Actually, they do. If selection pressure for a particular phenotype no longer exists, then you may lose those genes through genetic drift. edit: philwelch argu

Actually, they do. If selection pressure for a particular phenotype no longer exists, then you may lose those genes through genetic drift. edit: philwelch argues in a comment in this thread that this exact thing happens: https://news.ycombinator.com/item?id=8853769 https://news.ycombinator.com/item?id=8853769