Natural selection is a consistent, biased or nonrandom difference in the fitness among phenotypically different biological entities. It is the antithesis of chance. Natural selection can result in the replacement of one genotype by another.
Natural selection can result in the maintenance of polymorphism in a population.
When genotypes differ in fitness, selection determines the outcome if the population is large, but in a small population, genetic drift can be more powerful than selection. Strength of selection can vary, but selection is often strong and therefore a powerful evolutionary force. A feature is an adaptation for a particular function if it has evolved or is maintained by natural selection for that function by enhancing the relative fitness of the organisms with that feature.