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T4 - Theory of Evolution

I can explain the development of the theory of evolution by natural selection

Introduction

The Englishman Charles Darwin is one of the most famous scientists who ever lived. His place in the history of science is well deserved. Darwin’s theory of evolution represents a giant leap in human understanding. It explains and unifies all of biology.

An overview of evolution can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/0/GcjgWov7mTM (17:39).

Darwin’s Theory at a Glance

Darwin’s theory of evolution actually contains two major ideas:

  1. One idea is that evolution occurs. In other words, organisms change over time. Life on Earth has changed as descendants diverged from common ancestors in the past.
  2. The other idea is that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring than others do. This results in changes in the traits of living things over time.

In Darwin’s day, most people believed that all species were created at the same time and remained unchanged thereafter. They also believed that Earth was only 6,000 years old. Therefore, Darwin’s ideas revolutionized biology. How did Darwin come up with these important ideas? It all started when he went on a voyage.

The Voyage of the Beagle

In 1831, when Darwin was just 22 years old, he set sail on a scientific expedition on a ship called the HMS Beagle. He was the naturalist on the voyage. As a naturalist, it was his job to observe and collect specimens of plants, animals, rocks, and fossils wherever the expedition went ashore. The route the ship took and the stops they made are shown in Figure below. You can learn more about Darwin’s voyage at this link: http://www.aboutdarwin.com/voyage/voyage03.html.

Voyage of the Beagle. This map shows the route of Darwin

Darwin was fascinated by nature, so he loved his job on the Beagle. He spent more than 3 years of the 5-year trip exploring nature on distant continents and islands. While he was away, a former teacher published Darwin’s accounts of his observations. By the time Darwin finally returned to England, he had become famous as a naturalist.

Darwin’s Observations

During the long voyage, Darwin made many observations that helped him form his theory of evolution. For example:

  • He visited tropical rainforests and other new habitats where he saw many plants and animals he had never seen before (see Figure below). This impressed him with the great diversity of life.
  • He experienced an earthquake that lifted the ocean floor 2.7 meters (9 feet) above sea level. He also found rocks containing fossil sea shells in mountains high above sea level. These observations suggested that continents and oceans had changed dramatically over time and continue to change in dramatic ways.
  • He visited rock ledges that had clearly once been beaches that had gradually built up over time. This suggested that slow, steady processes also change Earth’s surface.
  • He dug up fossils of gigantic extinct mammals, such as the ground sloth (see Figure below). This was hard evidence that organisms looked very different in the past. It suggested that living things—like Earth’s surface—change over time.

On his voyage, Darwin saw giant marine iguanas and blue-footed boobies. He also dug up the fossil skeleton of a giant ground sloth like the one shown here. From left: Giant Marine Iguana, Blue-Footed Boobies, and Fossil Skeleton of a Giant Ground Sloth

The Galápagos Islands

Darwin’s most important observations were made on the Galápagos Islands (see map in Figure below). This is a group of 16 small volcanic islands 966 kilometers (600 miles) off the west coast of South America.

Gal

Individual Galápagos islands differ from one another in important ways. Some are rocky and dry. Others have better soil and more rainfall. Darwin noticed that the plants and animals on the different islands also differed. For example, the giant tortoises on one island had saddle-shaped shells, while those on another island had dome-shaped shells (see Figure below). People who lived on the islands could even tell the island a turtle came from by its shell. This started Darwin thinking about the origin of species. He wondered how each island came to have its own type of tortoise.

Influences on Darwin

Science, like evolution, always builds on the past. Darwin didn’t develop his theory completely on his own. He was influenced by the ideas of earlier thinkers.

Earlier Thinkers Who Influenced Darwin

  1. Jean Baptiste Lamarck (1744–1829) was an important French naturalist. He was one of the first scientists to propose that species change over time. However, Lamarck was wrong about how species change. His idea of the inheritance of acquired characteristics is incorrect. Traits an organism develops during its own life time cannot be passed on to offspring, as Lamarck believed.
  2. Charles Lyell (1797–1875) was a well-known English geologist. Darwin took his book, Principles of Geology, with him on the Beagle. In the book, Lyell argued that gradual geological processes have gradually shaped Earth’s surface. From this, Lyell inferred that Earth must be far older than most people believed.
  3. Thomas Malthus (1766–1834) was an English economist. He wrote an essay titled On Population. In the essay, Malthus argued that human populations grow faster than the resources they depend on. When populations become too large, famine and disease break out. In the end, this keeps populations in check by killing off the weakest members.

Artificial Selection

These weren’t the only influences on Darwin. He was also aware that humans could breed plants and animals to have useful traits. By selecting which animals were allowed to reproduce, they could change an organism’s traits. The pigeons in Figure below are good examples. Darwin called this type of change in organisms artificial selection. He used the word artificial to distinguish it from natural selection.

Artificial Selection in Pigeons. Pigeon hobbyists breed pigeons to have certain characteristics. All three of the pigeons in the bottom row were bred from the common rock pigeon.

Wallace’s Theory

Did you ever hear the saying that “great minds think alike?” It certainly applies to Charles Darwin and another English naturalist named Alfred Russel Wallace. Wallace lived at about the same time as Darwin. He also traveled to distant places to study nature. Wallace wasn’t as famous as Darwin. However, he developed basically the same theory of evolution. While working in distant lands, Wallace sent Darwin a paper he had written. In the paper, Wallace explained his evolutionary theory. This served to confirm what Darwin already thought.

Darwin’s Theory of Evolution by Natural Selection

Darwin spent many years thinking about the work of Lamarck, Lyell, and Malthus, what he had seen on his voyage, and artificial selection. What did all this mean? How did it all fit together? It fits together in Darwin’s theory of evolution by natural selection. It’s easy to see how all of these influences helped shape Darwin’s ideas.

For a discussion of the underlying causes of natural selection and evolution see http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/5/DuArVnT1i-E (19:51).

Darwin’s Theory

It took Darwin years to form his theory of evolution by natural selection. His reasoning went like this:

  1. Like Lamarck, Darwin assumed that species can change over time. The fossils he found helped convince him of that.
  2. From Lyell, Darwin saw that Earth and its life were very old. Thus, there had been enough time for evolution to produce the great diversity of life Darwin had observed.
  3. From Malthus, Darwin knew that populations could grow faster than their resources. This “overproduction of offspring” led to a “struggle for existence,” in Darwin’s words.
  4. From artificial selection, Darwin knew that some offspring have chance variations that can be inherited. In nature, offspring with certain variations might be more likely to survive the “struggle for existence” and reproduce. If so, they would pass their favorable variations to their offspring.
  5. Darwin coined the term fitness to refer to an organism’s relative ability to survive and produce fertile offspring. Nature selects the variations that are most useful. Therefore, he called this type of selection natural selection.
  6. Darwin knew artificial selection could change domestic species over time. He inferred that natural selection could also change species over time. In fact, he thought that if a species changed enough, it might evolve into a new species.

Wallace’s paper not only confirmed Darwin’s ideas. They pushed him to finish his book, On the Origin of Species. Published in 1859, this book changed science forever. It clearly spelled out Darwin’s theory of evolution by natural selection and provided convincing arguments and evidence to support it.

Applying Darwin’s Theory

Darwin's Theory of Evolution through Natural selection can be summarized in four basic steps:

  1. Overproduction - organisms tend to produce more offspring than are able to survive to maturity
  2. Variation - populations often contain a range of characteristics for many different inherited traits
  3. Selection - pressures from the environment will favor only certain traits in a population. These "fit" organisms will live longer and reproduce more thus passing on successful traits.
  4. Adaptation - over time traits of fit individuals will become more common in the population

The following example describes how these steps could have changed a population of giraffes.

  • In the past, giraffes had short necks. But there was chance variation in neck length. Some giraffes had necks a little longer than the average.
  • Then, as now, giraffes fed on tree leaves. Perhaps the environment changed, and leaves became scarcer. There would be more giraffes than the trees could support. Thus, there would be a “struggle for existence.”
  • Giraffes with longer necks had an advantage. They could reach leaves other giraffes could not. Therefore, the long-necked giraffes were more likely to survive and reproduce. They had greater fitness.
  • These giraffes passed the long-neck trait to their offspring. Each generation, the population contained more long-necked giraffes. Eventually, all giraffes had long necks.

African Giraffes. Giraffes feed on leaves high in trees. Their long necks allow them to reach leaves that other ground animals cannot.

As this example shows, chance variations may help a species survive if the environment changes. Variation among species helps ensure that at least one will be able to survive environmental change.

A summary of Darwin's ideas are presented in the Natural Selection and the Owl Butterfly video: http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/3/dR_BFmDMRaI (13:29).