Answer:
Geographical isolation
Explanation:
The evolutionary mechanism that can probably explain the difference, in this case, is geographical isolation.
Geographical isolation represents one of the reproductive isolation mechanisms - an array of physiological and behavioural processes that leads to speciation in living organisms.
Geographical isolation refers to the separation of organisms of the same species in terms of physical geographical features such as distance, mountains, water bodies, etc. The separation causes each group to take different adaptive paths, eventually leading to speciation, that is, they become separate species.
In this case, the small population of monkeys in the isolated forest must have been geographically isolated from their counterparts in some ways back. They took a different adaptive path from the larger group as a result of geographical difference and eventually became a new species.
