[Ahban]

The words "sudden appearance" are not in the article, but this is....

"Because fossils of the sagebrush vole are not found before the species appears full blown in Porcupine Cave, Barnosky thinks that the sagebrush vole had only recently evolved. "

....that is the same thing, in so many words.

The timescale for divergence in your graph is also too great for those mechanisms to explain the great diversity in the fossil record, unless those mechanisms also operate much, much, much faster than in these two examples. Its what, 1.6 million years total, with 90% of the divergence showing up in the last half million? That is still a long time for such a small difference.

On an issue unrelated to our disagreement, I noticed something odd about your diagram. I find it intersting that the data breaks toward the right, until one branch snaps back to the left, with another branch veering even more sharply right. I wonder if that pattern is consistent in other examples?

That would be consistent with the idea of a changing environment pushing a species from one niche into another. Say a species is fit for its niche. It is not "pulled" in any direction sofar as morphological change goes. As they exploit the new niche (along with the old one) for generations they are "pulled" in two directions, trending towards fitness in the new niche.

Eventually, the "pulling" of fitness for two niches results in a split, with one group quicly returning to its orginal starting point, and going back to its old niche (if it still exists). The other group, better adapted now to the new niche than the old, accelerates over into its new niche, now that it is no longer being dragged back to the mean by the "pulling" effect of exploiting two niches.

[VadeRetro]

"Because fossils of the sagebrush vole are not found before the species appears full blown in Porcupine Cave, Barnosky thinks that the sagebrush vole had only recently evolved. "

My sloppiness; I didn't register that. I should mention that while Barnosky says the sagebrush vole was pretty new at its appearance in the cave (presumably because there are no older fossils known elsewhere either), this is because prior divergence happened at some distance from the cave at some site not excavated or maybe not even preserved by fossilization. (Some places have deposition, some places have erosion.)

Eventually, the "pulling" of fitness for two niches results in a split, with one group quicly returning to its orginal starting point, and going back to its old niche (if it still exists).

Sounds like you are discovering the sympatric model of speciation. That's thought to happen in some cases, although it's perhaps not the most important model. (The model most often described on these threads is the allopatric one with geographic separation between sub-populations.)

Modes of Speciation.