4.1: Darwin, Wallace, and the Theory of Natural Selection

Figure \(\PageIndex{1}\): HMS Beagle in the Galapagos by John Chancellor (artist) (CC BY-NC-SA 2.0; Karen E. James via Flickr)

The theory of evolution by natural selection describes a mechanism for species change over time. That species change had been suggested and debated well before Darwin. The view that species were unchanging was grounded in the writings of Plato, yet there were other ancient Greeks philosophers that expressed evolutionary ideas.

Definition: Natural Selection

The process by which living organisms adapt and change in nature. Natural selection results from differential survival and reproduction of individuals due to differences in heritable traits. It is a key mechanism of evolution and is the only process that results in the formation of adaptive traits and behaviors.

In the eighteenth century, ideas about the evolution of animals were reintroduced by the naturalist Georges-Louis Leclerc, Comte de Buffon and even by Charles Darwin’s grandfather, Erasmus Darwin. During this time, it was accepted that there were extinct species. At the same time, James Hutton, the Scottish naturalist, proposed that geological change occurred gradually by the accumulation of small changes from processes (over long periods of time) just like those happening today. This contrasted with the predominant view that the geology of the planet was a consequence of catastrophic events occurring during a relatively brief past. Hutton’s view was later popularized by the geologist Charles Lyell in the nineteenth century. Lyell became a friend to Darwin and his ideas were very influential on Darwin’s thinking. Lyell argued that the greater age of Earth gave more time for gradual change in species, and the process provided an analogy for gradual change in species.

In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change that is now referred to as inheritance of acquired characteristics. In Lamarck’s theory, modifications in an individual caused by its environment, or the use or disuse of a structure during its lifetime, could be inherited by its offspring and, thus, bring about change in a species. While this mechanism for evolutionary change as described by Lamarck was discredited, Lamarck’s ideas were an important influence on evolutionary thought.

Charles Darwin and Alfred Wallace

The actual mechanism for evolution was independently conceived of and described by two naturalists, Charles Darwin and Alfred Russell Wallace, in the mid-nineteenth century. Importantly, each spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, visiting South America, Australia, and the southern tip of Africa (Figure \(\PageIndex{1}\)). Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys in the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands (west of Ecuador). On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species that each had a unique beak shape. He observed both that these finches closely resembled another finch species on the mainland of South America and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with very small differences between the most similar (Figure \(\PageIndex{2}\)). Darwin imagined that the island species might be all species modified from one original mainland species. In 1860, he 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 modified for different ends." (Darwin, 1860).

Figure \(\PageIndex{2}\): Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources. This illustration shows the beak shapes for four species of ground finch: 1. Geospiza magnirostris (the large ground finch), 2. G. fortis (the medium ground finch), 3. G. parvula (the small tree finch), and 4. Certhidea olivacea (the green-warbler finch). (CC 0; Open Stax Concepts in Biology)

Concept Check \(\PageIndex{1}\)

True or False: Darwin's most significant contribution to biology was proposing that species change over time (evolve).

Answer

False: By the time Darwin traveled the world it was already accepted by the scientific community that species had changed over Earth's history. What was not understood, was why this would happen in nature. Darwin's most significant contribution was proposing a mechanism, or process, for this change. His theory of natural selection is still today the most widely accepted, and supported, explanation of species change.

The Mechanism of Natural Selection

Wallace and Darwin both observed similar patterns in other organisms and independently conceived a mechanism to explain how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature:

• First, the characteristics of organisms are inherited, or passed from parent to offspring.
• Second, more offspring are produced than are able to survive; in other words, resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers.
• Thus, there is a competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading an essay by the economist Thomas Malthus, who discussed this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited.

Out of these three principles, Darwin and Wallace reasoned that offspring with inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called “descent with modification.”

Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnaean Society in London. The following year Darwin’s book, On the Origin of Species, was published, which outlined in considerable detail his arguments for evolution by natural selection.

Figure \(\PageIndex{3}\): (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858. (CC-0; OpenStax Concepts in Biology)

Variation and Adaptation

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons, such as an individual being taller because of better nutrition rather than different genes. The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation (changes in DNA sequence).

A heritable trait (one that is genetically, rather than environmentally determined) that aids the survival and reproduction of an organism in its present environment is called an adaptation. An adaptation is a “match” of the organism to the environment. Adaptation to an environment comes about when a change in the range of genetic variation occurs over time that increases or maintains the match of the population with its environment.

Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Whether or not a trait is favorable depends on the environment at the time. Traits that are helpful in one environment might actually be harmful in another.

Modern Evidence of Natural Selection in Galapagos Finches

Demonstrations of evolution by natural selection can take years to study. One of the best lines of evidence has been in the very birds that helped to inspire the theory, the Galápagos finches. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976. The Grants found changes from one generation to the next in the beak shapes of the medium ground finches on the Galápagos island of Daphne Major. The medium ground finch feeds on seeds. The birds have inherited variation in the genes for bill shape. Some individuals have an allele (a variation of the gene for bill shape) that causes a wide, deep bill, and others have an allele for thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During 1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically: the decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year. When the Grants measured beak sizes in the much-reduced population the year after the drought, they found that the average bill size was larger. This was clear evidence for natural selection (differences in survival) of bill size caused by the availability of seeds. The Grants had studied the inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and evolution of bill size in this species in response to changing conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare.

Figure \(\PageIndex{4}\): A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the average beak size of the medium ground finch between 1976 and 1978. (CC-0; OpenStax Concepts in Biology)

References

Darwin, Charles. 1860. Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the World, under the Command of Capt. Fitz Roy, R.N, 2nd. ed. (London: John Murray, 1860),

Attribution

This page is a modified derivative of:

• 11.1 Discovering How Population Change, Concepts of Biology (CC-BY; Open Stax)