Not all eusocial organisms have the same reproductive system that Hymenoptera species do. Termites, for example, live in colonies with a reproductive pair (one female and one male) and sterile female workers, but the workers aren't more closely related to their siblings than they are to their own offspring (were they to have any). For them and other such creatures, kin selection isn't what compels them to remain in the colony.
In trying to understand these other eusocial systems, biologists suspect that eusociality may have been the result of a dynamic between selection at the level of the individual (which favors the fitness of the individual and therefore tends to prevent the formation of such systems) and selection at the level of the group (which favors groups of organisms over individual ones).
If playing a minor role in a larger group makes an individual more fit than playing a solitary role, natural selection could favor genes for remaining in the group. Think of the dancers in a chorus line. They may never be the headliners that draw in the audiences, but the show can't go on without them — and they're dancing on Broadway.
So how do roles get assigned in such species? What mechanism dictates whether a budding young organism is the star or a background player? Genes. Specifically, genes for plasticity — those that allow individuals to develop into reproductive individuals or non-reproductive workers. If such genes led to increased efficiency in the division of labor within the colony, they would be selectively favored.
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