Here is a diagram of fundamental influences on the development of life on any planet.
A (black) represents the laws of physics, chemistry, i.e. those rules which cannot be affected by the development of life.
Hence, there is no change in the outline black-page.
And concerning overall logic
The reason that life does not regress back to or toward single cell life is
Thus, there may be an infinitude of life forms possible and they will occupy every possible position in the given domain in as many variations as can be physically supported. And the time taken to occupy all positions is as the time taken for the gas to disperse. And the types of animals found in the domain will be, in their variety, as the placement of those gas molecules.
There is a difference in the dispersion of animals from that of molecules in a gas
A dispersion of gas has no hierarchy in the relationship of its constituents as does a living system. Its constituents are different in size, shape and abilities. There is a "pecking" order ... of size, of shape and of ability.
So that our gas dispersal analogy must take place under hierarchical constraints which do not occur in the world of physics. First come the viruses then bigger, better, smarter toward the top. And in quantity more at the bottom and very few at the top.
And now ... to the heart of the matter
Just how is it that new species evolve? Not variations ... New species. This is the present bone of contention.
We are all aware that cats, dogs, human beings and all sorts of farm animals are bred into new varieties over just a few generations. Selective breeding. But all individuals of these breeds can mate with another of a different variety. How can it be that a new species arrives on the scene? [One which can't mate with another species.]
Where does it come from?
We have these possibilities:
Similarly with the aliens. Advanced beings such as these would follow a general rule - "If it ain't broke ... don't fix it.". And it would not be ethical to interfer except perhaps in the background so as to prevent a fetal planet disaster.
Successive variations was the method adopted by Darwin. This is entirely reasonable at first look. Obviously, a new species must be born. Ergo, by definition, its mother must not be of the same species. Ergo, all new species must become different by degrees. They just vary bit by bit until, at length, they can no longer mate with ... hmmmm ... wait a minute ... If they can't mate with the mother's species ... with whom do they mate to reproduce the new line. If they don't mate, the new species (of which they are the only member) dies with them.
That genetic information can be changed by acquired characteristics is another idea I would personally employ (if I were God). Thus, if you strive mightily during your life, your efforts are continued, generally, in the next generation. So if you learn to play the harp, your daughter will pick it up like a dog picks up fleas. Unfortunately, there is no evidence of genetic alteration later in life (at least none in higher animals that I know of). Nuts.
Lastly, it may be this. My best guess at "one of those things" is ... Mutate a single cell's genetic information and split that then fertilized cell into male and female identical twins (I mean identical except for sex). Then, there is someone to mate with to carry on the new species. Impossible? Yup ... you bet. As far as is known.
Of the above, I favor the gradual change of separated groups. I see no other presently viable alternative to basic Darwinism.
On the scale of mutation ...
There is a hierarchy of information in the genetic code. Whether it is from placement in a given gene or from redundant copies in the gene ... I don't know. You can see this hierarchy in the fetuses of higher animals. They all look the same or roughly the same in the early stages of development.
It has to do with what "life" problem was solved first and that has to do with the hierarchical nature of existence itself.
And, of course, birds can't stay in the air (except one albatross which has been aloft for 43 years). So everybody is stuck on a gravitational equipotential whether he can pronounce it or not. This defines left and right and therefore ... the need for bilateral symmetry.
Do you see where I'm going with this?
When an advanced species is developing, it goes through all the fundamental stages which it has in common with all other animals. Now then, we might conceivably have a mutation near the top of the hierarchy causing some very gross deformation and rendering the fetus unviable. However, there are less possibilities at the top of the hierarchy than at the bottom. So that, a beneficial alteration nearer the top would tend to result in two extremes - namely, death or a new species (for everything below the point of mutation may be a viable format). Like ... hmmmm ... mutate your hand so that the left is on the right and vice versa. It would be awkward and totally stupid ... but each hand would be viable (qua hand). Or, swap your ear for a "gill formation" which is latent in your genetic makeup.
Now it should at least be apparent to anyone reading this why there is much squabbling about this point ... the origin of species. It's still
Creationism (some type of instantaneous change)
No one yet knows