Darwin carefully studied the biogeographical patterns of existing species. (Biogeography is the study of the locations of different species through space.) Biogeography reveals that species that appear to be closely related tend to be geographically close as well, as though groups of species had a common origin at a particular geographic location and radiated out from there. Darwin was especially interested in the study of species on islands, and he observed that they seemed to be most closely related to species found on the nearest mainland. Darwin was interested in what, if anything, these biogeographical patterns revealed about evolutionary history.
In developing his ideas, Darwin focused on finches that lived on the Galapagos Islands, an archipelago in the Pacific Ocean off South America. Several species of finches live on the Galapagos, each species inhabiting a different island. The species seemed quite similar to one another and to a species on the mainland, leading Darwin to hypothesize that the different species of Galapagos finches were descended from individuals in the mainland species that had reached the islands sometime in the past. Because conditions on the islands differed from conditions on the mainland, the selective pressures acting on the finches also differed, resulting in new traits being favored in the new environment.
Archaeopteryxis one of the few transitional life forms that was known in Darwin's time. This early birdlike creature had many characteristics in common with some dinosaurs, yet it also had wings and feathers. Most obvious to the casual observer, Archaeopteryx had jaws full of sharp teeth, rather than the beak structure of birds. Archaeopteryxwas clearly more toward the bird end of the transition to flight. Recently, paleontologists have discovered feathered dinosaurs that did not have wings.
Another interesting creature, Tiktaalik, had a skeletal structure intermediate between fish and tetrapods (critters with four legs) and had both gills and lungs. This skeletal structure was sufficient to have allowed the organism to support itself, at least briefly, out of water. When the first creatures crawled onto the land, they might have looked like Tiktaalik You can read more about these and other fossil finds in Chapter 20.
As a result of the different evolutionary tracks between the mainland finches and island finches, the gradual changes accumulated to the point where the island finches were different enough from the mainland finches to be considered a new species. The concept of speciation, in which one species gives rise to a new species, can seem a bit slippery at first, but it's really not. Head to Chapter 8 for the details.
This process occurred on the various Galapagos Islands, which are far enough apart that travel among them by finches is uncommon. After those rare events when finches did make it to a new island — perhaps as a result of being blown there by a storm — they evolved separately from the population on the island from which they came, in response to whatever novel environmental factors were present in their new home.
When Darwin proposed that the Galapagos Islands had become inhabited by so many different but apparently related species of finches through the process of evolution, he had only his observations of existing variation to rely on. Today's scientists, by analyzing DNA, have confirmed these relationships, and detailed studies support Darwin's hypotheses about the existence of different selective pressures on different islands.
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