95 Ring Species: The Greenish Warbler

Key Idea: Genetic and song analyses provide strong evidence that greenish warblers form a ring species that originated south of the Himalayas. Greenish warblers (Phylloscopus trochiloides) are found in forests across much of northern and central Asia. They inhabit the ring of mountains surrounding the large area of desert which includes the Tibetan Plateau, and Taklamakan and Gobi deserts, and extends into Siberia. In Siberia, two distinct subspecies coexist and do not interbreed, but are apparently connected by gene flow around the Himalayas to the south. The greenish warblers may thus form a rare example of a ring species.

Populations spread both east and west along the Himalayas. Populations developed unique characteristics, but adjacent populations remained able to breed together. East and west populations eventually rejoined in Siberia, but because of morphological, behavioural, and genetic differences they do not interbreed.

E No gene flow

X

H

Genetic data and analysis of song spectra point to a single species establishing on the southern edge of the Himalayas about 10,000 years ago.

A B C D

C

B

A F G

Gene flow

Song spectra of the greenish warbler

D E

The greenish warbler has been touted as "Darwin's missing evidence", showing how one species can diverge and evolve into two when populations are separated and subjected to different selection pressures.

F G H

The two coexisting subspecies of greenish warblers are distinguished by their songs and the number of bars on the wings. The warbler in western Siberia has one light bar across the top of the wing, while the warbler in eastern Siberia has two. Analysis of the songs around the ring show that all songs can be traced to the population labelled A above. Songs become progressively different moving east or west around the ring. The songs of the eastern warblers (H) and western warblers (E) in Siberia are so different that neither recognises the other. Eastern and western forms have subspecies status.

1. How do the eastern and western Siberian populations of greenish warblers differ?

2. Explain how these differences might have occurred:

3. Explain why the greenish warbler is described as "evolution in action":