By Robert Hazen, George Mason University
That life has changed over almost four billion years of Earth history is as firmly established as any idea in science. But how does life change? More than 150 years ago, naturalist Charles Darwin discovered that life on Earth continues to evolve by the process of natural selection.
Darwin’s Observations
At the time of Darwin’s travels, it was widely accepted—primarily on religious grounds—that species can’t be changed, that species were the result of a single creation event. His observations caused him to question that idea, but he did it rather quietly, and in a personal way.
By 1837, less than a year after his return from that voyage, he had begun a personal Notebook on the Transmutation of the Species. Transmutation, here, refers to change, or evolution. He tabulated all sorts of evidence for evolution. He noted the similarities in animal behavior. He noted, for example, the yawning of dogs, horses, and men as all similar physiological reactions. He cited a whole litany of evidence from fossils, from comparative anatomy, from vestigial organs, and from the geographical distribution of closely related, but distinct, species.
Thomas Robert Malthus
Eventually, having convinced himself of the fact of evolution, Darwin set about trying to understand the mechanisms of evolution. He was greatly influenced by the British economist Thomas Robert Malthus (1766 to 1834). Malthus wrote the Essay on the Principles of Population, published in 1798, and this was extremely influential to Darwin.
What Malthus said is that there’s a relationship between food supply and human population. If you have unchecked population, Malthus argued, you’d lead to human misery. Darwin saw an exact parallel to animal and plant populations in the natural world.
The Origin Of Species
By 1838, Darwin’s theory of evolution had been framed. By 1842, he had written a brief outline of this new theory of evolution, which he called “natural selection”, and he had shared it with a few scientific colleagues. In spite the fact that this was 1842, he hesitated to publish the work, because he knew he’d meet great resistance to these ideas.
It was only in 1858—after considerable prodding from his friends, and also a letter from the naturalist Alfred Russell Wallace, who had independently arrived at a similar theory—that Darwin finally wrote what he called an “abstract” of his thesis. That was the revolutionary book, The Origin of Species, which was published in 1859.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Theory of Natural Selection
Darwin’s Origin of Species is presented in lucid and very modest prose. It describes the process of evolution by natural selection. This theory s based on four commonsense observations and arguments.
- The first of these is that all species produce more offspring than can possibly survive to maturity, because the resources of food and water and space are limited.
- The second idea is that individuals of a species display variations in their physical characteristics. These variations include coloring, speed, eyesight, and intelligence. Individual traits, furthermore, usually resemble those of the parents.
- The third idea is that advantageous traits provide an individual with a better chance of surviving to produce more offspring, and therefore, a better chance of passing on those traits from one generation to the next.
- The fourth point is that after many generations, gradual changes add up to large changes, that is, evolution. This is a biological example of the kinds of geological thinking that had gone on before; small changes adding up to very large changes, over long spans of time. Darwin labeled this process “natural selection”.
Parallels with Artificial Selection
Darwin presented his case in a carefully reasoned series of steps, presenting evidence in a very low-key way, like a legal brief. He began by drawing parallels with the process of “artificial selection,” or what we call selective breeding, well-known to British farmers and sportsmen. Humans, as you know, can produce a new breed of dog, or a new breed of cattle, in just a few dozen generations of very careful selective breeding, selecting for desirable traits.
In Darwin’s book, he talks at great length about domesticated pigeons, which are used for racing and for other ornamental displays, and how you can evolve new varieties of pigeons very quickly in this way. He said that nature does the same thing, though in a less-directed, more random process. During Darwin’s time, there was an exact example of this process that went on right in Britain, and this is the story of the peppered moth.
Observing the Peppered Moth
The peppered moth, a common moth in Britain, occurs in two varieties, a light variety and a darker variety. When Darwin first started thinking about this procedure, most of the tree trunks in England were rather light in color. The light-colored moth, which got its camouflage from tree trunks, was more dominant.
During the course of the 19th century, during the Industrial Revolution, as soot kept belching out of smoke stacks, the trunks of trees got darker and darker and darker. Gradually, the dark peppered moth became much more abundant, because the light-colored moths were being eaten selectively.
In more recent years, as coal burning is no longer allowed, tree trunks have lightened up, and so have the moth populations. This is a visible example, over a several-hundred-year period, of a change in the physical characteristic by natural selection.
This theory continues to receive much study and much research, as scientists attempt to understand the rates of evolutionary change, as well as the causes of extinction.
Common Questions about Darwin’s Theory of Natural Selection
Charles Darwin’s revolutionary book, The Origin of Species, was published in 1859.
Charles Darwin was greatly influenced by the British economist Thomas Robert Malthus, who lived from 1766 to 1834. Malthus wrote the Essay on the Principles of Population; it was published in 1798, and this was extremely influential to Darwin.
Charles Darwin concluded from his observations that after many generations, gradual changes add up to large changes, that is, evolution. This is a biological example of the kinds of geological thinking that had gone on before; small changes adding up to very large changes, over long spans of time. Darwin labeled this process “natural selection”.
