natural selection

Meaning: Individuals whose heritable traits suit their environment tend to leave more surviving offspring, so the alleles behind those traits rise in frequency. The logic has four steps: individuals vary, some variation is heritable, more offspring are born than survive, and survival or reproduction differs among variants. Mutation and recombination supply the variation; selection sorts what already exists. Fitness means relative reproductive success, not strength or lifespan.

On the MCAT: Passages describe a population and an environmental change, such as antibiotic exposure or a new predator, and ask which phenotype will increase. Trait-distribution graphs: a shifted mean suggests directional selection, a narrowed peak stabilizing selection, and two peaks disruptive selection. Hardy-Weinberg questions list "no natural selection" as an equilibrium condition; when selection acts, that condition is violated and allele frequencies change.

Common confusion: Organisms do not change their traits because they need to. Mutations arise without regard to need, and the environment favors variants already present. Traits acquired during life, such as muscle from training, are generally not passed on through genes. The fact that settles it: evolution is a change in allele frequency across generations, so the population changes, not the individual.