A proximate explanation focuses on gene expression and regulation, hormonal control, and other physiological or developmental processes.

An ultimate explanation focuses on natural selection, genetic drift, gene migration, and mutation . . . processes that can change gene frequencies in populations . . . in other words, processes that can produce evolution.

Ultimate explanation focuses especially on natural selection . . .

Do ecological differences between populations (or species) produce differences in natural selection that can explain differences in behavior or anatomy?

Some important terms . . .

An ecological difference is a difference in the external influences on a population . . . predators, food, other species, habitat, climate.

A population of organisms (in biology) is all the organisms in a defined area . . . especially an area over which individuals might meet for mating in their lifetimes . . . an area that depends on the distance of natal dispersal (the movement of individuals from where they are born/hatched to where they themselves reproduce).

Notice that social groups are almost always smaller than populations.   A population usually includes many social groups . . . for instance, a population of wolves includes many packs, a population of monkeys includes many troops, a population of Carolina wrens includes many monogamous pairs . . .