The Origin of ... (What?)

Fertile hybrids demonstrate the limitations of strict Darwinian taxonomy.  According to the easy definitions, a species is a genetically distinct group of plants or animals.  In fact, fertile hybrids are common. And they seem fundamental to evolution.

Dozens of Hybrid Sharks Found Off Australia
January 2nd, 2012 - 10:59 AM ET
The world's first hybrid sharks have been discovered in substantial numbers off the coast of Australia, and scientists say it may be an indication the creatures are adapting to climate change.
Australian researchers say they've found 57 animals that are a cross between the Australian blacktip shark and the common blacktip shark, two closely related but genetically distinct species.  (More here.)

Since 1874, at Halle, a series of successful matings of polar bears and brown bears were made. Some of the hybrid offspring were exhibited by the London Zoological Society. The Halle hybrid bears proved to be fertile, both with one of the parent species and with one another. Polar bear/Brown bear hybrids are white at birth but later turn blue-brown or yellow-white.   DNA studies indicate that some brown bears are more closely related to polar bears than they are to other brown bears. All the Ursinae species (i.e., all bears except the giant panda and the spectacled bear) appear able to crossbreed. (Wikipedia on Ursid hybrids here.)

Granting that hybrids are possible between species of the same genus, such as donkeys and horses, it is too easy to expect them all to be "mules", i.e., sterile.  Macroevolution.net is a blog by biologist Eugene M. McCarthy, PhD.  His site includes biographies of biologists, helpful glossaries of Greek and Latin for science, and much more.  He also provides an authoritative explanation of the prevalence of fertile animal hybrids.  Read here.

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