These sometimes get linked because they both have their deniers. I am ignoring those silly people who actually blame the holocaust on Darwin. Here I want to compare them as phenomena supported by immense amounts of converging evidence.
In “The Greatest Show on Earth: The Evidence for Evolution”, Richard Dawkins compared the denial of evolution to the denial of the history of the Roman civilisation. There is good evidence for both and yet some people seem to think it is OK to deny evolutionary science while accepting the existence of the Roman civilisation.
Same with the Holocaust. As Dawkins pointed out the evidence for evolution is as good as, if not better, than that for the holocaust. But there are still people who accept the Holocaust as a fact while denying evolution. ( actually I did come across someone the other day who disagreed with Dawkins because while he found the evidence for evolution convincing nhe denied the evidence for the Holocaust. Almost a mirror image).