Unit 7 (Natural Selection) accounts for approximately 13-20 percent of the AP Biology exam. It covers natural selection mechanisms, evidence for evolution, speciation, and phylogenetics. This is the second most heavily weighted unit after Unit 1 (Chemistry of Life combined with Cell Structure).
Natural Selection: Four conditions must be met for natural selection to operate: variation in the population, heritability of traits, differential survival and reproduction, and traits that affect fitness. The AP exam frequently presents scenarios and asks students to identify which condition is present or absent.
Types of Selection: Directional selection shifts the mean of a trait in one direction. Stabilizing selection reduces variation around the existing mean. Disruptive selection favors both extremes over the mean. AP exam questions typically present a graph of trait distribution before and after selection and ask students to identify the selection type.
Evidence for Evolution: The AP exam tests five categories of evidence: fossil record (transitional forms), comparative anatomy (homologous vs analogous structures), embryology (shared developmental patterns), molecular biology (DNA and protein sequence similarity), and biogeography (island species distribution).
Speciation: Allopatric speciation occurs when geographic barriers divide a population. Sympatric speciation occurs without geographic separation (often through polyploidy in plants or habitat differentiation). The AP exam expects students to distinguish between these and identify the reproductive isolation mechanism.
Phylogenetics: Reading phylogenetic trees is a high-frequency AP Bio skill. Key concepts: common ancestors are at nodes, sister taxa share the most recent common ancestor, and branch length may or may not represent time depending on the tree type. The AP exam asks students to identify shared derived characters (synapomorphies) that define clades.
Hardy-Weinberg Equilibrium: The mathematical foundation of population genetics. p + q = 1 (allele frequencies) and p-squared + 2pq + q-squared = 1 (genotype frequencies). The AP exam expects students to calculate allele frequencies from phenotype data and identify which H-W conditions are being violated in a scenario.
Practice with Cognify's AP Biology evolution questions to see how these concepts appear in exam-format MCQs and FRQs. The adaptive system identifies your specific gaps within this unit and focuses your practice there.