Chapter 7-1 Darwin’s Theory Science 7

7-1 Objectives Describe important observations Darwin made on his voyage. State how Darwin explained differences between similar species. Explain how natural selection leads to evolution.

The Voyage of the Beagle Darwin set sail on the HMS Beagle (a ship) in December 1831. 22 years old. Was originally employed as the captain’s companion, but eventually became the ship’s naturalist (someone who studies the natural world).

Darwin’s Observations Darwin’s most important observations included the diversity of organisms, the remains of ancient organisms, and the characteristics of organisms on the Galápagos Islands.

Diversity Darwin was amazed by the tremendous diversity of living things that he saw. In Brazil: insects that looked like flowers and army ants. In Argentina: sloths Currently, over 1.7 million species have been identified by scientists. Species is a group of similar organisms that can mate with each other and produce fertile offspring.

Fossils Fossils were among one of Darwin’s discoveries. A fossil is the preserved remains or traces of an organism that lived in the past. Darwin discovered fossil bones that resembled the bones of living animals (such as sloths and capybaras), only the fossil bones were much larger. Darwin also discovered fossilized seashells on cliffs, far above the ocean.

Galápagos Organisms Darwin reached the Galápagos in 1835. Darwin observed many unusual species such as giant tortoises, Galápagos finches, marine iguanas, and blue-footed boobies.

Comparison to South American Organisms Darwin found many similarities between Galápagos organisms (plants and animals) and those in South America. Darwin hypothesized that a small number of different plant and animal species had come to the islands from the mainland (via storms or set adrift on the ocean). Eventually the islands organisms’ offspring became different from the mainland relatives.

Comparisons Among the Islands Darwin notices difference in the animals on different islands in the Galápagos. For example: tortoises on one island had dome shaped shells on those on another had saddle-shaped shells.

Adaptations Finches (birds) in the Galápagos were different from one island to the other. Their beaks varied in sizes and shapes to suit the life it leads (type of food it eats). Adaptation is a trait that helps an organism survive and reproduce in its environment.

Evolution Upon returning to England, Darwin spent 20 years consulting with other scientists. Darwin reasoned that plants or animals that arrived on the Galápagos Islands faced environmental factors that were different from those on the mainland. Perhaps, Darwin hypothesized, the species gradually changed over many generations and became better adapted to the new environment. The gradual change in a species is called evolution.

Why Just a Theory? Darwin’s ideas are often referred to as the theory of evolution. A scientific theory is a well-tested concept that explains a wide range of observations. A well-established theory is generally considered to be true by the scientific community.

Selective Breeding Darwin studied selective breeding in order to understand how evolution might occur.

Natural Selection 1859: Darwin published a book called On the Origin of Species. Darwin proposed that evolution occurs in nature through a mechanism called natural selection. In Natural selection individuals that are better adapted to their environment are more likely to survive and reproduce that other member of the same species.

Overproduction Darwin was influence by Thomas Malthus. Malthus argued that organisms produce a lot of offspring. They over-reproduce in order to make sure that some of the offspring will survive to adulthood.

Variations A difference between individuals of the same species is called a genetic variation.

Competition Members of a species compete with each other for food and other resources in order to survive.

Selection Some variations make individuals better adapted to their environment. These individuals are better able to survive until adulthood and reproduce. Darwin proposed that, over a long time, natural selection can lead to change. Helpful variations may gradually accumulate in a species, while unfavorable ones may disappear.