Darwin and Evolution - Part 2
Let's continue here what we left in our left article about Darwin and evolution. It was his opportunity to serve as the naturalist on the Beagle that provided Darwin with his first insights into how evolution might work. In fact, it was this voyage that changed his worldview and convinced him that evolution occurred. Darwin began to observe and document the kinds of evidence for evolutionary change that I mentioned in my previous post: geological change, homologies among species, relationships between fossil forms and living forms.
Particularly illuminating to Darwin were groups of animals and plants occurring on islands. He would find that if you went to a group of islands, you will often find, on different islands, species that were similar enough to obviously be related to each other, and yet different enough to be considered other species. Furthermore, these species occurring on islands would often resemble a species occurring on the mainland.
Darwin’s Finches
The most celebrated example of this phenomenon is a closely related group birds species, now collectively known as Darwin’s finches, that are found on the Galapagos Islands, about 700 kilometers off the coast of Peru. Each island has a different species that appears to be uniquely suited for that habitat. These are really representative of Darwin and evolution itself.
In particular, these species differed largely in the size and shape of their beaks. They used their beaks in different ways to feed on different kinds of material. For example, there are some species with very large beaks that appear to be suitable for crushing large and hard seeds. Other species have smaller beaks that are more suitable for handling smaller seeds. The size and shape of the different beaks did correspond to what Darwin observed about their feeding ecology.
Darwin wondered how is it possible that there are different species on islands that were similar and yet clearly different, and that the differences related so clearly to the environment in which those species lived. This pattern made perfect sense to Darwin if the various species were all descended from the same common ancestor. This was presumably and ancestor that had come from the mainland. Eventually, all of the island species had gradually evolved and diversified in a way that matched the habitats in which they live. Darwin called this pattern “descent with modification”.
Working on the Species Problem
Darwin spent five years traveling around the world, collecting animals and plants and making observations. He returned from his trip in 1836, loaded with specimens and notebooks of his observations. He spent more than 20 years working on what he called the “species problem”. During the time that he was working on this problem, he spent most of his time in England. He nonetheless continued to amass evidence by talking to naturalists who were collecting plants and animals from other parts of the world, and specially by looking at the effects of domestication on species.
These observations convinced him not only that evolution occurred, but also suggested a particular mechanism by which evolution could occur. Darwin proposed this mechanism, the theory of natural selection, as early as 1844. He wrote a paper, but he never published it. He was urged by friends and colleagues to publish it, even by his wife, but Darwin preferred to perfect his ideas. He didn’t want to propose this idea until he had amassed so much evidence that the idea could simply not be refuted. So, he spent another ten or more years revising his ideas, collecting more specimens, evaluating more data.
The Publication of the Origin
Darwin hand was forced, however, in 1858, when he received a manuscript from Alfred Russell Wallace, a well-known naturalist. Wallace’s manuscript essentially presented the same idea of natural selection that Darwin had been working on for 20 years. Wallace asked Darwin to submit this manuscript he sent for publication.
I could not imagine what Darwin felt at that moment. He wrote to his friend Charles Lyell asking him for advice about what to do. Lyell arranged for Wallace’s paper, and an excerpted synopsis of Darwin’s 1844 manuscript to be published simultaneously, giving credit for the discovery of the theory of natural selection to both men.
Darwin then quickly finished the book-length version of his ideas, which was published the following year, 1859, with the title: “On the Origin of Species by Means of Natural Selection”.
What’s in On the Origin of Species
The essential observations behind the theory of natural selection are very straightforward. Darwin was struck by three things. First, he was struck by how much variation he observed among individuals of the same species. All individuals of the same species look alike, but no two were exactly alike. Darwin also recognized that some of this individual variation is passed on from parents to offspring. He observed that traits are heritable. Darwin didn’t know the mechanism responsible for this, but he argued that a mechanism must exist. We today know that DNA is the molecule that holds information in living things, how it is replicated, and all the stuff that makes it much clearer to us.
Darwin’s next important observation was that most species produced more offspring than ever survive. In this, Darwin was influenced by the writings of Thomas Malthus, who was an economist. Malthus wrote an essay arguing that much of human suffering was inevitable as the result of the fact that human population would always grow faster than the available resources needed to support it. In other words, the size of the human population is limited by competition. Darwin saw that this was true in animals and plants as well.
These observations led Darwin to the following conclusions. First, given that more individuals are born in a population that can ever live, there must be competition for limited resources, so that only some of those individuals are going to survive and eventually reproduce. Second, because individuals in a population differ in their characteristics, not all individuals are expected to be successful in this competition. Individuals with the more favorable variations would be more likely to survive and reproduce. Therefore, these naturally selected individuals, as Darwin called them, would contribute more offspring to later generations.
Finally, if the traits that contributed to the success of the parents are heritable, then over time, there would be more individuals possessing those traits. We call traits that evolved this way “adaptations”. Over time, natural selection is thought to cause populations to change their characteristics in such a way that would increase adaptation to the prevailing environment.
The first edition of Darwin’s book was published in 1859. All 1000 copies were sold on the first day. People knew this idea was coming and were eager to digest it. Despite the original controversy that it caused, Darwin’s work established the fact that evolution occurred. Darwin came to be considered one of the most famous and celebrated scientists of his day.
By the end of the century, pretty much every biologist accepted the idea that evolution occurs.
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