Natural Selection

Fitness
Stabilizing Selection
Directional Selection
Disruptive Selection
Speciation
Extinction

References:

Digital critters mimic behavior of real life
Macroevolution, The Fossil Record

Natural Selection

Charles Darwin introduced a concept that took hold of the scientific community, changing how we both perceive and examined nature, Natural Selection. In it most common definition, survival of the fittest, Natural Selection implies that those species which we see today survived the changes in their environment better than those who became extinct. In a more restricted definition, Natural Selection states that populations who best "adapt" to changes in the environment will eat more, grow faster, be in better shape, so that they could produce more offspring. Natural Selection looks at one product, the number of offspring that survives to enter the reproductive population. When one population has more offspring that survive to reproductive age than another population, then that population will have a better chance of outcompeting the later population. This is at the foundation of ecological theories (e.g. competition and succession).

Natural selection can be divided into three groups, stabilizing selection, directional selection, and disruptive selection. Each form describes how a population may be affected by the environment. For Evolution theory, how natural selection causes speciation is the important issue.