2.3: Charles Darwin and Natural Selection

In the mid-nineteenth century, the actual mechanism for evolution was independently conceived of and described by two naturalists: Charles Darwin and Alfred Russel Wallace. Importantly, each naturalist 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, including stops in South America, Australia, and the southern tip of Africa. 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 to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador.

Figure \(\PageIndex{1}\): 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.

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. The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey. 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.

Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, also known as “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits; this leads to evolutionary change.

For example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked
tortoises would be present in the population.

Figure \(\PageIndex{2}\): Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented
together before the Linnean Society in 1858.

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. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding.
• Second, more offspring are produced than are able to survive, so 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. 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. Darwin and Wallace reasoned that offspring with inherited characteristics which 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. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution.

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

Demonstrations of evolution by natural selection are time consuming and difficult to obtain. One of the best examples has been demonstrated in the very birds that helped to inspire Darwin’s theory: the Galápagos finches. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of natural selection.

The Grants found changes from one generation to the next in the distribution of beak shapes with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited variation in the bill shape with some birds having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, the large hard seeds that large-billed birds ate were reduced in number; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, survival and reproduction were much better in the following years for the small-billed birds. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the size of bills had evolved to be smaller. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

Comparison of Lamarckism vs. Darwinism

Jean-Baptiste Lamarck (1744-1829)

Lamarck was a French biologist who is best known for his Theory of Inheritance of Acquired Characteristics, first presented in 1801. He believed that evolution was the “acquired traits” of a species that is inherited by its offspring. His theory was that if an organism continually used a structure to carry out a certain task, the structure used would become physically modified over time to make the task easier. This modified structure would then be passed on to any offspring. For example, if a short-nosed elephant was continually stretching out its trunk to try to reach the leaves high up in trees, it’s trunk would stretch and become longer over time, and any babies that it had would be born with longer trunks. Lamarck also believed that when body parts were not being used, such as the human appendix, they gradually disappear. Eventually, people will be born without these parts. Lamarck believed that evolution happens according to a prearranged plan and that the results have already been decided.

Charles Darwin (1809 -1882)

Charles Darwin is famous for the theory of evolution and Natural Selection, or ‘Survival of the Fittest’. He dedicated his life to studying plants and animals and believed that the desires of animals have nothing to do with how they evolve. He said that organisms, even of the same species, are different in some ways, and over time those creatures which are adaptable, survive, while those that do not adapt to changing conditions, such as climatic and environmental change, do not
live to breed and pass on their genes. He came to the conclusion that there was a variation of physical and behavioral features within a species. Organisms which had features that helped them to adapt to their environment and circumstances had a better chance of survival than individuals who lacked these features.

These adaptable organisms survived to breed and produce offspring which generally inherited the ‘successful’ features of their parents. He called this process ‘natural selection’. Darwin knew that organisms evolved and changed from generation to generation, but did not know how traits were passed on from one generation to another. Only after more was understood about genetics, was this explained. Darwin also suggested that each species evolves over time and adapts to the environment in which they live. Thus, the same species living in different environments will evolve differently and become more and more differentiated
(different) over time. He believed that all species of life on Earth are interrelated and have a common ancestor.

Contributors and Attributions

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