In 1859, Darwin and Wallace proposed "natural selection" as the mechanism by which new species arise from pre-existing species. The theory can be summarized as below:

• Variations exist among the individuals of every species.
• Each species tends to increase its population in a geometric ratio. However, the population of a species is more or less constant over a long period of time because of two main reasons: a.) The necessities for life such as food and shelter are limited. b.) There are attacks from enemies and diseases. This means that the individuals in a species have to compete with each other for life necessities and have to endure enemies and diseases. This is the so called STRUGGLE FOR EXISTENCE.
• As a result, those individuals with variations advantageous to this struggle would be more likely to survive and reproduce, while those with unfavorable variations will be eliminated. This phenomenon is described as SURVIVAL OF THE FITTEST.
• Darwin claimed that it was the nature (that is all the environmental factors including both physical and biotic ones), which did the selecting of the fittest individuals for survival. This process is called NATURAL SELECTION.

• As a consequence of natural selection occurring from generation to generation over a long time, the species will become more and more adaptive to the environment.
• Furthermore, a population with one variation may become adapted to an environment in one way, while others, with another different variation may become adapted to another environment. If these two groups are prevented from interbreeding by some isolating mechanisms for some time, they will evolve independently, giving rise to two different species.