The adaptations we see in the natural world—the exquisite fit of organisms to their environment—were typically taken by pre-Darwinian biologists as evidence of a divine Creator’s existence. Each species, they argued, was so well adapted—the desert plant so physiologically adept at coping with minimal levels of rainfall and the fast-swimming fish so hydrodynamically streamlined—that it must have been designed by a Creator.

With the publication of On the Origin of Species in 1859, Darwin, pictured in Fig. 21.6, overturned the biological convention of his day on two fronts. First, he showed that species are not unchanging; they have evolved over time. Second, he suggested a mechanism, natural selection, that brings about adaptation. Natural selection was a brilliant solution to the central problem of biology: how organisms come to fit so well in their environments. From where does the woodpecker get its powerful chisel of a bill? And the hummingbird its long delicate bill for probing the nectar stores in flowers? Darwin showed how a simple mechanism, without foresight or intentionality, could result in the extraordinary range of adaptations that all of life is testimony to.

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For 20 years after first conceiving the essence of his theory, Darwin collected supporting evidence. In 1858, however, he was spurred to begin writing On the Origin of Species by a letter from a little-known naturalist collecting specimens in what is today Indonesia. By a remarkable coincidence, Alfred Russel Wallace, shown in Fig. 21.7, had also developed the theory of evolution by natural selection. Aware that Darwin was interested in the problem but having no idea that Darwin was working on the same theory, Wallace wrote to Darwin in 1858 to see what he thought of his idea.

Suddenly, Darwin was confronted with the prospect of losing his claim on the theory that he had been quietly nurturing for 20 years. But all was not lost. Darwin’s colleagues arranged for the publication of a joint paper by Wallace and Darwin in 1858. This was done without consulting Wallace, who nonetheless never resented Darwin and afterward was careful to insist that the idea rightly belonged to the older man. It was Darwin’s publication of On the Origin of Species in 1859 that brought both evolution and natural selection, its underlying mechanism of adaptation, to public attention. Wallace is only fleetingly mentioned in Darwin’s great work, and Darwin, not Wallace, is now the name associated with the discovery.

Both Darwin and Wallace recognized their debt to the writings of a British clergyman, Thomas Malthus. In his Essay on the Principle of Population, first published in 1798, Malthus pointed out that natural populations have the potential to increase in size geometrically, meaning that populations get larger at an ever-increasing rate. Imagine that human couples can have just four children (two males and two females), so the population doubles every generation. Starting with a single couple, by the twentieth generation, the population will have grown to over a million—1,048,576, to be precise.

However, this geometric expansion of populations does not occur. In fact, population sizes are typically stable from generation to generation. This is because the resources upon which populations are dependent—food, water, places to live—are limited. In each generation many fail to survive or reproduce; there simply is not enough food and other resources to go around. This implies in turn that individuals within a population must compete for resources.

Which individuals will win the competition? Darwin and Wallace suggested that those that are best adapted would most likely survive and leave more offspring. Genetic variation among individuals results in some that are more likely than others to survive and reproduce, passing their genetic material to the next generation. As a result, the next generation will have a higher proportion of these same advantageous alleles. Darwin used the term “natural selection” for the filtering process that acts against deleterious alleles and in favor of advantageous ones.

Competitive advantage is a function of how well an organism is adapted to its environment. A desert plant that is more efficient at minimizing water loss than another plant is better adapted to the desert environment. An organism that is better adapted to its environment is more fit. Fitness, in this context, is a measure of the extent to which the individual’s genotype is represented in the next generation. We say that the first plant’s fitness is higher than the second’s if it leaves more surviving offspring either because the plant itself survives for longer, giving it greater opportunity to reproduce, or because it has some other reproductive advantage, such as the ability to produce more seeds. Assuming that the trait maintains its advantage, natural selection then acts over generations to increase the overall fitness of a population. A plant population newly arrived in a desert may be poorly adapted to its environment, but, over time, alleles that minimize water loss increase under natural selection, resulting in a population that is better adapted to the desert.

Such changes in populations take time. Borrowing from the geologists of his day, Darwin recognized that time was a critical ingredient of his theory. Geologists had put forward a view of Earth’s history that argued that large geological changes—like the carving of the Grand Canyon—can be explained by simple day-to-day processes operating over vast timescales. Darwin applied this worldview to biology. He recognized that small changes, like subtle shifts in the frequencies of alleles, could add up to major changes given long enough time periods. What might seem to us to be a trivial change over the short term can, over the long term, result in substantial differences among populations.

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