Intelligent Design, Proof IV

[Angrboda]

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In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" species. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing a "ring".

Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special because they represent in living populations what normally happens over time between long deceased ancestor populations and living populations, in which the intermediates have become extinct. Richard Dawkins observes that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension."

Ring species also present an interesting case of the species problem, for those who seek to divide the living world into discrete species. After all, all that distinguishes a ring species from two separate species is the existence of the connecting populations - if enough of the connecting populations within the ring perish to sever the breeding connection, the ring species' distal populations will be recognized as two distinct species.

Ring species are living examples of transitional forms. They are speciation in action.

Greenish warblers

Another ring species that has provided valuable insights into speciation consists of the greenish warblers (Phylloscopus trochiloides). These small, insect-eating songbirds breed in the forests of central and northern Asia and eastern Europe. In the center of Asia is a large region of desert, including the Tibetan Plateau and the Taklamakan and Gobi Deserts, where the warblers cannot live. Instead, they inhabit a ring of mountains surrounding this region, as well as the forests of Siberia to the north. The warblers have remarkable geographic variation:

• In Siberia, two distinct forms of greenish warblers coexist, one in the west and one in the east, their distributions narrowly overlapping in central Siberia, where they do not interbreed. These forms differ in color patterns, the songs that males sing to attract mates, and genetic characteristics. Also, males of each form usually do not recognize the song of the other form, but respond strongly to their own.
  
• The traits that differ between the two Siberian forms change gradually through the chain of populations encircling the Tibetan Plateau to the south.
  
• Thus two distinct species are connected by gradual variation in morphological, behavioral, and genetic traits.
  

Claude Ticehurst, who during the 1930s studied variation in museum specimens of greenish warblers, hypothesized that the present pattern of variation arose when an ancestral species in the south, perhaps in the Himalayas, expanded northward along two pathways, one on the west side of Tibet and the other on the east. The two expanding fronts gradually became different, resulting in two distinct Siberian forms. More recently, studies of genetic variation and song variation have strongly supported this view.

The pattern of song variation is particularly interesting:

• Songs are short and simple in the south, but to the north songs become gradually longer and more complex along both pathways into Siberia.
  
• However, songs have also become different in structure, resulting in distinct differences in songs between the Siberian forms.
  

The birds distinguish between these differences; males respond aggressively to tape recordings of their own songs, thinking that another male has invaded their territory, but they do not respond to songs of the other form. In most species of songbirds, songs play an important role in mate choice; usually, only males sing, and females listen to songs when deciding which male to choose as a mate. Speciation is essentially the evolution of reproductive isolation between two populations, and song differences can cause reproductive isolation. Hence, the geographical variation in songs of greenish warblers provides a rare illustration of how gradual change in a trait can cause speciation.

— Ring Species: Unusual Demonstrations of Speciation, Darren E. Irwin,