Eusociality is a social system characterized by reproductive specialization. One or some individuals in the colony bear all the offspring, and non-reproductive individuals assist in caring for the offspring. This system sounds nice and tidy, but evolutionary biologists had to work long and hard to figure out why eusociality works or what benefits it confers to the workers to make them want to hang around rather than flit off to start their own colonies.
The biggest question is why a non-reproductive worker class exists at all. These individuals aren't postponing reproduction until sometime in the future (as is the case for birds helping at the nest). They're forgoing reproduction entirely. Their genes will make it into future generations only to the extent that they are able to help related individuals reproduce and their offspring survive.
For some eusocial species, like ants, bees, and wasps (all Hymenoptera species), kin selection and inclusive fitness explain why a nonreproductive worker class can develop, and it all goes back to a reproductive system that makes individuals more related to their siblings than to their own offspring. For the other organisms, researchers suspect that other forces are at play.
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