If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection. ~ Charles Darwin
Coinciding interests have repeatedly led to interspecies mutualism, where both parties contribute, and both gain from cooperation. The most intimate mutualism is between a macrobe and its microbiome.
The structural composition of multicellular eukaryotes comprises organs with cells specialized for certain functionality. Plants and animals regularly contribute to their microbiomes: helping themselves by helping their closest companions.
Secretions are a common symbiotic expression, produced by specific organs for intended effect. Human breast milk contains copious amounts of complex sugars that a suckling infant cannot metabolize, but that a baby’s microbiota can. Likewise, plants produce nutrients exclusively for its cooperative cohorts at its roots.
Bobtail squid have light organs which they house with specific luminescent bacteria. Without the bacteria, the light organ is nonfunctional. Legumes manufacture nodules in their roots to provide a private residence for nitrogen-fixing bacteria. Flowers are expressly designed for pollinators. Bullhorn acacia creates its namesake horns to house ants, some species of which may be provide protection in return for room and board, whereas other ants may do nothing to repay a plant’s generosity.
In these and other examples, proactive accommodation initiates an ongoing mutualism. Darwin’s simplistic hypothesis of competitive natural selection cannot account for cooperative coevolution, let alone freeloading commensalism.