It was the tortoises, finches, and mockingbirds he found on the Galapagos Islands off the west coast of Ecuador, beginning in 1831, that first got young Charles Darwin thinking. The animals varied from island to island, and also from similar species on the mainland. Why did they vary? And how did it happen?
Those questions would lead Darwin on a three-decade odyssey that culminated in 1859 in the publication of the British naturalist's most famous work, On the Origin of Species, which explained how animals, including humans, all evolved from a common ancestor. Darwin's theory of evolution by natural selection has survived for 150 years, though not without controversy that continues today.
Darwin was 22--and already infatuated with beetles and natural history--when one of his professors at Cambridge University recommended him as the resident botanist on a mapping expedition of South America. Darwin would spend five years on the H.M.S. Beagle on a voyage that circled the globe, mostly in the Southern Hemisphere, unlocking a new world of biology and geology along the way. His travels introduced him to previously undiscovered living species and to the fossilized remains of long-extinct ones that failed, it seemed, to adapt to a changing environment.
In the mid-19th century, there were several schools of thought about how humans and animals as varied as worms and dinosaurs had come to exist. One argued that species had simply originated wherever and however they were currently found, echoing the Biblical view that God had created humans and all animal life. Another maintained that new species were formed as previous generations of related species were killed off by natural catastrophes, such as the Ice Age and asteroid and comet strikes. And one theory hinted that animals somehow adapted to their environments and passed 'along new traits to their offspring.
But Darwin's observations on his Beagle voyage suggested other possibilities.
"It is a grand spectacle to see all nature thus raging," he wrote in a letter in 1833.
Darwin's mind was raging too. He found slight variations in turtles and birds in the Galapagos from one island to the next; in Australia, where the Beagle arrived in 1836, he saw unique life forms that seemed to exist nowhere else.
"These facts seemed to me to throw some light on the origin of species," Darwin later wrote, "that mystery of mysteries, as it has been called."
For Darwin, the voyage of the Beagle was only the beginning. Back in England, he began publishing journal articles (some of his letters were published even before he returned) with his painstakingly precise observations of flora and fauna. But the theory of evolution that would make him famous--which other scientists, too, were beginning to write about--itself evolved slowly.
He would later recall that one of his keenest insights was that the struggle for survival by plants and animals meant that "favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this would be the formation of new species."
While his personal observations from the Beagle voyage formed the backbone of his hypothesis, he continued to gather evidence from both living species and fossils that eventually affirmed the thrust of his argument: that species had evolved through a process he called "natural selection."
In his introduction to On the Origin of Species in 1859, Darwin wrote:
"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."
In other words, in each generation random mutations occur. Those more beneficial to the survival of a species endure and are incorporated into future generations. That's why, for example, Darwin concluded that male peacocks have such flamboyant tail feathers: Female peacocks choose males with the most showy ornamentation, and therefore the most flashy peacocks would produce the most offspring.
Darwin's theory of evolution--a word he barely mentioned and never took credit for originating-was quickly embraced by biologists, but he was denounced by religious authorities for challenging the biblical idea of divine creation. (Scientists were initially skeptical that evolution occurred precisely the way Darwin theorized, but by the 1870s natural selection had gained wider acceptance.)
The impact of the theory, not only on science, but on modem thought in general, was enormous.
'ALL ORGANISMS BECAME CONNECTED'
"Before the Origin," writes Olivia Judson, an evolutionary biologist and New York Times columnist, "similarities and differences between species were mere curiosities; questions as to why a certain plant is succulent like a cactus or deciduous like a maple could be answered only, 'Because.' Biology itself was nothing more than a vast exercise in catalog and description. After the Origin, all organisms became connected, part of the same, profoundly ancient, family tree."
Darwin had no detailed knowledge of genetics, which Gregor Mendel, a German priest and scientist, would develop later in the 19th century, or of DNA, which was not fully identified until the 20th century. But his conclusions largely anticipated the role they would play in explaining the physical basis for the variations behind evolution.
Those conclusions also changed the way humans saw themselves. "Darwin's theory challenged the notion of human exceptionalism," says Eugenie Scott, executive director of the National Center for Science Education, "and brought to light this idea that humans are a result of natural processes, meaning we were not as special as we once thought."
The fact that humans more or less haphazardly evolved from an ancestor common to all animals presented a challenge to religious orthodoxy--the idea that God created man and woman--that has yet to be reconciled.
Still, there was little formal opposition to teaching evolution in the U.S. until after World War I when some conservative Christians lobbied state legislatures to ban it from the public school curriculum.
In 1925, Tennessee legislators made it illegal for public schools to teach "any theory that denies the story of the Divine Creation of man as taught in the Bible," and the following year, in a trial that riveted the nation, 24-year-old John T. Scopes was convicted for teaching evolution in his high school biology class in Dayton, Tenn.
Scopes, whose trial was recounted in the popular play and film Inherit the Wind, was defended by famed lawyer Clarence Darrow; William Jennings Bryan, the Democratic nominee for President in 1896, 1900 and 1908, argued for the prosecution. Scopes was fined $100, but the verdict was later overturned on a technicality.
During the 1970s and '80s, some states mandated equal time for teaching "creation science" (based on the biblical account of creation) alongside evolution. But in 1987, the Supreme Court ruled in Edwards v. Aguillard that such requirements were unconstitutional because they violated the First Amendment's establishment clause, which says: "Congress shall make no law respecting an establishment of religion."
More recently, challenges to evolution, mostly on religious grounds, have arisen in Georgia, Kansas, Pennsylvania, Florida, and Texas. In 2005, an effort by the school board in Dover, Pennsylvania, to require the teaching of "intelligent design"--which holds that any complex design, such as that of the human brain mad body, requires an intelligent creator led to a lawsuit from parents and the board's defeat in the next election.
Three years later, Florida education officials voted to add evolution to the state's required curriculum but mandated that Darwin's conclusions be described as only a theory. (The previous standards didn't mention evolution.) And in March of this year, the State Board of Education in Texas, in a close vote, barely upheld the teaching of evolution as accepted mainstream science.
Indeed, even though the vast majority of scientists have embraced Darwin's theory, there are still large numbers of Americans who have doubts about evolution 150 years later. In February, a Gallup poll found that 39 percent of Americans believe in evolution, although the number rose to 53 percent among college graduates and 72 percent for those with postgraduate degrees.
While there may be a wide range of opinions about evolution, Darwin's influence on modem life can't be underestimated. His methods and conclusions laid the groundwork for much of modem science and even modem thought, according to Olivia Judson.
"In short," Judson writes, "Darwin gave us a framework for asking questions about the natural world, and about ourselves."
Ask students to define the term "theory," then brainstorm a List of theories on the board.
* How does a hypothesis become a theory?
* How do scientists test theories? Why do some theories gain acceptance while others do not?
* What is Darwin's theory of evolution? Do any other scientific theories contradict it? What other kinds of beliefs contradict it?
* Do the differing views necessarily exclude each other? Can they complement one another?
Explain your view.
Write a reflective essay discussing your beliefs about evolution and how you came to hold them.
Support or refute: John Scopes was rightly convicted in Tennessee in 1925.
In what ways did Darwin change how humans view themselves? Why do you think his theory has been the source of so much contention for 150 years?
The article discusses several court cases related to the teaching of evolution in schools, from the 1920s until today. Why has so much of the public debate about evolution played out in schools?
Do humans show signs of adaptation or demonstrate the idea of natural selection?
Why might polls show that belief in evolution increases with education level?
Do you think most Americans will eventually accept Darwin's views? Why or why not?
In 1794, Charles Darwin's grandfather, Erasmus Darwin, developed one of the first theories of evolution in his book, Zoonomia. He thought that all life had evolved from one common ancestor, which over time branched off into different species.
PBS's Evolution site is a comprehensive multimedia exploration of the ways in which art animals continue to adapt and evolve, and also presents alternate theories.
QUIZ 3 * TIME PAST
(1) Darwin's study of animals on the--Islands raised questions for him about the variation of species around the world.
(2) Darwin's evolutionary theories did not include the idea that
a all animals evolved from a common ancestor.
b only the fittest of the species will survive.
c some animals are better suited for their environment than others.
d species originated wherever they were found.
(3) Darwin's theory of natural selection holds that in each generation,
a an entire species can be wiped out because of failure to adapt to environmental, changes.
b random mutations occur, and the most beneficial are passed on to future generations.
c no animals in a species will be weaker than any from the previous generation.
d stronger, more aggressive animals always strengthen the gene pool.
(4) In 1925, the Scopes trial
a was the first to challenge a scientific theory.
b didn't attract much attention.
c Led to the conviction of a teacher for teaching the theory of evolution.
d overturned a previous ruling, which mandated the teaching of evolution.
(5) In Edwards v. Aguillard, the Supreme Court ruled that the teaching of "creation science" in public schools
a violated the First Amendment.
b needed to be explained better.
c should be given equal time with the teaching of evolution.
d was similar to "intelligent design," and so its teaching could be required by school districts.
(1) What are some examples of natural selection and adaptation of animal species?
(2) In what ways do you think the theory of evolution affected modern thought?
(3) What do the results of the recent Gallup poll discussed in the article seem to indicate about who believes in evolutionary theory? Why might this be?
(1) [a] Galapagos
(2) [d] species originated wherever they were found.
(3) [b] random mutations occur, and the most beneficial are passed on to future generations.
(4) [c] led to the conviction of a teacher for teaching the theory of evolution
(5) [a] violated the First Amendment.
Sam Roberts is urban affairs correspondent for The New York Times.
Roberts, Sam. "1859 Darwin and the 'Origin of Species': how one man redefined what it means to be human--and why his work is still a source of contention today." New York Times Upfront 14 Dec. 2009: 16+. Academic OneFile. Web. 17 Dec. 2009.
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