Question regarding "reverse mutations"
I am reading "A River out of Eden". On page 49 (I think) while talking about the relationship between pigs and horses and yeast, Richard makes reference to a possible case of "reverse mutation".
I remember reading in "Climbing Mount Improbable" that once a species starts a particular "trail up the Mountain" that species does not go back down and take another "trail". In view of that, I'm wondering what Richard means by "reverse mutation".
If anybody can shed some light on this, I would be grateful.