I learned in an nineth grade anthropology class that it is not survival of the fittest. It is survival of the most adaptable.
If a species of red bird develops a mutation that causes some to be blue, and they happen to live in an area rife with red flowers, the green birds will contrast with the environs and be easy prey. On the other hand, if the red flora succumbs to plant disease and is replaced with green foliage, the green mutation will have the upper hand. Neither, of their own accord, is more fit or more adaptable. They are either beneficiaries or victims of fate (as the secularist would call it) or design (as the creationist would term it.) In either case, there's still a very, very long leap to the red or green specimen becoming a new species, much less an entirely new class of vertebrae (ie. When does the green bird become a mammal?)