Internal causal factors have played a far more prominent role in the actualization of evolutionary innovations than cumulative selection
Pigs will never fly — not only because of functional reasons, but because of deep internal structural constraints in the way a pig is put together.
There is something incongruous about the very notion of genuine immutable “taxon-defining novelties” if Darwinian evolution is true.
There is no reason for believing the major Type-defining homologs, such as the feather, were achieved gradually via functional continuums.
If many of the taxa-defining homologs are non-adaptive, the whole Darwinian enterprise breaks down entirely.