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Quoting from Darren E. Irwin, et al.(1):
"The evolutionary divergence of a single species into two has never been
directly observed in nature, primarily because speciation can take a long
time to occur."
Let me elaborate a bit. I would challenge the idea that speciation takes a long time in populations of organisms that are out of equilibrium. There are several hundred endemic species of Drosophila in the Hawaiian Islands that probably arose from one or a few ancestral species in a very short time. However after many years of intensive study, we still do not have a good grasp of what the process of "speciation" entails. Most scientists would say that we know of many examples of speciation that have occurred in the past, and would perhaps refer to these prehistoric examples that clearly evidence speciation. Nonetheless, your question concerned whether any living scientist has witnessed such an event. The answer is, as Irwin quite correctly asserts, "no".
The recent book by Jonathan Wells, "Icons of Evolution" has laid bare the common examples that are most often referred to in science textbooks. I refer you to that source as well as previous questions in this section for information on such oft-cited examples as the peppered moth, Darwin's finches and others.
1. Darren E. Irwin, et al. "Speciation in a Ring", Nature 409, 333-337, 2001