In this topic you will learn about how species change over time.

Differences in size and shape are examples of variation. Variation is the difference in appearance of an inherited trait in the members of a species. Some variations don't have much of an effect on an organism. However, for some organisms living in the wild a trait like color can make a big difference.

An animal that blends in with color patterns of its surroundings may survive longer than one with color that does not blend in as well. This ability to blend in with the environment is an example of an adaptation. An adaptation is a variation that increases an organism's chances for survival.

Charles Darwin, an English scientist who lived in the 1800's, developed a theory about adaptations. His theory of natural selection says that organisms that are best adapted to their environment live longest and pass their traits to their offspring. This theory is based upon the following ideas: Living things produce more young than can live in the environment. There is always variation among offspring. Some variations in traits enable members of a species to survive and reproduce more successfully than others. Natural selection is always taking place.

To living things, competition is the struggle to obtain the resources they need to survive. Organisms compete for resources such as food, air, water, and space. Offspring that inherit variations that make it easier for them to find or use a resource are better able to survive.

A completely new variation may occur in an offspring. Such a change may take place because of mutations. A mutation is a change in a gene. If the mutation takes place in a sex cell, the new gene can be passed on to the offspring. Mutations are normal, random events. They can happen at any time.

Most new species evolve when two groups, or populations, of the same species stop breeding with one another. This usually happens by isolation. Isolation occurs when one population of a species is separated from others of the same species by a river, ocean, mountain, or other geographical barrier. The types of variations that emerge in one population may not emerge in the other. After many generations, the two populations become different species.

Some species probably evolved gradually, and others very rapidly. An event that is "rapid" in geologic time may extend over thousands of years.

Extinction is an important part of natural selection and the changing variety of living things. There have been at least five mass extinctions during which thousands of species died out, including the extinction of the dinosaurs. When one species dies out, it leaves room for other species to take its place.