Darwin's Theory

In 1831, the British ship HMS Beagle set sail from England on a 5-year trip around the world. Charles Darwin was on board. Darwin was a naturalist - a person who observes and studies the natural world.

DIVERSITY

Darwin was amazed by the diversity of living things that he saw during his voyage. He wondered why they were so different from those in England. Darwin saw insects that look like flowers. He also observed sloths, slow-moving animals that spent much of their time hanging in trees. Today, scientists know that organisms are even more diverse than Darwin thought. In fact, scientists have identified more than 1.6 million species of organisms on earth. A species is a group of similar organisms that can mate with each other and produce fertile offspring. The exact number of species have not yet been studied.

FOSSILS

Darwin saw fossils of animals that had died long ago. Is the preserved remains or traces of an organism that lived in the past. Darwin was puzzled by some of the fossils he observed. For example, he saw fossils that resemble the bones of living Sloths but we're much larger in size. He wondered what had happened to the ancient, giant ground sloth. See the figure below.

GALAPAGOS ORGANISMS

The Beagle made many stops along the Atlantic and Pacific coasts of South America. From the Pacific coast, the ship traveled West to the Galapagos Islands. Darwin observed many unusual life-forms there. He compared organisms from the Galapagos Islands to those on the other islands as well as organisms that lived elsewhere.

Comparisons to South American Organisms

Darwin discovered many similarities between Galapagos organisms and those found in South America. Many of the birds and the plants on the islands resemble those on the mainland. However, he also noted important differences between the organisms. For instance, you can see the differences between Island and mainland iguanas in the figure below.

Darwin became convinced that species do not always stay the same. Instead, he thought species can change and even produce new species over time. Darwin began to think that maybe the island species Were Somehow related to South American species. Perhaps, he thought, the island species had become different from the mainland relatives over time.

Comparisons Among the Islands

Darwin also discovered many differences among the organisms on the different Galapagos Islands. For example, the tortoises on one Island had dome-shaped shells. Those on another Island had saddle-shaped shells. A government official in the islands told Darwin that he could tell which island a tortoise came from just by looking at its shell.

Adaptations

Birds were all so different from one Island to the next. Look at the figure below. When Darwin returned to England, he learned that the different birds were all finches. Darwin concluded that the finch species were all related to a single ancestor species that came from the mainland. Time, different finches developed different beak shapes and sizes that were well-suited to the food that they ate. Beak shape is an example of an adaptation, a trait that increases an organism's ability to survive and reproduce.

DARWIN’S HYPOTHESIS

Darwin thought about what he had seen during his voyage on the Beagle. By this time Darwin was convinced that organisms change over time. The process of change in a population over time is called evolution. Darwin however wanted to know how organisms change. Over the next 20 years, he consulted with other scientists and gathered more information. On his observations, Darwin reasoned that plants or animals that arrived on the Galapagos Islands faced conditions that were different from those on the nearby mainland. Darwin hypothesized that species change over many generations and become better adapted to new environmental conditions.

Darwin's ideas are often referred to as the theory of evolution but are more correctly stated as the theory of evolution by natural selection. A scientific theory is a well-tested concept that explains a wide range of observations. From the evidence he collected, Darwin concluded that organisms on the Galapagos Islands had changed over time.

Artificial Selection

Darwin studied the offspring of domesticated animals that were produced by artificial selection in an effort to understand how Evolution might occur. In artificial selection, only the organisms with a desired characteristic, such as color, are bred. Darwin himself had bred pigeons with large, fan-shaped tails. By repeatedly allowing only those pigeons with many tail feathers to mate, Darwin produced pigeons with two or three times the usual number of tail feathers. Darwin thought that a process similar to artificial selection might happen in nature. He wondered what natural process selected certain traits.

What is Natural Selection?

In 1858, Darwin and Alfred Russel Wallace, another British biologist, both proposed the same explanation for how evolution occurs. The next year, Darwin described his explanation in his book The Origin of Species. In this book, Darwin proposed that evolution occurs by means of natural selection. Natural selection is the process by which individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species. Darwin identified factors that affect the process of natural selection overproduction, competition, variations, adaptations, and the passing of those variations to the next generation.

Overproduction (Overpopulation)

Darwin knew that most species produce far more offspring than can possibly survive. In many species, so many offspring are produced that there are not enough resources - food, water, and living space - for all of them.

Variation

Members of a species differ from one another in many of their traits. Any difference between individuals of the same species is called a variation. For example, sea turtles may differ in color, size, the ability to swim quickly, and shell hardness.

Competition

As food, space, and other resources are limited, the members of a species must compete with one another to survive. Competition does not always involve physical fights between members of a species. Instead, competition is usually indirect. For example, some turtles may not find enough to eat. A slower turtle may be caught by a predator, while a faster turtle may escape. Only a few turtles will survive to reproduce.

Selection

Darwin observed that some variations make individuals better suited to their environment. Those individuals are more likely to survive and reproduce. Their offspring may inherit the helpful characteristic. The offspring, in turn, will be more likely to survive and reproduce, and pass the characteristic to their offspring. After many generations, more members of the species will have the helpful characteristics (adaptations).

In effect, the environment selects organisms with helpful traits to become parents of the next generation. Darwin proposed that, over a long time, natural selection can lead to change. Helpful variations may accumulate in a species, while unfavorable ones may disappear.

Environmental Change

A change in the environment can affect an organism's ability to survive and therefore lead to natural selection. Environmental change can be gradual, such as when mountains or deserts form, other species die out, or new species evolve. These things can take millions of years. Change to an environment can also be quick, such as floods, volcanoes, or earthquakes. It can also be caused not by a change to the environment itself, but by the organism's movement to a different environment (by migration or evolution).

For example, if a species of animal is mostly limited to eating one kind of leaf, and a change occurs: a fungus attacks and kills most of that kind of plant, the species of animal has to evolve either to fight the fungus or to eat something else. Notice it is the species that is changing, not an individual animal. Remember that individuals cannot change their DNA or traits. The traits we have either help us compete and survive or they don’t.

Isolation means that organisms of the same species are separated, and happens when there is something between the organisms that they can't cross. Organisms become isolated as a result of environmental change. The cause of isolation can be gradual, like when mountains or deserts form or continents split apart. It can also be quick, such as organisms being blown to different places by a storm or tsunami (tidal waves).

Genes and Natural Selection

Without variation, all the members of a species would have the same traits and the same chance of surviving and reproducing. But where do variations come from? Are they passed on from parents to offspring? Darwin did not know about genetics and traits and heredity. he could not explain what caused variations or how they were passed on. As we know now, variations can result from changes in genes and the shuffling of different forms of genes when an egg and sperm join. Genes, such as those for hair color and height, are passed from parents to their offspring. Only traits that are inherited, or controlled by genes that are passed to offspring, can be acted upon by natural selection.