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Natural selection occurs when heritable phenotypic differences cause some individuals to survive and reproduce more than others, changing trait frequencies across generations.
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.
Natural selection occurs when heritable phenotypic differences cause some individuals to survive and reproduce more than others, changing trait frequencies across generations.
For natural selection to produce adaptation and speciation, populations must have heritable variation affecting fitness, and environmental pressures determine which traits are favored.
Selection can act in different ways (directional, stabilizing, disruptive/balancing), while genetic drift can also influence neutral variation.
The differential survival and reproduction of individuals due to heritable differences in traits that affect fitness.
A measure of an individual's reproductive success relative to others, influenced by how well its traits support survival and reproduction.
Genetic differences among individuals that lead to phenotypic differences and can be passed from parents to offspring.
Selection that increases the frequency of alleles associated with higher fitness, shifting trait values in one direction.
Selection that reduces the frequency of alleles producing lower fitness, favoring intermediate trait values.
Selection that favors extreme trait values over intermediate ones, potentially promoting niche partitioning and divergence.
Selection driven by differences in mating success due to traits preferred by mates.
Selection based on differences in reproductive output and reproductive health costs that affect how many offspring are produced.
Random changes in allele frequencies due to chance events, which can affect neutral or nearly neutral genetic variation.
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