AP Biology Unit 7 Essentials

Natural selection requires three ingredients: heritable variation within a population, and differential reproductive success based on that variation. Individuals don't "try" to evolve, and evolution has no inherent direction toward complexity or improvement — it's simply the statistical result of which heritable variants happen to survive and reproduce better in a given environment. Change the environment, and what counts as "fit" changes too. This is why the same population can be pushed in completely different directions by different selective pressures.

The Hardy-Weinberg equations (p² + 2pq + q² = 1) describe a population that is NOT evolving — one with no mutation, random mating, no selection, an infinite population size, and no gene flow. Virtually no real population meets all five conditions perfectly, which is exactly the point: by comparing a real population's observed genotype frequencies to the Hardy-Weinberg expectation, biologists can detect that evolution IS happening and start investigating which force (selection, drift, gene flow, mutation) is responsible.

Fossils, biogeography, comparative anatomy (homologous structures), and molecular data (shared DNA sequences and the universal genetic code) are independent sources of evidence — yet they all point toward the same conclusion: all life shares common ancestry, and that ancestry can be reconstructed as a branching phylogenetic tree. When unrelated lines of evidence (a fossil record AND a DNA comparison, for instance) agree on the same evolutionary relationships, that convergence is much stronger support than any single line of evidence alone.