What’s the point of your more serious note?
My point is that the numbers don’t tell you which differences are important. They don’t tell you which differences were selected. If we start the discussion with the differences “between the mammals that share a common ancestor” as you requested, but begin with the time interval zero, you might see my point more easily.
Are you beginning to see mine, perhaps? Please understand that I’m not denying the existence of new mutations nor the rate of new mutations. You are claiming that new mutations are the primary substrate for evolution, I am pointing you to the evidence that shows that existing variation is the primary substrate for evolution, a perfectly mainstream concept among population geneticists.
It’s presented in another way here:
starting with slide 28.
The point I’m making is highlighted on slide 33:
"Genetic polymorphism provides a much greater source of genetic variation than do the relatively few new mutations that arise each generation."
Merely measuring the numbers or ratios of differences between species or individuals does not help one to understand this extremely important mechanistic point. Denialists ignore and obscure it because it makes evolution easier to understand and takes away the theological club of randomness.