About the question:
I failed to find the complete question. However, I propose the following answer and explanation according to the given statement.
Answer:
Natural selection
Explanation:
Natural selection is the product of the interaction between phenotype and environment, which determines gene destiny in space and time, selecting beneficial alleles and increasing their frequency in the population.
Natural selection results in adaptation, which means the increase of the aptitude phenotype. Aptitude is the contribution of each genotype to the next generation.
In many cases, adaptations -as a result of natural selection-, can be correlated to environmental factors or selective pressures applied by other organisms or habitats. These organisms might change or modify their habits according to the environmental pressures that are acting on them. Natural selection favors these new habits and behavior. The better an organism can behave in an environment, the higher chances to survive.
In the exposed example, if assuming that these two populations belong to the same species, they might behave differently from each other if they are, for instance, occupying different niches, changing their feeding habits, competing with other species, among other factors. If these two groups are separated in space and/or time, they might receive different environmental pressures that lead them to change the original behavior to a new one. And as both groups are separated, those pressures probably differ, modeling a behavior adapted to a specific niche.
