The Evidence for Natural Selection

The Evidence for Natural Selection

Lecture 14

If you look at a lot of species, one of the strange things you nd is you can often nd very similar features across a wide range of different species.

Because his ideas seemed so radical to his contemporaries, Darwin defended them with great care. Yet he did so in language accessible to a general readership, which is why On the Origin of Species is one of the most readable of all scienti c classics. We will nd that many of Darwin’s arguments are easier to understand if we appreciate that he was arguing against an alternative theory of life: the idea that all living things had been created by a deity. This lecture discusses the evidence available in Darwin’s time, as well as the evidence that has become available since his death.

For Darwin himself, the rst argument for his theory was that it made sense of what he had encountered on his travels. Darwin was repeatedly puzzled by nding many neighboring species that were clearly related but also slightly different. These differences were particularly striking among the birds, tortoises, and lizards of the Galapagos Islands, a group of 19 small volcanic islands in the Paci c Ocean about 600 miles west of Ecuador. For example, there seemed to be 14 distinct species of nches, each exquisitely adapted to one of the islands. In 1846, Darwin wrote, “Seeing this gradation and diversity of structure in one small, intimately related, group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modi ed for different ends” (Eldredge, Darwin, p. 89).

Only on his return did Darwin realize how similar the different species of nch were. This suggested they had all once belonged to a single species. Yet in each island, slightly different beaks must have been favored because of slight differences in the trees and fruit of the island. Over many generations, those individuals with the best-adapted beaks would have fed and bred more successfully—and left more offspring, with similar beaks. Eventually,

such processes would have generated new species, closely related, yet each adapted to its own island. The Galapagos nches offered a paradigm of the working of natural selection. Natural selection could explain both why the nches were well adapted and why they were closely related. Deistic arguments could offer no reason why they should be related.

The rapid evolution of

disease vectors such as What direct evidence did Darwin AIDS also provides direct offer for his theory? First, he pointed evidence of natural selection, to the fossil record. It was already clear that the fossil record contained as does the declining many species that no longer existed. effectiveness of antibiotics But there were so many gaps in as new, resistant strains of the fossil record that to refute the bacteria have evolved. argument that God had repeatedly created new species, Darwin needed to show the existence of “transitional” species, demonstrating the slow evolution of one species into another. While a deistic theory of life did not need transitional species, the theory of natural selection did. Fortuitously, such a fossil was found in 1861, just two years after his book was published. It belonged to a bird-like species now known as Archaeopteryx, which lived about 150 million years ago. It was about the size of a magpie and had feathered wings and a wishbone, features one would expect of a bird. But it also had some distinctly reptilian features, including teeth and a bony tail. These made it similar to a small dinosaur. Archaeopteryx was the perfect “transitional” fossil. Second, Darwin pointed to “homologies,” or unexpected similarities between species. Many striking features can be found across many different species, such as the ngers of mammals. A deistic theory of life naturally supposed that homologies re ected good design. But not all homologies could be explained in this way. For example, whales have ngers, though they have no use for them. There was no reason why a rational creator should have given whales ngers. But natural selection, which worked by constantly tweaking existing species, provided a natural explanation for such “survivals.” It suggested that such species were similar because they were related. (We now know that whales are indeed descended from mammal ancestors.)

Third, Darwin argued that the geographical distribution of species supported his theory. Why did most marsupials live in Australia? Why were there no wolves in Australia, even though there was a “marsupial wolf,” the Thylacinus, living in environments that would have suited wolves very well? Darwin argued that this was because Australian species were descended from local ancestors. Yet a deistic theory of life could offer no reason why a deity should not have placed species such as wolves in all parts of the world for which they were designed.

As a scrupulous scientist, Darwin was also painfully aware that there were gaps in his evidence. However, most of the gaps have been lled in since his death. The fossil record is now much richer. Even today, it is impossible to produce a series of fossils showing all the links between one species and another because so few organisms are ever fossilized. However, many more “transitional” fossils have been found. Modern dating techniques also enable us to date fossils precisely, putting them on timelines that demonstrate clearly how species have changed over time. A particularly striking example is the detailed reconstruction of the evolution of modern horses from a small, fox-sized animal—Hyracotherium—that lived about 60 million years ago. Darwin feared that contemporary estimates for the age of the Earth (20–400 million years, according to Lord Kelvin) allowed too little time for natural selection to generate the many species present in the modern world. As we have seen, modern estimates give an age of more than 4 billion years for the Earth, providing plenty of time for natural selection to create new species.

Darwin and his contemporaries had little understanding of how heredity works. Darwin assumed that qualities inherited from parents were normally blended in their offspring. Unfortunately, this seemed to mean that unusual but advantageous mutations, far from being preserved, would be diluted generation by generation. And that suggested that species ought to remain stable over time. Modern genetic studies began with the work of Gregor Mendel (1822–1884), whose work was not widely known before the 20th century. Mendel showed that many features, such as the colors of eyes, were not blended. The discovery of DNA in 1953, and subsequent clari cation of how genes work, has demonstrated that heredity works in just the ways required by Darwin’s theory. While some parental features are blended, many are not. Furthermore, though the copying of genes is almost perfect,

there are occasional errors, which create the tiny variations that provide the raw material on which natural selection works.

Darwin feared that we could never observe natural selection directly because it was such a gradual process. In the 20th century, work on rapidly reproducing species such as fruit ies (Drosophila) has allowed us to observe evolution at work in great detail. The rapid evolution of disease vectors such as AIDS also provides direct evidence of natural selection, as does the declining effectiveness of antibiotics as new, resistant strains of bacteria have evolved.

Since Darwin’s time, the evidence that living organisms adapt and evolve through natural selection has become overwhelming, which is why Darwin’s idea is the key to modern biological science, with applications in many different areas of life—from the design of pharmaceuticals to the use of DNA in the courtroom. But how were the very rst living organisms created? That is the question tackled in the next lecture.

Essential Reading

Supplementary Reading

Questions to Consider

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

Darwin, On the Origin of Species. Jones, Almost Like a Whale. Watson, The Double Helix.

1. How did Darwin counter contemporary objections to the theory of natural selection?

2. What new evidence in support of Darwin’s theory has emerged since his death?