Does natural selection increase genetic diversity?
A good question, and the short answer is, it depends.
In many and probably most cases, no, natural selection usually decreases genetic diversity. Why? Well it depends on the environment- if conditions are the same throughout the area where a species is found, then one particular type will be at an advantage, and individuals which are different from that will be disadvantaged and not pass on their genes. Result - less diversity.
It might help to think in terms of gene pools, meaning the total number of (different) genes existing in a species. Mutations add to this pool, and generally speaking, selection 'weeds out' or removes genes that are not advantageous. One famous example of selection reducing diversity is birth weight in humans, babies being born much smaller or larger than the mean having a much lower chance of surviving. (or at least that was true, when medical science wasn't so advanced)
However, there are cases where selection favours diversity. A famous example of this is in polymorphic butterflies - check out