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During the 19th century when Europeans were exploring their own countries as well as mounting large expeditions to explore foreign lands, natural historians such as Charles Darwin and Alfred Russel Wallace were making observations of the geographic distributions of species. They found that different species were found in different environments. Species in the tropics were well suited, or adapted, to live in the tropics and species in England were well adapted to live in England. Finches on different Galapagos Islands were well adapted to their particular environments. Independent of one another, Darwin and Wallace both developed the theory of natural selection to explain the geographic distributions of species. That theory states that there are variations in organisms of the same species. Some are tall, some are short, some have larger mouths, keen eyesight, etc. The organisms that are most well suited to live in a particular environment are more likely to survive. The survivors produce the next generation and those offspring are more likely to have their parent's special traits (such as keen eyesight, etc.)

Here are the main points of the theory of natural selection:

1. Everyone’s Different: There are variations among individuals of the same species (better eyesight, extra feathers, longer legs, etc.)
2. Resources are Limited: Resources such as food and shelter are limited
3. Lots of Babies: Organisms make more babies (young) than could actually live in the environment.
4. Organisms Compete: Organisms compete for food and other resources in the environment.
5. The "Fittest" Survive: The organisms whose variations best fit the environment are most likely to survive, reproduce, and pass useful traits to the next generation.

The evidence of evolutionary changes in life forms through time has been well established and accepted for more than a century. Natural selection is an explanation for the mechanism of evolution. In addition, scientists continue to investigate more specific hypotheses that might further develop our understanding of how change through time happens. One of the main hypotheses, developed by Stephen J. Gould and Niles Eldridge, is called punctuated equilibrium. This hypothesis suggests that the rate of change in a population is not constant. No changes, or very few, may happen to a population of organisms over millions of years and then changes may happen rapidly over many thousands of years. Advances in technology over the past century have allowed scientists to continue to test hypotheses that further our understanding of evolutionary processes.