Unit 7: Natural Selection

Executive Summary

Unit 7 formalizes evolution: populations evolve, not individuals, and natural selection acts on phenotypes shaped by genotypes.

Natural selection

Variation exists in populations; some variants survive and reproduce more in a given environment; those alleles increase in frequency. Selection requires heritable variation and differential reproductive success. Populations — not individuals — evolve.

Evidence and common ancestry

Fossils show change over time; comparative anatomy reveals homologous structures (shared ancestry) vs analogous structures (convergent evolution); molecular data (DNA/protein sequences) quantify relatedness; biogeography matches distributions to history. Vestigial structures are reduced remnants. All life shares a common ancestor — seen in the universal genetic code.

Population genetics

Hardy–Weinberg equilibrium (p² + 2pq + q² = 1) describes a non-evolving population: no mutation, no selection, random mating, no gene flow, and a large population. Violations — drift, gene flow, selection, mutation, non-random mating — drive evolution. Genetic drift (bottlenecks, founder effects) is strongest in small populations and can fix alleles randomly.

Speciation and phylogeny

Reproductive isolation splits lineages. Allopatric speciation needs geographic barriers; sympatric speciation happens without them (often polyploidy in plants). Prezygotic barriers block mating; postzygotic barriers reduce hybrid fitness. Phylogenetic trees and cladograms group species by shared derived traits; the most parsimonious tree needs the fewest changes.

Continuing evolution

Evolution is ongoing: antibiotic resistance, pesticide resistance, and viral antigenic shifts are observable natural selection. Populations continue to evolve — allele frequencies keep changing.