I probably shouldn’t have used the word natural, it’s too broad. I just feel there is a risk with allowing an artificial gene (for example the bacterial beta carotene gene) to spread through a population it never would have been in. Now I can’t think of any particular reason this might actually be bad, but we are still introducing a variable into a system we do not understand fully. When that system is what feeds us, I’d rather not mess with it without complete understanding.