Define natural selection and explain how it leads to adaptation.

• A. Differential survival and reproduction based on heritable variation, causing advantageous traits to become more common.
• B. Random genetic drift changes allele frequencies over time.
• C. Traits acquired during a lifetime are inherited by offspring.
• D. A single organism adapts immediately to its environment and passes that trait.

Answer: (A)

Natural selection is differential survival and reproduction based on heritable variation in traits. In a population, individuals differ in characteristics, and those differences can affect how well they survive and mate in a given environment. The individuals with traits that give them higher fitness are more likely to reproduce and pass those traits on to their offspring. Over many generations, these advantageous traits become more common, shifting allele frequencies in the population and producing adaptations—traits that improve the population’s ability to live in that environment.

This explanation works best because it links variation that can be inherited to differential success, and to gradual changes across generations that lead to better-fitting traits. The other ideas don’t fit: random genetic drift changes allele frequencies by chance, not because of fitness differences; traits acquired during a lifetime aren’t usually passed to offspring; and a single organism can’t instantly adapt and pass on a new trait to its descendants.