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Charles Darwin's evidence for evolution

by Dr. Niles Eldredge

Young naturalist

Charles Darwin was only 22 years old in 1831 when he sailed as ship's naturalist on the H.M.S. Beagle on what would turn out to be a five-year voyage circumnavigating the globe. A hunter and specimen collector (he especially liked rocks and minerals—and beetles), Darwin was an all-around outdoorsman. He had not especially liked school, though by the time he left England on the Beagle he had taken the botany course taught by Prof. John Stevens Henslow at Cambridge University three times. He had also profited from a short excursion to learn the rudiments of field geology from Prof. Adam Sedgwick only a few months earlier. But Darwin was far from a highly trained or seasoned scientist.
Although bored at times by formal schooling, the young Charles Darwin poured enormous energy into his fascination with the natural world. By the time he attended the Cambridge University, Charles had been singled out by elite circle of prominent professors who recognized his potential as a scientist. 
© Darwin Heirlooms Trust, English Heritage Photo Library
In those early days of professional science in England, many believed that the idea of "transmutation" was false and anti-religious. Transmutation was the contemporary term for what we now call "evolution," or the idea that all species now and formerly alive are descended from a single common ancestor that lived in the remote geological past. The concept was more widely accepted on the continent by scientists such as the well-known biologist Jean-Baptiste Lamarck and by Etienne Geoffroy, who worked at the natural history museum in Paris. In England, the tendency was still to reconcile the findings of natural history—primarily geology, paleontology, botany, and zoology—with a literal reading of the biblical account of creation. Thus, most people in England—scientists, clergy, and the public at large—were creationists. They believed that God had created each species separately, in more or less its present form, no more than 10,000 years ago.
But there were a few exceptions. Darwin's own grandfather, the successful physician Erasmus Darwin, wrote Zoonomia, a book on physiology and medicine that included evolutionary ideas. And when Darwin's father (also a physician) sent Darwin to Edinburgh to study medicine at the tender age of 16, Charles met the young zoologist Robert Grant, a devotee of Lamarck's evolutionary ideas and also a fan of Darwin's own grandfather. Years later, in his Autobiography, Darwin wrote that he did not think that either his grandfather's or Grant's pro-evolutionary views had much "effect on my mind," though he conceded that the ideas of such important figures in his life must have meant something to him!
When Darwin set sail, then, as a Cambridge graduate and also a cautious and conservative young gentleman, he was a creationist, but one who had already heard arguments in favor of transmutation. It is difficult to tell precisely when he decided in favor of evolution, though it is certain that by the time he reached home in the fall of 1836, he was a convert. Several factors—what Darwin saw on the trip, how he reconciled his experiences with his onboard reading, and the intelligence and ambition of a young man anxious to make his mark in the scientific world—combined to lead him to the conclusion that the origin of new species could be explained by natural law.
While the primary purpose of the Beagle voyage, which was sponsored by the British government, was to survey and map the coast of South America, Darwin's task was to make scientific observations. In fact, two-thirds of Darwin's time was spent on dry land, largely in the wilderness in Brazil, Argentina, and Chile, and in remote areas such as the Galápagos Islands.

A trip around the world

What was it about Darwin's experiences that eventually led him to turn against the entrenched scientific view of his mentors and peers—as well as the religious beliefs of almost all his countrymen? For one thing, the young scientist conducted five years of close, incessant, almost obsessive fieldwork during the voyage. Darwin observed and collected whatever was at hand, wherever he was: fossils at one place, birds at another, the local geology in places when it was well exposed. Few of his predecessors had ever enjoyed such rich and prolonged experience in the field.
Paradoxically, it was commonplace by the 1830s for biologists and paleontologists to talk in what we would now call "phylogenetic" terms, terms that reflect the evolutionary development of organisms. Going as far back as Linnaeus in the preceding century, people recognized, whether they were evolutionists or creationists, the existence of "natural groups," and they classified organisms accordingly. They also saw connections between species. For example, Darwin wrote openly to his creationist mentor Henslow about the intermediate characteristics of a snake he had collected: a fer-de-lance with a hard casing at the end of its tail that Darwin saw as intermediate between the plain tail of a European viper and the full-blown rattle of a rattlesnake—which he considered the "more perfect" organ. But such talk was commonplace, and not in itself "evolutionary."
Darwin read the anti-evolutionary books of both the Frenchman Georges Cuvier, who believed in numerous separate creations following episodes of extinction, and his fellow Englishman Charles Lyell, who is remembered as the founder of modern geology. Lyell's Principles of Geology was published in three volumes: The first volume was presented to Darwin by the Beagle's captain, Robert FitzRoy, before the ship set sail. The second volume, sent to Darwin while he was at sea, was a diatribe against Lamarck's transmutational ideas. Darwin always saw himself first and foremost as a geologist, and Lyell's work inspired him as he labored to understand the geology of South America. But Lyell's attacks on evolution arguably inspired Darwin in precisely the opposite direction.

The first pattern

The glyptodont was an immense shelled animal with a morphology similar to the modern armadillo, but on a massive scale. Darwin was delighted by the armadillos he saw scurrying about in Argentina. But most intriguing to him was the striking similarity of these small armored mammals to some of the fossils he was unearthing. © AMNH
Both Lyell and Cuvier, who is also remembered as the "father of comparative anatomy," discussed extinctions and new appearances of species. Both thought in terms of the migration of faunas. In addition, Lyell had a distinctive (and rather brilliant) ecological bent. He thought of new species being created in terms of ecosystem roles: a new species of carnivore could not be created unless and until a suitable prey species was already in place.
Not so Darwin, who focused on the appearance of new species as replacements for extinct species of the same natural group. At his very first stop, on St. Jago in the Canary Islands, he saw that the fossilized invertebrates exposed in the rocks on the beach were the very same species whose shells littered the beach. These observations drew on his training in geology and paleontology with Sedgwick, his reading of Lyell, and his focus on comparing fossil and modern specimens of the same natural group.
In 1832, as the Beagle was exploring the Bahia Blanca along the coast of Argentina, Darwin discovered the bones of fossil mammals at two localities. In a place where armadillos abounded, he was especially struck by what looked liked the outer shell of an extinct form of armadillo. In another location he found the bones of extinct giant ground sloths. (Armadillos and sloths belong to the Order Edentata, a primitive group of mammals restricted to the South American continent.) Why would fossil remains and modern species found on the same continent resemble each other so closely, he wondered?

The second pattern

In 1837, bird expert John Gould named this rare South American ostrich Rhea darwinii in honor of its collector. Gould's confirmation that this ostrich was indeed a distinct species fueled Darwin's curiosity. Darwin knew that a larger rhea was common in an adjacent region. Were the two derived from a common ancestor?
© AMNH Library
This diverse evidence from the mammalian fossil record led Darwin to speculate about the causes of extinction and to pay special attention to endemic species, or those that belong to groups that occur nowhere else on the planet. By focusing on endemic species, he was eliminating the possibility that they had migrated from elsewhere. Whatever had happened to put new, replacement species in place had occurred right there in South America.
Darwin was also struck by a second pattern. In different parts of South America, many species appeared to have been replaced by species that were similar yet distinct. For example, the common rhea, an ostrich-like bird that Darwin had been studying—and eating!—in northern Argentina, is replaced south of the Rio Negro by a smaller, browner species. In this case, Darwin focused on "allied" species that replace each other geographically—that is, across space rather than time.
The Beagle reached the Galápagos Islands in the fall of 1835. There Darwin saw the same patterns he had already encountered on the South American continent, though on a smaller, more refined scale. Though he is famous for not having paid attention to the birds that have become known as "Darwin's finches," he did observe that distinct forms of otherwise similar animals occur on the islands. For example, there were three or four distinct "varieties" or species of mockingbird on the different islands, all closely "allied" not only to one another but also to those on the mainland. His attention was also drawn to differences in the shapes of the shells of giant tortoises, with each island home to a slightly different form.
In the summer of 1836 Darwin wrote up detailed notes on the specimens he had collected. He concluded that the pattern of differences between closely allied forms on different islands, as well as between those on the islands and those on the mainland, had great potential significance. He saw the slightly different forms of mockingbirds and tortoises on the different islands as beginning stages in the development of new species from common ancestors. He wrote: "If there is the slightest foundation for these remarks the zoology of Archipelagoes will be well worth examining; for such facts (would) undermine the stability of Species." In the cautious manner that would remain typical of him until he was finally goaded to publish his evolutionary ideas in 1859, Darwin went back and inserted the word "would." Nevertheless, there's no real doubt that by the time he penned these words, Darwin had become a committed transmutationist.
Galápagos tortoises are adapted for different feeding habits. The "saddle-backed" tortoises have shells that rise in the front like a saddle. This adaptation makes it easier for them to lift their necks and feed on taller cactus. "Dome-shaped" tortoises live on islands where most of the vegetation is close to the ground, making it unnecessary for them to raise their heads to feed.
© Minden Pictures, and © Woods Hole Oceanographic Institution

The third pattern

Thus, it was three patterns—the replacement of extinct species by modern ones (as in the case of armadillos and giant sloths); the geographic replacement of one species by another over large portions of the South American continent (rheas); and the micro-version of the same geographic replacement patterns on different islands in a chain (mockingbirds and tortoises)—that led Darwin to entertain, and then accept, the idea of evolution. It was his focus on endemic species in "allied groups" that set him apart from those who could not—or perhaps would not—accept transmutation.
When he arrived home in 1836, Darwin immediately began the search for a mechanism that could cause evolutionary change. After several false starts, Darwin hit upon the idea of natural selection, which combined his knowledge of variation and inheritance with a realization derived from reading the economist Thomas Malthus: that more organisms are born each generation than can possibly survive and reproduce. The organisms with the heritable variations that make them best suited for surviving are the most likely to produce offspring—and to pass on their "recipe for success" to succeeding generations. When environments change, different variants will be favored, and evolutionary change will occur.
Darwin had arrived at a complete theory of evolution by 1839, but it was to be another 20 years before he published his ideas of evolution through natural selection in his epochal book On the Origin of Species by Means of Natural Selection.
This essay was developed for the AMNH online course Evolution, part of Seminars on Science, a program of online professional development courses for educators.
Additional resource:
Darwin's Manuscript Project

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