What would happen if a pentadactyl limb evolved to have an additional digit?

When a pentadactyl limb, which normally has five digits, evolves to have an additional sixth digit, it would indeed be referred to as a hexadactyl limb. The term "hexadactyl" specifically describes any limb or structure that has six digits. The evolution of an additional digit signifies a direct change in the anatomical makeup of the limb itself.

Considering the other potential outcomes, it’s important to note that an additional digit could serve a purpose depending on the specific context of the organism’s environment and its evolutionary adaptations. For example, a hexadactyl limb might enable improved grip, locomotion, or manipulation of objects, thus potentially aiding survival. However, this possibility isn't addressed in the correct choice.

Regarding the notion of no changes occurring or the additional digit serving no purpose, these imply situations where the limb structure would have neither evolutionary significance nor functional improvement, which would generally be inconsistent with evolutionary principles that favor beneficial adaptations. Thus, while those options explore different concepts, they don't address the factual naming convention for the limb upon the addition of a digit.