|MadSci Network: Evolution|
It has often happened that in geographically separated regions, the evolutionary process has produced strikingly similar animals to fill similar ecological niches. The tendency for animals in similar "jobs" to evolve similar features is called "evolutionary convergence." The old-world and new-world buzzards mentioned in your question, for example, work under the job description of "flying carrion-eater," and it is often suggested that species filling this niche tend toward baldness because their fluffier-headed siblings are significantly hampered by globs of rotting goo stuck in their head-feathers.
An article by Natalie Angier in the December 15 New York Times (go here and search for "convergent evolution") discusses convergent evolution at length, citing the example of the spiny anteater of Australia, the pangolin of Africa, and the giant anteater of Latin America, three species that seem to have evolved independently to work the job of "eater of tiny, colonial insects." The three species share many distinctive features, such as long, sticky tongues; but they are such distant relatives of one another that these features cannot plausibly have been inherited from a common ancestor. The spiny anteater and the pangolin, for example, are no more closely related to each other than they are to you.
Another example is the Argentine tuco-tuco, which looks much like a gopher and fills the same niche as the gopher: "tunnel-dwelling eater of plants." Yet the tuco-tuco and the gopher are related only to the extent that they are both rodents.
Some examples of evolutionary convergence that involve marsupials are especially interesting, since the marsupial branch separates from the mammalian familiy tree long before "our" branch split into primates, rodents, whales, bats, carnivores, and more. The marsupial branch produced some animals very similar to their better-known, non-marsupial counterparts, including marsupial stand-ins for the sabre-toothed tiger (now extinct, just like its placental cousin) and the weasel. When two chains of evolutionary action, separated for so long, both produce weasels, there is a strong suggestion that the "weasel design" in some sense captures important evolutionary lessons.
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