Before famine impacted monkeys with selective pressure. They were strong happy animals, swinging in trees and being wild.
There were some monkeys that were omnivores and others that were herbivores, a form of genetic variation found in monkeys.
Famine occurs within all living plants and trees, which causes there to be no source of food for the herbivore monkeys (selective pressure). On the other hand the omnivores are able to eat small animals and insects. Which becomes their source of food now.
The population of Omnivores manage to survive the Famine on trees and plants. The herbivore monkeys start to die off at a fast rate.
The omnivore population of monkeys have reproduced at greater rates.
The monkeys whom were omnivores survived the famine that occurred to the plants and trees. They had the mutation of being omnivores and therefor were able to survive by consuming small animals and insects. The mutation served as an advantage.