The three basic requirements for evolution through natural selection (as told by the anthropology textbook used in ANTH 1001 at the University of Minnesota):
  • Genetic variation in a species
  • Heritability of the variation
  • Differential reproductive success
    This is essentially the same as what melodrame stated, but simpler.
    Contrary to popular belief, natural selection does not necessarily imply increased survivability of a species, but only a change in survivability. It works both ways, wherein a trait can be selected for or against (or have no change in the survivability of the species).
    There are three basic types of natural selection as well:
    1. Directional Selection: Selecting for taller people, resulting in overall higher mean heights of a population.
    2. Disruptive Selection: Selecting for either the tallest or the shortest, resulting in a similar average height, but two populations with much higher or much lower local averages.
    3. Stabilizing Selection: Opposite of Disruptive selection; selection against the tallest/shortest of a population, reducing variation of that trait.