Natural Selection

Fitness
Stabilizing Selection
Directional Selection
Disruptive Selection
Speciation
Extinction

References:

Digital critters mimic behavior of real life

Natural Selection

Fitness

A species' fitness lies at the heart of Darwin's original theory. A species fitness is measured solely by the number of offspring that an individual has. But the offspring must survive to contribute to the following generation, so fitness as a generational value. For example, take two male Zebras, Zebra 1 and Zebra 2, who live on the African savanna. Both are the same age, but live in different areas of the savanna. Zebra 1 and Zebra 2 lives in an area where a large lion pride roam, so they both must avoid being eaten by the lion pride on a regular bases. They must also find time to eat and find water. Zebra 1 has a deformed rear leg. Zebra 2 is fast and strong. As Zebra 1 and Zebra 2 were growing, their herd protected them by keeping their young in the middle of the herd. But as the zebras became a juveniles, the head did less to protect them, having the juvenile run on the outside of the heard. Soon after they both became juveniles, the lion pride attacked the heard. Zebra one, not being able to run fast, was caught by the lion pride. Zebra 2 escaped this, and many more attacks. Lived to have many offspring who were also fast and strong runners. So Zebra 2 was more fit than Zebra 1, by surviving.

With a stronger understanding of the mechanisms of inheritance, the ideas of fitness has been expanded in the following way. An individual's fitness became defined as to how well that individual contributed its genes to the next generation. The bottom line is still how many successful offspring an individual has.