THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
14.3. The Theory of Natural Selection
It is one thing to observe the results of evolution but quite another to understand how it happens. Darwin’s great achievement lies in his formulation of the hypothesis that evolution occurs because of natural selection.
Darwin and Malthus
Of key importance to the development of Darwin’s insight was his study of Thomas Malthus’s Essay on the Principle of Population (1798). In his book, Malthus pointed out that populations of plants and animals (including human beings) tend to increase geometrically, while the ability of humans to increase their food supply increases only arithmetically. A geometric progression is one in which the elements increase by a constant factor; the blue line in figure 14.6 shows the progression 2, 6, 18, 54, . . . and each number is three times the preceding one. An arithmetic progression, in contrast, is one in which the elements increase by a constant difference; the red line shows the progression 2, 4, 6, 8, . . . and each number is two greater than the preceding one.
Figure 14.6. Geometric and arithmetic progressions.
An arithmetic progression increases by a constant difference (for example, units of 1 or 2 or 3), while a geometric progression increases by a constant factor (for example, by 2 or by 3 or by 4). Malthus contended that the human growth curve was geometric, but the human food production curve was only arithmetic. Can you see the problems this difference would cause?
Because populations increase geometrically, virtually any kind of animal or plant, if it could reproduce unchecked, would cover the entire surface of the world within a surprisingly short time. Instead, population sizes of species remain fairly constant year after year, because death limits population numbers. Malthus’s conclusion provided the key ingredient that was necessary for Darwin to develop the hypothesis that evolution occurs by natural selection.
Sparked by Malthus’s ideas, Darwin saw that although every organism has the potential to produce more offspring than can survive, only a limited number actually do survive and produce further offspring. Many examples appear in nature. Sea turtles, for instance, will return to the beaches where they hatched to lay their eggs. Each female will lay about 100 eggs. The beach could be covered with thousands of hatchlings, like in figure 14.7, trying to make it to water’s edge. Less than 10% will actually reach adulthood and return to this beach to reproduce. Darwin combined his observation with what he had seen on the voyage of HMS Beagle, as well as with his own experiences in breeding domestic animals, and made an important association: Those individuals that possess physical, behavioral, or other attributes that help them live in their environment are more likely to survive than those that do not have these characteristics. By surviving, they gain the opportunity to pass on their favorable characteristics to their offspring. As the frequency of these characteristics increases in the population, the nature of the population as a whole will gradually change. Darwin called this process natural selection. The driving force he identified has often been referred to as survival of the fittest. However, this is not to say the biggest or the strongest always survive. These characteristics may be favorable in one environment but less favorable in another. The organisms that are “best suited” to their particular environment survive more often, and therefore produce more offspring than others in the population, and in this sense are the “fittest.”
Figure 14.7. Sea turtle hatchlings.
These newly-hatched sea turtles make their way to the ocean from their nests on the beach. Thousands of eggs may be laid on a beach during a spawning, but less than 10% will survive to adulthood. Natural predators, human egg poachers, and environmental challenges prevent the majority of offspring from surviving. As Darwin observed, sea turtles produce more offspring than will actually survive to reproduce.
Darwin was thoroughly familiar with variation in domesticated animals and began On the Origin of Species with a detailed discussion of pigeon breeding. He knew that breeders selected certain varieties of pigeons and other animals, such as dogs, to produce certain characteristics, a process Darwin called artificial selection. Once this had been done, the animals would breed true for the characteristics that had been selected. Darwin had also observed that the differences purposely developed between domesticated races or breeds were often greater than those that separated wild species. Domestic pigeon breeds, for example, show much greater variety than all of the hundreds of wild species of pigeons found throughout the world. Such relationships suggested to Darwin that evolutionary change could occur in nature too. Surely if pigeon breeders could foster such variation by “artificial selection,” nature through environmental pressures could do the same, playing the breeder’s role in selecting the next generation—a process Darwin called natural selection.
Darwin’s theory provides a simple and direct explanation of biological diversity, or why animals are different in different places—because habitats differ in their requirements and opportunities, the organisms with characteristics favored locally by natural selection will tend to vary in different places. As we will discuss later in this chapter in section 14.9, there are five evolutionary forces that can affect biological diversity, although natural selection is the only evolutionary force that produces adaptive changes.
Darwin Drafts His Argument
Darwin drafted the overall argument for evolution by natural selection in a preliminary manuscript in 1842. After showing the manuscript to a few of his closest scientific friends, however, Darwin put it in a drawer and for 16 years turned to other research. No one knows for sure why Darwin did not publish his initial manuscript—it is very thorough and outlines his ideas in detail. Some historians have suggested that Darwin was wary of igniting public, and even private, criticism of his evolutionary ideas—there could have been little doubt in his mind that his theory of evolution by natural selection would spark controversy. Others have proposed that Darwin was simply refining his theory, although there is little evidence he altered his initial manuscript in all that time.
Wallace Has the Same Idea
The stimulus that finally brought Darwin’s theory into print was an essay he received in 1858. A young English naturalist named Alfred Russel Wallace (1823-1913) sent the essay to Darwin from Malaysia; it concisely set forth the theory of evolution by means of natural selection, a theory Wallace had developed independently of Darwin. Like Darwin, Wallace had been greatly influenced by Malthus’s 1798 book. Colleagues of Wallace, knowing of Darwin’s work, encouraged him to communicate with Darwin. After receiving Wallace’s essay, Darwin arranged for a joint presentation of their ideas at a seminar in London. Darwin then completed his own book, expanding the 1842 manuscript that he had written so long ago, and submitted it for publication.
Publication of Darwin's Theory
Darwin’s book appeared in November 1859 and caused an immediate sensation. Although people had long accepted that humans closely resembled apes in many characteristics, the possibility that there might be a direct evolutionary relationship was unacceptable to many. Darwin did not actually discuss this idea in his book, but it followed directly from the principles he outlined. In a subsequent book, The Descent of Man, Darwin presented the argument directly, building a powerful case that humans and living apes have common ancestors. Many people were deeply disturbed with the suggestion that human beings were descended from the same ancestor as apes, and Darwin’s book on evolution caused him to become a victim of the satirists of his day—the cartoon in figure 14.8 is a vivid example. Darwin’s arguments for the theory of evolution by natural selection were so compelling, however, that his views were almost completely accepted within the intellectual community of Great Britain after the 1860s.
Figure 14.8. Darwin greets his monkey ancestor.
In his time, Darwin was often portrayed unsympathetically, as in this drawing from an 1874 publication.
Key Learning Outcome 14.3. The fact that populations do not really expand geometrically implies that nature acts to limit population numbers. The traits of organisms that survive to produce more offspring will be more common in future generations—a process Darwin called natural selection.