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.