Natural Selection operates with four conditions:

1. Reproduction
2. Heredity
3. Variation in individual characters (size, shape, height, color)
4. Variation in the fitness of organisms (number of offspring, reproductive success)

What are the forces at work that bring about evolutionary change?

Directional Selection - Directional selection favors one extreme value for a particular trait in a distribution of these values. (example: smaller body size birds produce more offspring than larger birds, decreasing body size in a population)

Stabilizing Selection – Reduces phenotypic variance in a population (example: medium body size over extreme sizes keeping the population constant through a long period of time)

Disruptive Selection - Disruptive selection increases the population’s phenotypic variance.