Final answer:
The Siberian greenish warbler example illustrates 'Allopatric speciation', which occurs when populations are geographically isolated from each other, preventing gene flow and leading to speciation. The correct option is A.
Step-by-step explanation:
The Siberian greenish warbler, a ring species, provides evidence for 'Allopatric speciation'. Allopatric speciation is a form of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with genetic interchange.
In the case of the Siberian greenish warbler, populations that spread in a ring around the Tibetan plateau have diverged genetically and morphologically where the ends of the ring meet but not elsewhere along the ring, illustrating allopatric speciation through geographic isolation.