(a) Geographical isolation is the isolation of population by physical barriers such as stretches of water or mountain ranges.
(b) There is a population of red beetles living in a mountain area covered with green bushes. Various sub-population in the large-population exists in the neighbourhood. Individuals of a sub-population reproduce sexually among themselves and numerous variations accumulate in these sub-populations of red beetles.
There may be some reproduction among these sub-populations. If a beetle from one sub-population strays into other's area and some individuals from one sub-population may be carried by predators to the area of other sub-population, then random reproduction takes place. It may lead to migration of genes from one sub-population to other.
If the sub-populations are isolated due to the presence of river between them:
(i) Variation will accumulate in isolated sub-population.
(ii) Over a period of generation, the sub-populations would get very different from each other due to the processes of genetic drift and natural selection and they will not be able to reproduce themselves.