Termites are eusocial insects that have as their closest relatives, cockroaches in the Genus Cryptocercus. The cockroach relatives have overlapping generations but do not have castes. What were the steps that termites took on their evolutionary path? What were the selection pressures that led to wingless worker and soldier castes and a winged reproductive caste?
Bourguignon and colleagues* suggest the early steps in termite evolution may have involved differentiation of the reproductive caste. A winged reproductive caste can fly and disperse long distances but suffers heavy mortality due to predation. In some environments, a reproductive caste that disperses by walking short distances to start new colonies could be adaptive. Bourguignon and colleagues propose that an initial step in termite evolution was a split of the reproductive caste into winged and wingless phenotypes. According to their hypothesis, only the winged phenotypes retained the ability to reproduce. The loss of reproduction in the wingless phenotype would have led to the worker caste we now observe.
*Thomas Bourguignon, Ryan A. Chisholm & Theodore A. Evans. 2016. The Termite Worker Phenotype Evolved as a Dispersal Strategy for Fertile Wingless Individuals before Eusociality. The American Naturalist. 187 DOI: 10.1086/684838