Final answer:
The primary agent favoring eusociality is kin selection, which explains altruistic behaviors in social insects that increase their inclusive fitness. Studies in primates and evolutionary relationships between species further underscore the importance of kin selection in understanding complex social behaviors.
Step-by-step explanation:
Eusociality and Kin Selection in Biology
The subject of eusociality is a fascinating topic in the field of evolutionary biology. The question posed suggests that the primary agent favoring eusociality is not something unidentified but is kin selection because this concept explains the seemingly altruistic behavior in social insects like bees, ants, wasps, and termites. Despite their sterility, these worker insects care for the queen and her offspring, thereby indirectly passing on their own genes, an example of inclusive fitness. This behavior is favored by natural selection, as the reproductive success of relatives indirectly contributes to the genetic fitness of the individuals displaying these behaviors.
Analyzing social structures in primates further supports kin selection, as cooperation is often more beneficial than direct competition. Similarly, understanding the role of phylogeny is key to understanding species' evolutionary relationships, as certain physiological and ecological factors affect their chances of extinction or invasive risks.
Therefore, social behavior across various animal species, including humans, suggests an evolutionary advantage, which is significant in shaping complex social systems observed today.