There is literally tons of fossil evidence to support it actually. The evolution of Equids, for example, Horses and Asses and Ponies and Zebras and their many extinct relatives are pretty thoroughly mapped out for the last ~60million years with fossils, and it's very informative. You should check it out.
"In your example, divergent populations B and C are still fertile with each other. "
Not forever. Speciation is a funny concept to really grasp though. It reminds me of quantum physics. We as humans like and need to have clear conceptual lines, so we draw them. We draw this line called species, and we test the line by attempting to breed, and that is pretty much pass/fail, most of the time, and the definition works well enough for us, most of the time. But it's not a perfect reflection of existential reality, you see. There are Horses and Asses and Mules and Jennies. There is the Great Pyrenees and the Chihuaha; the Liger... it could grow to a very long list without being exhaustive.
But the aspect that is like quantum physics - you cannot tell exactly when it happens. Because it will only happen if the populations are separated, yet you cannot test whether it has happened or not until they are reunited.
People today typically credit Charles Darwin with 'evolution' but in fact evolution in some form or another has been talked about since biblical times. Everyone in biology already believe in evolution, but there were varied ideas as to what caused it to happen. Lamarck's was the best accepted at the time, and of course quite off-track as to the main mechanism, but the point needs to be understood that evolution did not come from Darwin, he received it from prior centuries and even millenia.
What brought some better sense into this debate which hitherto had concerned philosophers and bishops and the like, was the intrusion of someone who worked, and specifically with domesticated plants. He was a monk named Mendel, and he did some very interesting work with peas. And when Darwin read Mendel, a lightbulb went off in his head, but he was still missing the final piece of the puzzle.
And then he sailed around the south pacific. Islands, each with their own little species, isolated from those on other islands. And it finally clicked. Mendels genetics allowed traits to be inherited. A small founder population on an island at the beginning, followed by selection for different traits on different islands, produced a dizzying number of species, many closely and obviously related. And that was the *mechanism* for evolution - genetics and selection.
"A great example is the artificial construct of race in humanity. Groups that diverged based on isolation and adapted based on climate, yet any fertile male and any fertile female can still produce offspring."
Sure we can, but we are a species that has existed for only ~250k years, and it was only ~70k years ago that the ancestors of all non-africans left africa. We're an infant species and on top of that there have been very few cases where any population was totally isolated from others for very long, so it's not at all surprising we have no subraces. We're not birds, our generations take much longer, and we tend to adapt quickly via culture before selection at a physical level can really do its worst, so we evolved more slowly even long before we invented things like e.g. modern medicine.
