Natural Selection
Natural selection is the process by which individuals with traits suited to their environment survive and reproduce more, making those traits more common.
Definition
Natural selection is the idea, proposed by Charles Darwin, that individuals with heritable traits improving survival and reproduction in a given environment leave more offspring than others, so those traits become more common in the population over generations; it requires variation among individuals, heritability of that variation, and differences in reproductive success, and it acts only on variation that already exists rather than creating new traits on demand. "Survival of the fittest" means best fit to the current environment, not strongest or fastest, so a different trait can become the fit one when conditions change. More formally, natural selection is differential reproductive success caused by heritable phenotypic differences, changing allele frequencies across generations; its strength is captured by selection coefficients, and its mode can be directional (shifting the mean), stabilizing (reducing variance), or disruptive (favoring extremes), with sexual selection, frequency-dependent selection, and kin selection as special cases, while the response of a quantitative trait follows $R = h^2 S$.
Example
Peter and Rosemary Grant measured Galapagos ground finches during a drought and found that birds with beaks about $0.5\ \text{mm}$ deeper survived better by cracking large, hard seeds, so the next generation had measurably deeper beaks. The peppered moth in industrial England shifted from mostly light to mostly dark within decades as soot darkened tree bark, then reversed after clean-air laws, with field studies estimating selection coefficients near $0.1$ to $0.3$ from differential bird predation on the mismatched form.
Key Insight
Natural selection is not a force with foresight but a statistical consequence of variation, inheritance, and differential reproduction, which is why it produces adaptations that are good enough rather than optimal and why vestigial structures can persist for generations.