Fitness is the reproductive success of organisms relative to others.
Selection is one of the main mechanisms of evolution. It acts on heritable traits, enabling organisms with phenotypes that favor survival to produce more offspring than others with less favorable phenotypes.
Survival rate of a phenotype is the percentage of individuals with that phenotype to survive to reproductive age. Reproduction rate of a phenotype is the number of offspring generated by an average individual of that phenotype.
Relative fitness is the product of survival and reproduction rates of a phenotype divided by the value of the population’s most successful phenotype. Strength of selection for a phenotype depends on its relative fitness, since selection coefficient can be calculated by subtracting relative fitness value of a phenotype from 1.
Main types of selection are directional, disruptive, and stabilizing. Directional selection favors extreme values of a trait in one direction. Disruptive selection favors two extremes of a phenotype in opposing directions. Stabilizing selection favors an optimum value.
