Darwin studied that different populations of finches on Galapagos island that are descended from the same species. Some species are evolved to eat nuts and large seed, some eat small seeds and some feed on insects and nectar.
So genetic variation occurred in different populations of finches which enabled them to bring changes in their beak according to the food resource they have chosen to feed on.
So in Darwin's finches, genetic variation leads to new species which is a type of natural selection. Therefore subset of finches that can feed on large seeds and small seeds is an example of genetic variation.
