In this forest there are a community of guinea pigs that are all a light tan color.
Genetic Variation: There is a mutation in the population that give that certain guinea pig a dark color.
Overproduction and struggle for survival: The light tan guinea pigs are struggling to survive in their habitat and they're reproducing more than the environment can handle but not all of them will make it because of the predators.
Differential survival and reproduction: The guinea pigs that are light tan are dying by predators leaving the dark ones to survival and reproduction.
Change in the gene pool: Due to the dark being less easily seen by predators, they are more fit to that habitat and will reproduce changing the whole population in that area.
The allele for light tan has slowly faded away leaving only dark guinea pigs that are very suited to that environment