Darwin Visits the Islands and Discovers a New World
“When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America….”
Thus begins Charles Darwin’s The Origin of Species By Means of Natural Selection, a book that has changed the way humans understand the world and our place in it.
Around the globe, 2009 is being celebrated as the Year of Darwin in recognition of his birth on February 12, 1809, and the publication of his landmark book, Origin of Species, on November 24, 1859.
In more than 25 years of teaching science in high schools and science education in universities, William F. McComas, the Parks Family Professor of Science Education at the University of Arkansas, has introduced hundreds of students to Darwin and evolution. He has prepared university students to become science educators, confident and able to teach a subject that has had a history of controversy. The author of three books about teaching science, he has traveled extensively worldwide – including to the Galapagos Islands — pursuing an interest in photographing ecologically significant places.
In his science textbooks and in the classroom, McComas uses teaching about evolution as an opportunity to address misconceptions about science. In particular, one of the most misunderstood notions surrounding evolution is the distinction between the reality of evolution as a series of ongoing events in the history of life and the explanation for how evolution has occurred, the theory of natural selection.
“Does life seem to have changed through time since its origins? The answer is yes, yes and yes,” McComas said. “There is no question that the record in the fossils is one of change through time. And that’s what evolution is. Period. Evolution has occurred — it is a fact. The big question for Darwin was how change through time could take place.”
Darwin’s answer, the theory of natural selection, has been subject to extensive questioning and testing.
“If there’s any argument about evolution, it deals with fine-tuning the elements of the mechanism. Even after 150 years of testing and criticizing natural selection, the theory holds up beautifully,” McComas said.
One of the reasons to celebrate Darwin’s genius, McComas explained, is that he developed an accurate theory of the mechanism of evolution without knowing much about how traits are inherited. The first true test came with Gregor Mendel’s discovery of the mechanism for inheritance, which we now call genetics. Natural selection as the explanation for evolution “passed this test brilliantly.”
Darwin had “put the puzzle together without all the pieces and with no picture on the cover of the box,” McComas said.
Darwin and the Islands
While Darwin’s discoveries are popularly linked with the Galapagos Islands and its flora and fauna, McComas noted that Darwin did not suggest the mechanism of natural selection while in the Galapagos Islands or even on his trip.
“The most important thing to understand about the Galapagos Islands is that by the time Charles Darwin got to those islands he’d already spent a good bit of time on other islands,” McComas said.
For Darwin, the Galapagos were another example of populations reproducing in isolation and an opportunity to see whether the pattern he’d observed on other islands was repeated. On the Galapagos Islands, he again found something strange and thought provoking. While the creatures on the islands come from the tropics, the Galapagos are an entirely different environment — remote volcanic islands surrounded by salt ocean. This raised a question for Darwin about why a creator would take something from the lush tropics of South America and make extensive and sometimes dramatic changes to it so that it could live successfully on such arid islands. That question, McComas said, “was the spark that finally led Darwin to accept that evolution had occurred.”
As Darwin wrote in his introduction to Origin of Species, “These facts seem to throw some light on the origin of species — that mystery of mysteries…. On my return home, it occurred to me in 1837, that something might perhaps be made out of this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it.”
Darwin’s realization that evolution had occurred “gave him something to think about for the rest of his life,” McComas said. With the reality of evolution established, Darwin could turn his attention to revealing its mechanism. The inhabitants of the Galapagos Islands had provided him important evidence.
Although Darwin’s finches have become a hallmark of his discoveries, “Darwin was much more taken with the mockingbirds than the finches,” McComas said. Before he arrived at the Galapagos, he had formed the idea that the same species would be present on all the islands. The mockingbirds challenged that notion. He was surprised to find that there were four distinct species of mockingbirds in the Galapagos Islands.
His assumption about the distribution of species led to “one of the most interesting mistakes in the history of science.” As McComas tells the story, while Darwin went from island to island collecting finches, he did not document the name of the island from which each bird came. The good news is that two other people from the ship were helping him collect specimens. Because they didn’t have any prior notion about what they might find, every time they shot a finch, they wrote down the location. Later, after Darwin had left the Galapagos and realized his mistake, he was able to reconstruct a lot of the data from the specimens that were properly labeled by his two assistants.
When Darwin returned to England, he gave his entire finch collection to John Gould, a famous British ornithologist. While studying and cataloging the collection, Gould realized that all the finches were actually from a common stock and that some of the unidentified birds Darwin had collected were actually finches.
It’s clear that many of the animals living on the Galapagos Islands did not originate there but carried the traits necessary to survive and for their progeny ultimately to evolve into new forms. For example, a tree-living iguana from the tropical coast of South America somehow got washed out to the islands. Even if those iguanas had been conscious of their plight, McComas said, “There wasn’t anything they could do about it beyond the traits that they brought with them. The only tool that any plant or any animal ever brings with it is in its genetic composition. It cannot produce new traits just because it might be useful to do so.”
On the Galapagos Islands, evolution took the iguanas in two different directions, McComas explained. Today hardy land iguanas with pointed faces get in amongst the cactus needles to feed. The others are the world’s only species of marine iguanas that live on the shore and feed in salt water. With their blunt faces they are able to nip close to underwater rocks and shave off the algae, their primary source of food. While the two iguanas look entirely different today, their genes show they came from a common ancestor, the South American tree iguana.
When Darwin arrived in the Galapagos Islands, the British governor there pointed out that some of the tortoises tasted better than others,” McComas said. “Darwin wondered why there would be such differences between the tortoises.” Again, Darwin assumed that with islands so close together, he would find the same creatures on them. Yet, McComas said, “With just a bit of help, even a third grader could look at the pictures of the tortoises and say that one came from a dry island, and that one came from a wet island.”
The shells of the dry island tortoises have pronounced flared plates, known as phalanges. Their shells flare up at the feet and are phalanged up dramatically where the neck sticks out. This is an adaptation: tortoises that were able to stretch to get their heads farther up into the trees on the dry islands were able to feed more effectively.
On the other hand, tortoises on the wet islands “look like common box turtles on steroids.” On the wet islands, McComas said, there was no evolutionary pressure to evolve the flared shells, so those individuals with even a hint of the trait did not have any advantage over those lacking phalanges.
Back in England
Even before Darwin returned to England, he shipped his collections to various experts, such as Gould, the ornithologist. Certainly, he did his senior colleagues a favor by sending them source material, McComas noted, while at the same time, he built his own reputation. When Darwin left for the voyage as a young man, no one knew who he was. By the time he returned home, he was one of the most famous naturalists in England.
Darwin spent 20 years after his return examining evidence and preparing detailed arguments to support the theory of evolution by natural selection. While Darwin was preparing the manuscript for Origin of Species, Alfred Russell Wallace, a naturalist-collector working in the Malay Archipelago, wrote him offering a parallel and nearly identical theory. Subsequently, papers from Darwin and Wallace were presented together at a meeting of the Linnaean Society in London in 1858. Their insights opened a new door for science.
In the concluding paragraphs of Origin of Species, Darwin pulls back from the detailed arguments of his book to consider the wonder of evolution:
“It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.”
And he concludes, using the word “evolved” for the first time in the book: “There is a grandeur in this view of life” in which “endless forms most beautiful and most wonderful have been, and are being, evolved.”
William F. McComas
William F. McComas is the Parks Family Professor of Science and Technology Education in the College of Education and Health Professions at the University of Arkansas. As a result of his professional interest in photography, he has developed photo-based instructional and resource units, a museum exhibition titled “The Galapagos Islands: Evolution’s Showcase” and assignments to ecology research sites in Central America, New England and Colorado.