Wednesday, March 08, 2006

When Darwin Went Galápagos

Smithsonian Magazine made the 5,000-mile journey to the Galápagos Islands, to follow in Charles Darwin’s footsteps. Darwin came to the archipelago at age 26, which straddles the Equator some 600 miles west of Ecuador, as part of the Beagle’s five-year mission to survey the coast of South America and to conduct a series of longitudinal measurements around the globe. Darwin’s five-week visit to the islands catalyzed the scientific revolution that now bears his name.

The article recounts the tale of the finches that misled Darwin. There are 14 finch species in the Galápagos that have all evolved from a single ancestor over the past few million years. They have become one of the most famous cases of species adapting to different ecological niches. From Darwin’s specimen notebooks, it is clear he was fooled into thinking that some of the unusual finch species belonged to the families they have come to mimic through a process called convergent evolution. For example, Darwin thought the cactus finch, whose long, probing beak is specialized for obtaining nectar from cactus flowers (and dodging cactus spines), might be related to birds with long, pointed bills, such as meadowlarks and orioles. He also mistook the warbler finch for a wren. Not realizing that all of the finches were closely related, Darwin had no reason to suppose that they had evolved from a common ancestor, or that they differed from one island to another.

The author's own discovery, more than 30 years ago, that Darwin had misidentified some of his famous Galápagos finches led him to the Darwin Archive at Cambridge University Library, in England. There he found a manuscript trail that poked further holes in the legend that these birds precipitated an immediate “aha” moment. It was only after Darwin’s return to England, when experts in herpetology and ornithology began to correct his Galápagos reports, that he realized the extent of his collecting oversights and misidentifications. In particular, Darwin had failed to label most of his Galápagos birds by island, so he lacked the crucial evidence that would allow him to argue that different finch species had evolved separately while isolated on different islands of the Galápagos group.

Five months after his return to England, in March 1837, Darwin met with ornithologist John Gould. Five years older than Darwin, Gould was just beginning to become known for his beautifully illustrated monographs on birds, which today are highly prized collectors’ items. One of my most unexpected discoveries in the Darwin archives was the piece of paper on which Darwin recorded his crucial meeting with Gould. This manuscript clearly shows how Darwin’s thinking began to change as a result of Gould’s astute insights about the Galápagos birds. Unlike Darwin, Gould had instantly recognized the related nature of the Galápagos finches, and he also persuaded Darwin, who questioned him closely on the subject, that three of his four Galápagos mockingbirds were separate species rather than “only varieties.” Gould also informed Darwin that 25 of his 26 land birds from the Galápagos were new to science, as well as unique to those islands.

Gould’s taxonomic judgments finally caused Darwin to embrace the theory of evolution. Stunned by the realization that evolving varieties could break the supposedly fixed barrier that, according to creationism, prevents new species from forming, he quickly sought to rectify his previous collecting oversights by requesting island locality information from the carefully labeled collections of three Beagle shipmates. Two of these collections, by Captain FitzRoy and FitzRoy’s steward, Harry Fuller, contained 50 Galápagos birds, including more than 20 finches. Even Darwin’s servant, Covington, had done what Darwin had not, labeling by island his own personal collection of finches, which were later acquired by a private collector in England. The birth of the Darwinian revolution was a highly collaborative enterprise.

The case for evolution presented by this shared ornithological evidence nevertheless remained debatable for nearly a decade. Darwin was not entirely convinced Gould was right that all the finches were separate species, or even that they were all finches. Read more.

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