Natural selection and its evolutionary history

Natural selection as differential survival and reproduction

Natural selection is the differential survival and reproduction of individuals because they differ in observable traits that affect their relative fitness. Individuals with heritable traits that improve survival and lifetime reproductive success leave more offspring, so those traits become more common in the population over generations. When environmental conditions favor certain traits consistently, their frequencies increase; when conditions change, different traits may be favored, producing microevolution (changes within lineages) and potentially macroevolution (larger-scale evolutionary change). Darwin framed natural selection as the outcome of inheritance, variation, and a struggle for existence (including competition and cooperation). For selection to drive adaptive evolution, populations must contain heritable variation affecting phenotypic fitness. Traits can be favored not only by survival advantages (natural selection) but also by mating preferences (sexual selection) and by reproductive health and fecundity effects (fecundity selection). Over time, selection can lead to population divergence and speciation, especially when reproductive isolation evolves. Mechanistically, natural selection acts on phenotypic differences that are under genetic control. Directional selection increases the frequency of advantageous alleles, stabilizing selection removes deleterious variants, and disruptive (diversifying) selection favors extreme trait values over intermediates. Although genetic drift can change allele frequencies for neutral variants, natural selection remains the primary explanation for adaptive evolution by increasing the representation of beneficial heritable traits and decreasing maladaptive ones.

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Heritable traits and variation within populations

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