Chronology's not reliable either, different taxonomic groups and different
genes evolve at different rates... the origin of an absolute reproductive
barrier (either pre- or post- zygotic) can occur with very little
divergence (and practically no relation to phenotype).
This is very true. In Drosophila melanogaster it is possible to create a
strain of flies in one generation that can no longer interbreed with any
other strain of melanogaster. Is this speciation?