Although the animals are the same species, they are geographically isolated and therefore do not commonly breed with each other. Natural selection refers to proecesses that result in the survival and reproduction of individuals best suited for the environment in which they live. All species tend to produce more offspring than the environment can sustain. Not all individuals can survive. Some fish lay millions of eggs, only a few of which will live to be adult fish and reproduce.
Variety is the basis for natural selection.
Individuals within a population are not all exactly the same. Genetically, they vary. Individuals with favorable variations tend to survive, and those with less favorable variations die. Survivors reproduce others with favorable traits. A gene pool contains all the traits or genes in any one population that can be passed on through generations.