Let me see if I understand your point.
You observe that, in many-worlds, there is a branch for every outcome, and no branch has the honor of being the "canonical" one. Let us consider a fictional animal called a "grobling" that is about to be conceived. Let us say that an extra arm would be very beneficial to a grobling, if it had one.
- There is a branch in which the grobling mutates and gets an extra arm, and prospers in its life.
- There is a branch in which the grobling is born normally, and prospers.
- There is a branch in which the grobling gets the extra arm, and dies to a predator without reproducing.
- There is a branch in which the grobling is born normally, and dies to a predator without reproducing.
Now how do we "prefer" one of those four branches? In many-worlds, no branch is the canonical one, right? So they should all happen at once, and it's difficult to see how there is selection pressure to prefer certain mutations.
That is how I understand your point.
My answer to it is this: in many-worlds, not all possible worlds are equally likely. No single branch is canonical, but some are more likely than others. In quantum physics we assign a probability amplitude to each possible outcome, which is a complex number. The squared magnitude of the probability amplitude is interpreted as a probability.
We would expect that the probability amplitude of branch #1, in which the grobling gets the extra arm and lives, is greater than the probability amplitude of branch #3, in which the grobling gets the extra arm and dies horribly - because the arm is useful to it. So evolution still happens, in the sense that branches in which the grobling is more suited to its environment are assigned greater probability.
There is some controversy over why we should interpret the squared magnitude of the probability amplitude as a probability, especially in the context of many-worlds. See some comments about that here. But the glib answer is, we do make that interpretation, and thus we privilege certain branches over others even in many-worlds.