[microgood]

The difference being?

Common descent refers to descent from earlier species. Universal takes us all back to a singularity of life creation. Obviously the amount of evidence for the latter is more lacking than the former and of course you cannot get there at all from DNA evidence since we have no idea what that singularity consisted of.

Saying we all came from the same original life form because all life has similar characteristics is non-falsifiable and would basically make universal common descent a non-theory.

How about a new allele or two? How about an existing allele under new conditions?

You go me there.

[b_sharp]

"You go me there"

I'm not sure what this means. I simply answered your question: "From what I hear, natural selection has to have something to select.