Certain populations worldwide are much more lactose intolerant than others. Why might this be the case?

Lactose persistence, and therefore lactose tolerance, is inherited as a dominant trait. Lactose intolerance is the result of  having two identical alleles of a particular gene or genes. for the recessive lactase allele that is poorly expressed after early childhood.

You basically can get it from your parents.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms