Evolution by natural selection describes a mechanism for how species change over time. Scientists, philosophers, researchers, and others had made suggestions and debated this topic well before Darwin began to explore this idea. In the eighteenth century, naturalist Georges-Louis Leclerc Comte de Buffon reintroduced ideas about the evolution of animals and noted that various geographic regions have different plant and animal populations, even when the environments are similar. Also, during the eighteenth century, James Hutton, a Scottish geologist and naturalist, proposed that geological change occurred gradually by accumulating small changes from processes operating like they are today over long periods of time. Nineteenth-century geologist Charles Lyell popularized Hutton’s view and Lyell’s notion of the greater age of Earth gave more time for gradual change in species, and the process of change provided an analogy for this change. In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change. We now refer to this mechanism as an inheritance of acquired characteristics by which the use or disuse of a structure during an organism’s lifetime could bring about long-term changes in traits in species. While many discredited this mechanism for evolutionary change, Lamarck’s ideas were an important influence on evolutionary thought.

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.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring, though neither Darwin nor Wallace (or anyone else) knew exactly how this happened at the time. Second, more offspring are produced than are able to survive, because 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 competition for those resources in each generation. Third, offspring vary in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics that allow them to best compete for limited resources are more likely to survive and thus likely to produce more offspring than individuals with variations that are less able to compete. Since the characteristics that allow one individual to reproduce more successfully are inherited, and those individuals produce more offspring, thus their traits will be better represented in the next generation as a result of having more offspring. Over generations, this can lead to changes in traits in populations, a process that Darwin called “descent with modification”. Ultimately, natural selection can lead to greater adaptation of the population to its local environment. Natural selection is the only mechanism that will necessarily result in adaptive evolution.

Natural selection can only take place if there is variation, or differences, among individuals in a population (Fig. 2). Importantly, these differences must have some genetic basis; otherwise, the selection will not lead to change in the next generation. This is critical because nongenetic reasons can cause variation among individuals such as an individual’s height because of better nutrition rather than different genes.

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 the organism. Sexual reproduction can also lead to changes genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each offspring.

A heritable trait that helps an organism survive and/or reproduce in its present environment is often referred to as an adaptation. A platypus’s webbed feet are an adaptation for swimming. A snow leopard’s thick fur is an adaptation for living in the cold. A cheetah’s fast speed is an adaptation for catching prey. Whether or not a trait is favorable depends on the current environmental conditions. The same traits are not always selected because environmental conditions can change. For example, consider a plant species that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

We can see such divergent evolution in the forms of the reproductive organs of flowering plants. The flowers share the same basic anatomies that may include sepals, petals, pistils, and stamen. However, the individual shapes of the structures can result in very different appearances. This is often the result of selection in different physical environments and adaptations to different kinds of pollinators.

We call this phenomenon convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestor. The wings in the two species came to the same function, flying, but did so separately from each other (Fig. 4).

For living organisms to adapt to changing environmental pressures, genetic variation must be present. This variation can be the result of mutations in individuals as well as sexual reproduction in the population. As a result of this genetic variation, individuals have differences in form and function that allow some to survive certain conditions better than others. Those organisms that have increased survival, and thus are more likely to reproduce, pass their favorable traits to their offspring. This differential reproductive success can result in changes in the frequencies of traits in populations, and over time increased adaptation to environmental conditions. However most natural environments are not static ( often change), and what was once an advantageous trait may no longer be as advantageous. This alters the survival and reproduction of individuals within the population and can result in different adaptions over time. Evolution is a continuously ongoing process. When similar traits evolve in unrelated species, such as wings in birds, bats, and insects, it is called convergent evolution.  Divergent evolution occurs when diverse traits evolve in multiple species that came from a common ancestor.