Surprisingly, even the famous philosopher of science Karl Popper raised a suspicion of tautology against evolution by natural selection (albeit a more subtle one). Popper basically questioned natural selection’s explanatory power based on the following argument: If certain species exist, this means that they were adapted to their environment (since those that were not adapted became extinct). In other words, Popper asserted, adaptation is simply defined as the quality that guarantees existence, and nothing is ruled out. However, since Popper published this argument, a number of philosophers have shown it to be erroneous. In reality, Darwin’s theory of evolution rules out more scenarios than it leaves in. According to Darwin, for instance, no new species can emerge without having an ancestral species. Similarly, in Darwin’s theory, any variations that are not achievable in gradual steps are ruled out. In modern terminology, “achievable” would refer to processes governed by the laws of molecular biology and genetics. A key point here is the statistical nature of adaptation—no predictions can be made about individuals, just about probabilities. Two identical twins are not guaranteed to produce the same number of offspring, or even to both survive. Popper, by the way, did recognize his error in later years, declaring, “I have changed my mind about the testability and the logical status of natural selection; and I am glad to have an opportunity to make a recantation.”

Finally, for completeness, I should mention that although natural selection is the main driver of evolution, other processes can bring about evolutionary changes. One example (which Darwin could not have known about) is provided by what has been termed by modern evolutionary biologists genetic drift: a change in the relative frequency in which a variant of a gene (an allele) appears in a population due to chance or sampling errors. This effect can be significant in small populations, as the following examples demonstrate. When you flip a coin, the expectation is that heads will turn up about 50 percent of the time. This means that if you flip a coin a million times, the number of times you’ll get heads will be close to a half million. If you toss a coin just four times, however, there is a nonnegligible probability (of about 6.2 percent) that it will land heads each time, thus deviating substantially from the expectation. Now imagine a very large island population of organisms in which just one gene appears in two variants (alleles): X or Z. The alleles have an equal frequency in the population; that is, the frequency of X and Z is ¹/₂ for each. Before these organisms have a chance to reproduce, however, a huge tsunami wave washes the island, killing all but four of the organisms. The surviving four organisms could have any of the following sixteen combinations of alleles: XXXX, XXXZ, XXZX, XZXX, ZXXX, XXZZ, ZZXX, XZZX, ZXXZ, XZXZ, ZXZX, XZZZ, ZZZX, ZXZZ, ZZXZ, ZZZZ. You will notice that in ten out of these sixteen combinations, the number of X alleles is not equal to the number of Z alleles. In other words, in the surviving population, there is a higher chance for a genetic drift—a change in the relative allele frequency—than for keeping the initial state of equal frequencies.

Genetic drift can cause a relatively rapid evolution in a small population’s gene pool, which is independent of natural selection. One oft-cited example of genetic drift involves the Amish community of eastern Pennsylvania. Among the Amish, polydactyly (extra fingers or toes) is many times more common than in the general population of the United States. This is one of the manifestations of the rare Ellis-van Creveld syndrome. Diseases of recessive genes, such as the Ellis-van Creveld syndrome, require two copies of the gene to cause the disease. That is, both parents have to be carriers of the recessive gene. The reason for the higher-than-normal frequency of these genes in the Amish community is that the Amish marry within their own group, and the population itself originated from around two hundred German immigrants. The small size of this community allowed researchers to trace back the Ellis-van Creveld syndrome to just one couple, Samuel King and his wife, who arrived in 1744.

There are three points that need to be emphasized about genetic drift. First, the evolutionary changes that are due to genetic drift occur entirely as a result of chance and sampling errors—they are not driven by selection pressure. Second, genetic drift cannot cause adaptation, which remains entirely the province of natural selection. In fact, being entirely random, genetic drift can cause certain properties to evolve whose usefulness is otherwise very puzzling. Finally, while genetic drift clearly occurs to some degree in all populations (since all the populations are finite in size), its effects are most pronounced in small, isolated populations.

These are, very concisely, some of the key points of Darwin’s theory of evolution by natural selection. Darwin revolutionized biological thinking in two major ways. He not only recognized that beliefs held for centuries could be false but also demonstrated that scientific truth can be achieved by the patient collection of facts, coupled with bold hypothesizing about the theory that binds those facts together. As you must have realized, his theory does a superb job in explaining why life on Earth is so diverse and why living organisms have the characteristics they have. The nineteenth-century English suffragist and botanist Lydia Becker beautifully described Darwin’s achievement: