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Learning Goals
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Explain how natural selection can result in adaptive evolution
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Identify sources of genetic variations in populations
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Define adaptation as it relates to natural selection
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Distinguish between convergent and divergent evolution
Introduction
Darwin, Wallace and Natural Selection
In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. On the ship H.M.S. Beagle, Darwin traveled around the world, including stops in South America, Australia, and the southern tip of Africa. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains. Darwin’s 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 with a unique beak shape (Fig. 1). What Darwin noted was that the species on the islands had a graded series of beak sizes (small to large) and shapes (width and depth). In addition, between the most similar species, there were very small differences. Darwin also observed that these finches closely resembled another finch species on the South American mainland. As a result of his observations, Darwin hypothesized that the island species might be the result of modifications of one original mainland species. Upon further study, he realized that each finch’s varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks that helped break seeds. Insect-eating finches had spear-like beaks, allowing them to stab their prey.
Over their travels, Wallace and Darwin both observed similar patterns to those seen in the finches in other organisms. Based on their observations they each independently developed the same explanation (natural selection) for how and why such changes could take place. Natural selection, or “survival of the fittest” is the increased reproductive success of individuals with traits that enhance survival and reproduction. This differential reproduction in individuals within a population can lead to evolutionary change.
Natural Selection
Variation in Populations
Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes that mutation causes can have one of three outcomes on the phenotype.
(1) A mutation affects the organism’s phenotype in a way that gives it reduced fitness—the lower likelihood of survival or fewer offspring.
(2) A mutation may produce a phenotype with a beneficial effect on fitness.
(3) A mutation may also have no effect on the phenotype or the organism’s fitness.
Most mutations do not affect the phenotype of t