Natural selection is based on the theory that in any given generation some individuals are more likely to survive and reproduce than others. When a certain trait improves the survivability of an organism, the environment is said to "favor" that trait and "naturally select" for it. The theory of natural selection is often referred to as "survival of the fittest."
NATURAL SELECTION VS. ARTIFIICAL SELECTION
Everyone has seen the process and results of artificial selection whether they realize it or not. Artificial selection occurs when people select plants or animals and breed them for certain desired charateristics. Examples of this are roses bred for a particular color, milk cows bred for highest milk production, and so-called "pocket dogs" bred for small size. This sort of selection has created many different "breeds" within one species.
Natural selection occurs environmental, or natural, factors "select" which plants or animals will survive and reproduce.
CONDITIONS UNDER WHICH NATURAL SELECTION OCCUR
1. Individuals in the population must produce more offspring than can survive
Example: A given species of moths produces twice as many offspring as their environment is able to support.
2. Those individuals must have different characteristics
Example: Within that single species of moth some moths are white and some moths are brown
3. Offspring must inherit some characteristics from their parents
Example: A moth's color has a genetic basis and is determined by genes inherited from its parents.
4. Organisms with the best-suited characteristics for their environment are more likely to survive and reproduce
Example: The white moths tend to get eaten by birds and so they survive to reproduce less often than brown moths do. The surviving brown moths reproduce more often and have more brown baby moths because this trait has a genetic basis. Eventually, the advantageous color trait of brown dominates within the population.
In this example, the birds that eat white moths more often would be referred to as the "selection pressure,". A selection pressure is an environmental factor that causes some organisms to survive better than other organisms of the same species.
FORMULA FOR NATURAL SELECTION
Based on the example provided above, one can see that the "formula" for natural selection is as follows:
Variation + Differential Reproduction + Heredity = Natural Selection
TYPES OF NATURAL SELECTION
Natural selection can cause several different types of changes in a particular population. How the population changes depends upon the type of selection pressure the population is under and which particular traits are favored by that pressure. There are four types of natural selection:
Stabilizing selection eliminates extreme or unusual traits. Those organisms with the most common traits are considered best adapted and will survive and reproduce in greater number.
Directional selection maintains that genetic traits at one end of a spectrum will be selected for, whi traits at the other end of the same spectrum will be selected against. An often-given example is that of horses. Ancestral horses were much smaller than modern day horses. The ancestral horses were adapted to a heavily wooded environment where small size resulted in greater survival. As horses moved into open grasslands, they evolved into larger animals with longer legs.
As opposed to stabilizing selection, disruptive selection occurs when environmental factors favor extreme or unusual traits while selecting against common traits. There's a good example of this right under your feet! If you compare the heigth of weed plants in a lawn to weed plants in the wild, you'll find that shorter weed plants are selected for in lawns where the grass is kept mowed short. In the wild, you find that selection favors taller weed plants that are better able to compete for sunlight.
Sexual selection occurs because of the different ways that the two sexes use to select their mates. Females generally select the "fittest" males of the species with which to mate, and the fittest males are determined by which males survive contests of strength. This has led the males of some species to develop things like horns, antlers, and larger muscles Additionally, in some species males have evolved other factors with which to attract females, such as brighter coloration and the practice of certain courtship behaviors.
A great subject-related book by one of my favorite authors - Stephen Jay Gould!