THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

14. Evolution and Natural Selection

These four finches live on the Galapagos Islands, a cluster of volcanic islands far out to sea off the coast of South America. All descendants of a single ancestral migrant, blown to the islands from the mainland long ago, the Galapagos finches gave Darwin valuable clues about how natural selection shapes the evolution of species. The two upper finches are ground finches, their different beaks adapting them to eat different-sized seeds. The finch on the left consumes smaller, slender seeds. The stouter beak of the finch on the right enables it to crack open larger, drier seeds. On the lower left is a woodpecker finch, a kind of tree finch that carries around a cactus spine, which it uses to probe for insects in deep crevices. On the lower right is a warbler finch that like its namesake, eats crawling insects. Each of these species utilizes food resources differently. Their different ways of interacting with the community within which they live generate the selective pressures that shape the evolution of groups like Darwin’s finches.

 14.1. Darwin's Voyage on HMS Beagle

The great diversity of life on earth—ranging from bacteria to elephants and roses—is the result of a long process of evolution, the change that occurs in organisms’s characteristics through time. In 1859, the English naturalist Charles Darwin (1809-82; figure 14.1) first suggested an explanation for why evolution occurs, a process he called natural selection. Biologists soon became convinced Darwin was right and now consider evolution one of the central concepts of the science of biology. In this chapter, we examine Darwin and evolution in detail, as the concepts we encounter will provide a solid foundation for your exploration of the living world.

Figure 14.1. The theory of evolution by natural selection was proposed by Charles Darwin.

This rediscovered photograph appears to be the last ever taken of the great biologist. It was taken in 1881, the year before Darwin died.

The theory of evolution proposes that a population can change over time, sometimes forming a new species. A species is a population or group of populations that possess similar characteristics and can interbreed and produce fertile offspring. This famous theory provides a good example of how a scientist develops a hypothesis—in this case, a hypothesis of how evolution occurs—and how, after much testing, the hypothesis is eventually accepted as a theory.

Charles Robert Darwin was an English naturalist who, after 30 years of study and observation, wrote one of the most famous and influential books of all time. This book, On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life, created a sensation when it was published, and the ideas Darwin expressed in it have played a central role in the development of human thought ever since.

In Darwin’s time, most people believed that the various kinds of organisms and their individual structures resulted from direct actions of the Creator. Species were thought to be specially created and unchangeable over the course of time. In contrast to these views, a number of earlier philosophers had presented the view that living things must have changed during the history of life on earth. Darwin proposed a concept he called natural selection as a coherent, logical explanation for this process. Darwin’s book, as its title indicates, presented a conclusion that differed sharply from conventional wisdom. Although his theory did not challenge the existence of a Divine Creator, Darwin argued that this Creator did not simply create things and then leave them forever unchanged. Instead, Darwin’s God expressed Himself through the operation of natural laws that produced change over time—evolution.

The story of Darwin and his theory begins in 1831, when he was 22 years old. The small British naval vessel HMS Beagle that you see in figure 14.2 was about to set sail on a five-year navigational mapping expedition around the coasts of South America. The red arrows in figure 14.3 indicate the route taken by HMS Beagle. The young (26-year-old) captain of HMS Beagle, unable by British naval tradition to have social contact with his crew, and anticipating a voyage that would last many years, wanted a gentleman companion, someone to talk to. Indeed, the Beagle’s previous skipper had broken down and shot himself to death after three solitary years away from home.

Figure 14.2. Cross section of HMS Beagle.

HMS Beagle, a 10-gun brig of 242 tons, only 90 feet in length, had a crew of 74 people! After he first saw the ship, Darwin wrote to his college professor Henslow: "The absolute want of room is an evil that nothing can surmount."

Figure 14.3. The five-year voyage of HMS Beagle.

Although the ship sailed around the world, most of its time was spent exploring the coasts and coastal islands of South America, such as the Galapagos Islands. Darwin's studies of the animals of these islands played a key role in the eventual development of his theory of evolution by means of natural selection.

On the recommendation of one of his professors at Cambridge University, Darwin, the son of a wealthy doctor and very much a gentleman, was selected to serve as the captain’s companion, primarily to share his table at mealtime during every shipboard dinner of the long voyage. Darwin paid his own expenses, and even brought along a manservant.

Darwin took on the role of ship’s naturalist (the official naturalist, a man named Robert McKormick, left the ship before the first year was out). During this long voyage, Darwin had the chance to study a wide variety of plants and animals on continents and islands and in distant seas. He was able to explore the biological richness of the tropical forests, examine the extraordinary fossils of huge extinct mammals in Patagonia at the southern tip of South America, and observe the remarkable series of related but distinct forms of life on the Galapagos Islands. Such an opportunity clearly played an important role in the development of his thoughts about the nature of life on earth.

When Darwin returned from the voyage at the age of 27, he began a long period of study and contemplation. During the next 10 years, he published important books on several different subjects, including the formation of oceanic islands from coral reefs and the geology of South America. He also devoted eight years of study to barnacles, a group of small marine animals with shells that inhabit rocks and pilings, eventually writing a four-volume work on their classification and natural history. In 1842, Darwin and his family moved out of London to a country home at Down, in the county of Kent. In these pleasant surroundings, Darwin lived, studied, and wrote for the next 40 years.

Key Learning Outcome 14.1. Darwin was the first to propose natural selection as the mechanism of evolution that produced the diversity of life on earth.