But Prof, you said this:
There are no particularly worrisome gaps in the modern theory of evolution, barring the gap at the very beginning of life.
Perhaps I read too much into this statement. It seems to acknowledge a "gap" in need of an explanation.
As for species descending from other species: Darwin, I'm told, drew many of his insights from selective breeding experiments. But none of these have ever shown descent of one species from another. I think when he got to Galapagos, and saw the concentrated, rich riot of birds and reptiles of so many different species running around this isolated piece of geography, he drew the perhaps unwarranted conclusion that they must all be related somehow by line of descent. (That's a speculation on my part.) But what "looks plausible" ain't the same thing as establishing a fact.
So, forgive me, but I'm a skeptic; for me, the "jury's still out" on the issue of species being derived from other species. I don't think this sort of thing has ever been observed. But it makes for a great "just-so story."
I am convinced that evolution is a fact, however.
And so, we get back to where we always begin, you and I, Prof. Though I sincerely respect your right to see things differently, and to disagree with me.
You can see it happening, BB:
Ring Species. We can observe two species and the intermediate forms connecting them.
Ensatina eschscholtzi: Speciation in Progress. A Classic Example of Darwinian Evolution.
The talk.origins website has a list of instances of observed biological speciation, in the sense of development of non-interfertility as a result of muatation followed by natural selection. Most of these instances are pretty limited - involving polyploidy, or mutation in a couple of genes, as one would expect when we've had a small number of generations to observe divergence. Nonetheless, they are speciation, according to the standard definition.
There are much more impressive instances of speciation on a near-historical timescale. The most impressive are the development of flightlessness in bird species over a time scale of a few thousand years. Birds that belong to usually sedentary species, such as rails, that arrive on new islands (produced usually by volcanism; it's straightforward to date recently formed volcanic islands), tend to become flightless on a time scale of a few thousand years. The 'just-so' story that explains this is that flight requires a lot of energy, and is needed primarily to escape predators. If there are no predators, birds tend to become flightless very quickly. New Zealand, which had no predatory land mammals at all, was full of flightless birds before the gentle, nature-attuned Maoris arrived and slaughtered most of 'em; so were the Hawaiian Islands. So, if you want macroevolution, we have strong circumstantial evidence bird wings have become vestigial appendages in a couple of thousand generations.