A polymorphic trait
I guess that's the closest I can get to answering because you didn't add enough details.
Answer:
chemical
physical
Grey Mice have a higher fitness because their phenotype allows them to blend in with their environment, thus helping them to avoid predators. This allows them to survive and reproduce better than the other mice.
The major evolutionary changes was overtime Pakicetus had an earbone with a feature unique to whales and an ankle bone that linked it to artiodactyls.