You are not emphasizing the descendants part enough. It still requires existing members of the species to propagate the gene with natural breeding, same as 'na

You are not emphasizing the descendants part enough. It still requires existing members of the species to propagate the gene with natural breeding, same as 'natural' genetic mutations. So if it is a gene detrimental to survival, it can still very easily die off. Which is not in violation of what natural selection would predict.