Selection generally refers to the pressures on cells and organisms to
evolve. These pressures include
natural selection, and, in
eukaryotic cells that reproduce
sexually,
sexual selection. Certain
phenotypic traits (characteristics of an
organism)—or, on a
genetic level,
alleles of
genes—segregate within a
population, where individuals with
adaptive advantages or traits tend to succeeded more than their peers when they
reproduce, and so contribute more
offspring to the succeeding generation. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring
inherit them from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved.