochlorococcus and Pelagibacter, and in these cases the
reduction appears to be driven by natural selection rather than drift. The
loss of certain genes in free-living organisms may leave them dependent on
co-occurring microbes for the lost metabolic functions. We present the
Black Queen Hypothesis (BQH), a novel theory of reductive evolution that
explains how selection leads to such dependencies; its name refers to the
queen of spades in the game Hearts, where the usual strategy is to avoid
taking this card. Gene loss can provide a selective advantage by
conserving an organism’s limiting resources, provided that the gene’s
function is dispensable. Many vital genetic functions are leaky, thereby
unavoidably producing public goods that are available to the entire
community. Such leaky functions are thus dispensable for individuals,
provided they are not lost entirely from the community. The BQH predicts
that the loss of a costly, leaky function is selectively favored at the
individual level and will proceed until the production of public goods is
just sufficient to support the equilibrium community; at that point, the
benefit of any further loss would be offset by the cost. Evolution in
accordance with the BQH thus generates “beneficiaries” of reduced genomic
content that are dependent on leaky “helpers,” and it may explain the
observed non-universality of prototrophy, stress resistance, and other
cellular functions in the microbial world.