Artificial selection has got to be possible, given we’ve done it to our fruit merely by happenstance over the generations. And our dogs, too.
I wonder if plastic would be more like diamond. We don’t see enzymes breaking down diamond, even though that’s been around for plenty of time.
Natural selection could probably get there if the conditions are right, just as artificial selection could probably force those conditions in a lab. Yet I feel the it’s probably much less likely than was the case with wood.