Member Rara Avis
LR ...it is completely compatible with Micro-evolution that (A20) and (A) may be so dissimilar that re-production between the two is not possible -- thus -- a new species. (A20)=(B) Speciation.
Defining "species" by the inability to reproduce is one way, LR, to reach speciation, but it's a definition not all naturalists would agree is accurate or useful. Your example, after all, would suggest that breeding a horse and a donkey will produce a new species of animal. Many within the biological community use quite different definitions of species, including those based on morphological, genetic, evolutionary (a bit circular, that one), paleontological, phylogenetic or biosystematic models.
How can we agree that speciation has been reliably observed when we can't even agree what constitutes a species?
The problem with every example of speciation I've seen published (and there aren't that many) is that there is never a test for viability. Even if we agree on the definition of a species, and even if we use that definition to discover the introduction of a brand new one, that alone is not enough to support the claims of evolution. Mutations are not in dispute, after all.
Virtually every claim, prediction, and retrodiction of evolution rests on the single premise that the functions of Life could follow multiple paths.
There are hundreds of different polymers, for example, but only three are used to support Life. The DNA in all organisms uses only four nucleotides out of nearly a hundred known. The list goes on and on and on, and all of this evidence is used to prove the validity of common descent.
Occam's razor would suggest a much simpler explanation. Maybe we've only seen one way for Life to function because there IS only one way for Life to function?
There is absolutely nothing in evolutionary history to predict a single point of origin. On the contrary, if there are a hundred different ways for DNA to exist in a viable form, evolution would tell us the only thing required is sufficient time.
You want to convince me the commonalities of life prove common descent? Fine. Show me the exception evolution insists must exist. Find or build me a form of life that falls outside the nested hierarchy. Do that and I'll have no choice but to accept that everything within the hierarchy sprang from a single source.
We have an infinitude of integers, yet only zeroes and ones are used to write today's digital computer programs. Gather all those programs into a taxonomy and I strongly suspect cladistics would give us a phylogenetic tree that very closely resembles the universal common descent tree attached to Life. Should I then theorize that, given a little background radiation and enough time, the word processor I'm using to write this post will one day evolve into a sapient spreadsheet?
While I understand your reluctance, LR, to differentiate between micro- and macro-evolution, I think it is nonetheless a necessary distinction. Survival of the fittest is about emphasizing characteristics that already exist in a species and is very nearly inevitable. Bacteria don't suddenly develop a resistance to penicillin through magic, but rather because a few already had a resistance that allowed their progeny to survive where others couldn't. It does not, in my opinion, naturally follow that bacteria will one day exhibit entirely new characteristics that enhance their survival, perhaps turning into green algae or horn worts in the process.
Micro-evolution is inevitable. Macro-evolution is a matter of completely random chance, and even within the context of a hundred million years, the odds aren't real good.