Darwin and Natural Selection

Darwin and Natural Selection

Lecture 13

The result of being born into a family that had money and a commitment to research was that [Darwin] was able to spend most of his life studying the one thing he most wanted to study, which was the natural world. You really do have to envy him. How many of us would love to be so privileged?

We’ve seen repeatedly that modern scienti c accounts of the Universe are historical; they tell a story of change at all scales, from the scale of the Universe to the scale of human history. Darwin’s great achievement was to show that this is also true of living species. This lecture describes Darwin’s elegant solution to the riddle of adaptation. One of Darwin’s grandfathers, Erasmus Darwin, was a doctor who was intrigued by how living organisms seemed to change over time. His other grandfather, Josiah Wedgwood, was a well-known scientist, a friend of James Watt and chemist Joseph Priestly, and the founder of the Wedgwood pottery works.

As a child, Darwin (1809–1882) was fascinated by the natural world. His father despaired of him, writing, “You care for nothing but shooting, dogs and rat-catching, and you will be a disgrace to yourself and all your family” (Eldredge, Darwin, p. 22). Darwin resisted pressure to become a surgeon (he was appalled by the screams of patients undergoing operations without anesthesia) or a clergyman. He wrote in his autobiography, “No pursuit at Cambridge was followed with nearly so much eagerness or gave me so much pleasure as collecting beetles” (Eldredge, Darwin, p. 24). Darwin described as the most important event of his life an invitation that he received in 1831 from Captain Robert Fitzroy to travel around the world as the naturalist on a ship called the Beagle. The voyage lasted from 1831 to 1836. It took Darwin to South America, around Cape Horn, across the Paci c via the Galapagos Islands, Tahiti, and New Zealand, to Australia, Mauritius, around South Africa, to Cape Verde Island, and back to Britain. Darwin collected specimens and fossils and took detailed biological and geological notes.

When Darwin left on the Beagle, he shared the orthodox belief that living species had been made by the creator, more or less in their existing forms. What he saw on his travels undermined this conviction. First, he observed the staggering variety of living organisms and the many subtle variations between species. Second, he noted many examples of similar but not identical species living close to each other, such as the nches, tortoises, and iguanas of the Galapagos Islands. In South America, he also found fossils (such as those of armadillo-like creatures) that were similar but not identical to species still living in the same areas. He concluded that all species, including humans, must be the products of slow, continuous change. In a letter to a friend, he wrote, “I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable” (Eldredge, Darwin, p. 58). Third, Darwin understood that few would accept this conclusion unless he could explain how species changed. In other words, he had to solve the riddle of adaptation.

Darwin stumbled on the solution two years after returning home, after reading Thomas Malthus’s Essay on the Principle of Population (1798). Malthus was a pioneer of demography, or the study of populations. He pointed to the terrifying fact that in every generation and every species, most individuals die before they can reproduce. Darwin immediately saw an analogy with pigeon breeding. Breeders only allowed individuals with particular features to breed, in the expectation that these features would become more common in subsequent generations. Darwin concluded that nature “selected” individuals to breed in a similar way.

But what were the criteria for selection used by nature? His answer was “ tness”—how well they tted their environment. Those individuals whose features best tted them for their environment would survive and reproduce just as those best tted to the speci cations of the breeder survived in the arti cial world of pigeon breeding. In his autobiography, he wrote, “it at once struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result would be the formation of new species” (Eldredge, Darwin, p. 52). Repeated over many generations, this mechanism could explain why species changed and why those changes that were preserved tended to be “adaptive”: They tended to aid the species’ survival.

Though his ideas had crystallized by 1838, Darwin didn’t publish for fear that many would nd them offensive. He published only after another naturalist, Alfred Russel Wallace (1823–1913), sent him a letter in 1858 that made it clear that he had stumbled on the same explanation. Darwin’s friend, geologist Charles Lyell, arranged for the ideas of Darwin and Wallace to be presented together at a meeting of the Linnaean Society in London on July 1, 1858. The society’s journal published the two presentations in August. Darwin then set to work to complete the book he had been thinking about for 20 years, and in 1859, he published On the Origin of Species by Means of Natural Selection. Priced at 15 shillings, it sold out immediately.

Though quite simple, the idea of natural selection is slippery because it is statistical. This explains why it is so easily misunderstood. This section will summarize the argument, step by step, emphasizing that natural selection is about change not in individuals, but in the average qualities of entire species.

Species. Species are groups of organisms that can breed with each other.

Variations. Though members of the same species are similar, there are always tiny differences, as you can see by looking at the faces of those around you.

Heredity. Individuals inherit features from their parents.

Fitness. Some features may increase or reduce an individual’s chances of surviving and reproducing. Poor eyesight is a serious problem for an eagle!

Differential reproduction. Because of tness, some individuals have less chance of surviving and reproducing.

Gradual change. The qualities of individuals with a lesser chance of reproducing will vanish in subsequent generations while the qualities of those with greater chances of reproducing will tend to increase. In this way, the average qualities of entire species will slowly change.

Why were these ideas so shocking in Victorian Britain? First, they implied that all species are related. Perhaps humans were related to apes—or even to plants and bacteria! In class-conscious Britain, this was a serious problem. Second, the idea of natural selection implied that complex organisms such as human beings could be created by blind, statistical processes acting over huge periods of time. Natural selection (“Darwin’s dangerous idea,” as philosopher Daniel Dennett has called it) seemed to leave no room for a divine creator. This terri ed Darwin’s wife, Emma, who feared her husband’s impiety would ensure that they were separated in the afterlife! A third consequence was that biological change was endless, because environmental change ensured that adaptation would continue forever. This meant there were no perfect organisms, despite the claims of some contemporaries that humans (particularly British humans) were clearly the “ ttest” of all organisms.

We have summarized Darwin’s idea of natural selection. But what evidence is there that Darwin was right? The next lecture describes why Darwin’s idea remains at the center of modern biological thought.

Essential Reading

Supplementary Reading

Questions to Consider

Christian, Maps of Time, chap. 4. Mayr, One Long Argument.

Darwin, On the Origin of Species. Eldredge, Darwin.

1. What was the crucial insight that allowed Charles Darwin to solve the problem of adaptation?

2. Why do so many people still nd Darwin’s solution to the riddle of adaptation unpalatable?