1. Variation among individuals in some attribute or trait.
2. A consistant relationship between that trait and some measure of reproductive success or survivorship (fitness differences).
3. Inheritance or a consistant relationship (for that trait) between parents and their offspringvwhich is at least partly independent of common environmental effects then:
- There will be a within-generation effect; individuals of a given age will differ from the individuals that do not survive til that age,
- Uf the population is not at equilibrium, there will also be a between-generation effect; the offspring generation will differ from their parental generation.
Elimination of extreme phenotypes from phenotypes at both ends of the trait distribution.