If Darwinian evolution means no new mutations, then Darwinian evolution won’t work as an explanation for the biodiversity we see today evolving from a common ancestor.
That’s what happened to rock pocket mice in the wild. A new mutation resulting in black fur allowed the mice to take up a new niche in environments dominated by black basalt rocks. The surrounding population is homozygous for brown fur, and the black genotype did not exist in pre-existing variation.