Imagine that everyone who posted non-bioinformatics related questions to this forum was sterilized. This could be "natural selection", that acted on the poster gene pool. So none of these non-bioinformatics related questions posters would be able to reproduce and pass on their trait of posting non-bioinformatics related questions to this forum. Eventually you would then have a better forum with no non-bioinformatics related questions posters, unless perhaps some mutation brought back the annoying trait. However, if natural selection was still acting strong on this trait, it would die as quickly as it arose. This is evolution, i.e. natural selection acting on the variation that exists within nature. Darwin figured this out, but he was puzzled, because eventually this ought to lead to 100% homogeneous species. He didn't know about mutations and other methods that create diversity, which are an essential part of evolution, the stuff that natural selection acts upon.