There needs to be pressure for animals with a mutation to reproduce more or animals without the mutation to die before reproduction. Like a disease for example. Otherwise the genes don’t spread and just disappear in the soup of all that species genes and never become dominant. Without any evolutionary pressure the mutation will only spread in one family and probably be gone after a few generations. Like there are human families that are more likely to produce offspring with 6 digits on their hands, but since it isn’t more advantageous than 5 digits (6 digit people don’t produce more offspring and 5 digit people aren’t more likely to die before reaching reproductive maturity) that mutation doesn’t spread across the entire species. Sure if you could sample the genome of every human on earth and identify every advantageous gene mutation you could build the ultimate human DNA. But that’s artificially created, something like that will never happen through evolutionary pathways.