Before the Origin, most naturalists believed that species were fixed entities created by a deity to fit their particular environments. Darwin himself started out with this belief. Doubt began to seep in while he was on board the Beagle, as he observed the living world firsthand and began to notice puzzling facts. If living beings have been specially created for their environments, why do the animals and plants that live on islands resemble those from the nearest mainland, where the environment is often quite different? Why is it possible for an animal “created” for Europe to thrive when introduced to Australia? Some geese almost never swim, yet they have webbed feet. Why? Many fish that live in the total darkness of caves are blind, yet they have the remnants of eyes. Why? Woodpeckers have many traits that make them good at climbing trees and ferreting insects out from under the bark, yet some woodpeckers with those traits eat only fruit, while others live on treeless plains. Why? Why? Why? Such questions perplexed Darwin and gradually led him to ponder evolution.
He was not the first to do so. Toward the end of the eighteenth century, Darwin’s grandfather, Erasmus Darwin, published speculations about evolution (in the fashion of the times, many of them were in poetical form). A few years later the great French biologist Jean-Baptiste Lamarck wrote the first major treatise arguing that evolution takes place. Like Darwin, Lamarck started out believing species were immutable and then changed his mind as a result of what he saw in nature. Darwin knew of Lamarck’s ideas: during his brief stint as a medical student, he became friends with the comparative anatomist Robert Grant, who spoke of Lamarck enthusiastically.
In 1844 Darwin wrote a long sketch of his own ideas about evolution. But fearing the ruckus it would cause, and feeling that he did not yet have enough evidence to be convincing, he did not make it public. Instead, he told a few friends about it, including the botanist Joseph Hooker, and wrote a letter to Emma, asking her, in the event of his death, to arrange to have it published. He then resumed work he was doing on other subjects.
Someone else was not so timid. Later the same year a Scottish writer, journalist, and publisher named Robert Chambers published – anonymously – a book called Vestiges of the Natural History of Creation, in which he put forward a mass of disconnected ideas about evolution, all completely speculative. The book was a sensation. Yet Chambers failed to persuade most people that evolution takes place. Indeed, none of these earlier writers and thinkers had the impact of the Origin.
One reason for this is obvious: they all lacked plausible mechanisms by which evolution could occur. Such as natural selection.
I will describe natural selection in a moment. But before I do, consider the fact that humans can, over time, produce huge changes in domestic animals. Just think of the different breeds of sheep, horse, and cow. Or think of the difference between a massive dog like a Great Dane and a tiny one like a Chihuahua. A single Great Dane can weigh more than twenty-five Chihuahuas. Such differences are caused by repeatedly selecting which individuals are allowed to breed with one another. To breed a big dog, then, you repeatedly select the biggest animals of each sex to breed with one another; to breed a little one, you do the reverse. This system works because size can be inherited – that is, it has a genetic component – and because different dogs have different genes. If the breeding is sustained for many generations, the resulting animals will be much larger (or smaller) than their ancestors.
Darwin saw that something similar happens in nature. But rather than humans selecting which traits are desirable, the selection happens as a natural consequence of the fact that different individuals have different genes.
To see what I mean, consider blue tits, small songbirds found throughout Europe and parts of Asia and North Africa. A typical pair has ten chicks in a brood, and some pairs have two broods a year – which is a lot of baby blue tits. So you might expect that the number of blue tits will grow rapidly from one year to the next.
But it does not. Bird census data shows that the number of blue tits does not increase dramatically. What happens to them all? They experience natural selection. That is, they get eaten by cats, or crows, or sparrow hawks – or they escape. They die of hunger, or cold, or disease – or they make it. They fail to find a place to nest, or, having found a nest, fail to attract a mate – or they have a successful mating season. Now, if one bird survives the cold because it is a little fatter or has warmer feathers, or if it escapes from the paw of the cat because it is a little flightier, or if one finds a mate because it is a little sexier, then the fatter, the featherier, the flightier, and the sexier individuals will be more likely to leave offspring. And if the reason for the fatness, featheriness, flightiness, or sexiness has a genetic component, then those traits will start to spread. Conversely, the thin, the unfeathery, the slow, and the unsexy will die – and their genes will be lost. Over time, if the same traits continue to be favored, the characteristics of the whole population will shift.
To put it more brutally, in all populations more individuals are born than can possibly survive and reproduce. Therefore, most individuals die prematurely. But individuals differ from one another in their inherited characteristics – that is, in their genes. If the difference between dying and surviving, between leaving no descendants and leaving some, is in part genetic, then the traits that enhance success – whatever they are – will start to spread, and those that hinder it will start to vanish. This is evolution by natural selection. The ingredients are genetic variation, the death of many coupled with the survival and reproduction of a few, and time.
Moreover, natural selection does not just shape obvious traits, such as how many flowers a plant produces or the number of legs possessed by a starfish. It can shape every aspect of living beings, from complex details of internal physiology to subtle aspects of behavior, as long as there is genetic variation for the traits in question.
Natural selection is the idea most closely associated with Darwin. Yet he was not the first to discover this either. For example, in 1831 – the same year that Darwin embarked on the Beagle – a Scottish journalist named Patrick Matthew published a clear and concise account of how natural selection works. Unfortunately for Matthew, however, he included it as an appendix to his book On Naval Timber and Arboriculture, and it went unnoticed. Darwin, for one, knew nothing about it until after the Origin was published, when Matthew wrote to a magazine to draw it to public attention. (In later years Matthew insisted on putting “Discoverer of the Principle of Natural Selection” on the title pages of his books. This irritated Darwin.)
But there was another discoverer of the principle of natural selection: Alfred Russel Wallace. In the history of biology the story is famous. In February 1858 Wallace wrote to Darwin from the island of Ternate, in what is now Indonesia, where he was collecting birds, beetles, butterflies, and anything else he could catch. The letter contained a manuscript in which Wallace had outlined the idea of evolution by natural selection (though he did not call it that; he referred instead to “a general principle in nature”). He asked Darwin, if he thought the manuscript sufficiently interesting, to forward it to the great geologist Sir Charles Lyell, who also happened to be a close friend and confidante of Darwin. Indeed, Darwin had already revealed his own ideas about evolution by natural selection to Lyell, and Lyell had already urged him to publish them.
On receiving Wallace’s letter, Darwin was devastated: he was about to be scooped. But as Wallace had asked, he sent the manuscript to Lyell, along with an anguished letter of his own. (“My dear Lyell . . . Your words have come true with a vengeance when you said I shd. [sic] be forestalled . . . all my originality, whatever it may amount to, will be smashed . . . I hope you will approve of Wallace’s sketch, that I may tell him what you say.”)
The upshot