ALLEN MACNEILL: These are blue-footed boobies doing their courtship dance in the Galapagos Islands. They were filmed by evolutionary biologist and historian of science William Provine. He was there for an international meeting of evolutionary biologists who gathered together to give papers and to talk about the current state of Darwin's theory of evolution.
They met in the Galapagos because 170 years earlier in the summer of 1835 a young Englishman visited there. Four years earlier, Charles Darwin had graduated from Cambridge University with a bachelor's degree in Anglican theology. He had been invited along as a gentleman's companion for the captain of HMS Beagle, a British Naval ship whose mission was to survey the coast of South America. When the voyage began in the winter of 1831, Darwin was essentially a talented amateur naturalist. As a student, he had not distinguished himself, earning a gentleman's C in theology, a degree that qualified him to become a simple country parson in the Anglican Church.
However, Darwin was as dedicated to natural history as he was desultory in theology. He was, what his uncle Josiah Wedgwood called him, a man of enlarged curiosity, who spent all of his free time collecting specimens-- especially beetles-- and studying nature. The five years he spent on board the Beagle changed his life forever and laid the foundation for his theory of evolution by natural selection. In 1836 when Darwin returned from the voyage of the Beagle, he began to put his notes and specimens in order.
He first collaborated with the captain of the Beagle on a technical report running to four volumes about the geology and natural history of the trip. He then rewrote the third volume of that edition as a book to stand alone by itself. We now know it as The Voyage of HMS Beagle. And it was his first tremendous public success. It was read widely throughout England and is still in print today. But many of his readers and even some of his closest friends did not know that at the same time he was entertaining an almost unthinkable idea-- that species were not fixed but had rather changed over time.
In 1839, he married his cousin, Emma Wedgwood, and moved to this beautiful house in Kent, called Down House, about 16 miles outside the city of London. There he began a meticulous study of the specimens that he had sent back to England from the voyage. In particular, he noticed something very curious about some of the small birds that had been collected on this trip. Here they are. We now know them as Darwin's finches.
What he noticed is that on the mainland South America there were many different species of birds unrelated to each other doing many different things. There were finches, of course, but there were also thrushes and warblers and woodpeckers. On the Galapagos there were just finches.
But those finches did everything that the birds on the mainland did. There were finches that acted like thrushes and finches that acted like warblers. There's even a finch on the Galapagos that breaks the thorns off of cactuses and uses it to dig grubs out from under the bark of trees like woodpeckers.
Darwin realized that a plausible explanation for this pattern of diversity in the finches was that finches were the only birds that had made it to the islands. And without competition, they had he diversified into all of the other species of finch, doing all the things that other birds did on the mainland. But this suggested something almost unthinkable-- that this species of finches were not fixed, that instead they had changed over time from an ancestral form of finch to all of the various different kinds of finches found there now. Darwin had discovered what he called descent with modification, what we now call evolution.
Darwin lacked what Lamarck had provided-- a mechanism by which descent with modification could have occurred. The solution finally came to him on the evening of September 28, 1838 when, in his own words, "I happened to read for amusement Malthus on population and being well-prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants. It had once struck me that under these circumstances, favorable variations would tend to be preserved and unfavorable ones to be destroyed. The results of this would be the formation of a new species. Here, then, I had at last got a theory by which to work." Darwin had discovered natural selection.
Why didn't Darwin simply recapitulate Lamarck's theory of evolution by the inheritance of acquired characteristics? Well, it turns out that he did. If you read the Origin of Species, you find that Darwin often referred to use and disuse as a process by which new variations could appear in populations.
But Darwin contrasted his theory with Lamarck's for a very important reason. Lamarck's theory was relentlessly purposeful. As we saw in a previous segment, Lamarck's theory was progressive moving to evermore complex adaptations. And everything that Lamarck explained he explained with the concept of purpose.
To understand how Darwin's theory was fundamentally different from Lamarck's, let's take a look at the four conditions that Darwin set out as the conditions that have to be met for evolution to occur as the result of natural selection. In the first two chapters of The Origin, Darwin gave numerous examples of variations in different species of animals, especially pigeons. He did this to establish the first and most important condition for evolution by natural selection.
Variety-- there are real variations between individuals in all known populations of living things. Darwin pointed out that variations that were not heritable from parents to offspring did not matter. Only heritable variations could produce changes over time.
Heredity-- some variations can be inherited from parents to offspring. Darwin also realized that only individuals who reproduce can pass on such variations. Individuals who do not reproduce are invisible to natural selection.
Fecundity-- some individuals reproduce at a rate greater than replacement. Finally, Darwin pointed out that if these three conditions are met, there would be an inevitable outcome. Demography-- some individuals survive and reproduce more often than others. As a result, the characteristics of such individuals become more common in populations over time. This demographic process is the core of natural selection.
None of these four conditions requires any kind of purpose whatsoever. It's not that they disprove that purpose is part of evolution. It's simply that purpose is not necessary for these explanations. The source of all evolutionary variation, of the raw material that evolution uses, is in the first condition-- that is, variety. Variation-- the engines of variation that produce all the new forms.
Notice as well that natural selection itself is not a creative process. The engines of variation are the creative processes in evolution. Natural selection, in fact, has the opposite effect. It tends to reduce the variation in populations by eliminating those individuals who are not well-adapted to their environment. As a result, natural selection is the process by which adaptations evolve.
It took Darwin 20 long years to assemble his evidence and begin the writing of his big species book, which he planned to call Natural Selection. Then, in the late spring of 1858, Darwin received a letter from Alfred Russel Wallace, a professional collector and naturalist. In the letter, Wallace included a manuscript for a paper "On the Tendency for Varieties to Depart Indefinitely from the Original Type-- Darwin's Theory of Evolution by Natural Selection."
Darwin's friends prevailed upon him to have his theory, as yet unpublished, read before a meeting of the Linnean Society along with Wallace's paper in the summer of 1858. And so Darwin was forced to share priority for his discovery of the theory of evolution by natural selection. Immediately, he set to work furiously condensing the book that he had been writing for 20 years. And in November of 1859, he published what is quite possibly the most important book ever written in the science of biology-- The Origin of Species.
This is it-- Darwin's little book on descent with modification. It was a tremendous success. The first print run sold out in one afternoon. In all, it ran through six editions and has been in print continuously ever since the fall of 1859. It established his reputation as the premier naturalist of England and established evolution as the primary theory to explain the origin of species.
It is therefore all the more ironic that in the 50 years following the publication of this little book, Darwin's theory fell from grace among scientists. Not that evolution fell from grace. On the contrary, within the first decade or so after the publication of The Origin of Species, almost all scientists worldwide had adopted his theory.
But what they did not adopt was his theory of evolution by natural selection. It was Darwin's theory of evolution by natural selection that fell out of favor. By 1900, so few biologists and scientists accepted the theory of natural selection that it was widely said among scientists and members of the public that Darwinism was all but dead. In the next segment in this series, we'll find out what happened to the theory.
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Published 150 years ago, Charles Darwin's
On the Origin of Species provided the foundation for the modern science of biology. It also set in motion a revolution in the sciences and in our understanding of ourselves and our place in nature.
This CyberTower Study Room is a brief introduction to Darwin's theory and its implications. Beginning with an overview of Darwin's predecessors, we learn how Jean Baptiste Lamarck set the stage for Darwin's monumental achievement with his Philosophie Zoologique, which advanced a theory of evolution by means of the inheritance of acquired characteristics.
Darwin, whose academic training at Cambridge University was in Anglican theology, became an acclaimed naturalist and science writer following the five-year voyage of HMS Beagle. Using the notes and specimens that he had collected during the voyage, Darwin spent twenty years refining his theory, first published in 1859, of evolution by natural selection.
In the last segment of this Study Room, we visit the Museum of the Earth in Ithaca, New York, whose director, Dr. Warren Allman, discusses the importance of such museums to the science of evolutionary biology. We also hear from Cornell professor William Provine, who discusses Darwin's work and its importance to the history and philosophy of biology.
This video is part 4 of 6 in the Darwinian Revolutions series.