Voyage of Discovery
Many people were disappointed in Charles
Darwin—his father (who wanted him to be a minister or, at least, a prominent
physician), his
boss (the fundamentalist Sea Captain Fitzroy), his young
protégé, Thomas Huxley (who argued for him.) Yet the model that Darwin proposed has
stood the test of time—and hasn’t disappointed biologists, who have built
most of their understanding of the modern science of life on the basic
principals he proposed. Darwin’s voyage of discovery took him to
South America in 1835, and by that fall to the Galapagos Islands. Trace his
travels with your cursor at
http://www.britannica.com/eb/art-19262?articleTypeId=1 He spent several
weeks in Valparaiso, because he was ill. His observations in Lima, Peru were
severely limited by “anarchy.” He seems to have been unimpressed at first,
perhaps because he was still ill. But thirty years later, in Origin of
Species, his memories of South America were still vivid.
(http://etext.library.adelaide.edu.au/d/darwin/charles/beagle/complete.html)
But interpreting, Darwin didn’t always sound “like Darwin.” This description of the logger-headed duck sounds a lot like Lamarck:
At the same time, Henry Walter Bates (who was to become Darwin's lifelong friend) was spending 11 years in South America cataloging thousands of butterflies and documenting the well-known evolutionary strategy of mimicry.
The Great American Interchange
The
first (Mesozoic) mammals were egg layers, and then came marsupials, whose
young were born in an extremely immature state and remained in pouches as
they developed. When the giant supercontinent Pangaea broke apart, the more
advanced “placental” mammals evolved in North America and Europe, but
marsupials remained dominant in South America and Australia for millions of
years. They were the “fittest” around.
South America’s isolation came to an end in the Tertiary Period (Cenozoic Era) some 3 million years ago with the rise of the Isthmus of Panama. Tapirs and peccaries, horses and wild cats, camel-like llamas, alpacas, guanacos and vicuñas, migrated south. Armadillos and giant ground sloths migrated north, but the sloths never got much farther than Florida. Soon the marsupials found themselves at a disadvantage; their immature young were just too fragile and unprotected to survive. Horse-like liptoterns, rhino-like notoungulates, and ground sloths were less fit than placental distant cousins. They are gone.
At
the time of the interchange, the armadillo was a giant—a glyptodont,
to be specific. It had “tweedy” caps on its head and a spiked club on the
end of its tale, just in case a sabertooth tiger came along. Like the
penguins on the coast, smaller variations found it easier to survive. With
the opossum, the armadillo is a persistent reminder of the old South
American fauna.
Biologists often ask: “Why do some organisms evolve quickly, with many intermediate forms over time, while others seem to be ‘living fossils?’” Paleontologists have studied fossils of dozens of North American horse ancestors, showing changes in their size, gait, and dentation over hundreds of thousands of years. Yet the armadillo, crocodile and leatherback turtle look much like their Tertiary ancestors. The answer is not completely clear, but seems to involve the match between the organism’s traits and the environment. The (smaller) armadillo and the crocodile were still the best suited for the environment in which they lived. No fitter competitor evolved there. More on evolution in the next few sections. But for now, just keep in mind that old adage: “When the going gets tough, the tough get going!” And what’s tougher than an armadillo? Or more stubborn than a vicuña (at left) when you have something to get going?