Individuals with extreme trait values have greatest fitness. E.g. selection on bill size in the black-bellied seedcracker – Pyrenestes o. ostrinus. This species of African finch exhibits non-sex related polymorphism in bill size. Members of the species have either one of two bill morphs; bills are relatively small or relatively large. The morphs feed on different sized seeds and this is directly related to natural selection on bill size.

The ability of an individual to survive and reproduce relative to conspecifics.

Any remains, trace, or imprint of an organism that has been preserved in the Earth's crust.

The principle that the founders of a new colony carry only a fraction of the total genetic variation in the source population.

Selection of a genotype depending on its frequency in the population, e.g. scale eating fish. See resource polymorphism.

The incorporation of genes into the gene pool of one population from one or more other populations.

A severe, temporary reduction in variation in a population.

Random changes in the frequency of two or more alleles or genotypes within a population.

The genetic makeup, as distinguished from the physical appearance (phenotype), of an organism or a group of organisms.

An individual resulting from mating between genetically distinct populations/species. See also hybrid zone.