Natural selection
Natural selection is an evolutionary process in which heritable traits become more or less common because they affect survival and reproduction in a particular environment.
What natural selection is
Natural selection is a mechanism of evolution. Individuals in a population vary, some of that variation is heritable, and some traits affect how likely organisms are to survive and reproduce. Over generations, traits linked to higher reproductive success can become more common.
Variation comes first
Selection does not create useful traits because a species needs them. Variation already exists through mutation, recombination, gene flow, and other sources. Natural selection sorts among that existing variation when different traits affect survival, mating, fertility, or offspring success.
Fitness is about reproduction
In evolution, fitness means reproductive contribution to future generations, not physical strength or moral worth. A trait can improve fitness if it helps an organism survive long enough to reproduce, attract mates, care for offspring, resist disease, use resources, or avoid predators in a specific environment.
Populations change
Individuals do not evolve during their lifetimes by natural selection. Populations evolve when inherited variants change in frequency across generations. That is why population genetics focuses on alleles, genotype frequencies, phenotype variation, and the environmental conditions that make some variants more successful than others.
Different modes of selection
Directional selection favors one end of a trait range. Stabilizing selection favors intermediate forms. Disruptive selection favors both extremes over the middle. Sexual selection can favor traits that improve mating success even when they carry survival costs. These patterns can overlap in real populations.
Adaptation and limits
Adaptations are heritable traits shaped by selection because they helped ancestors reproduce in particular conditions. Selection is powerful, but it is limited by available variation, tradeoffs, history, chance events, genetic linkage, development, and changing environments. It does not produce perfect organisms.
Misread examples
Natural selection is often summarized as survival of the fittest, but that phrase can be misleading if it sounds circular or purely about strength. A better question is: which heritable traits leave more descendants under these conditions? The answer can change when environments, competitors, predators, diseases, or mates change.
Why it matters
Natural selection matters because it explains how populations become adapted without planning or design. It helps scientists understand antibiotic resistance, pesticide resistance, disease evolution, conservation, domestication, biodiversity, and the deep history connecting living organisms through common descent.