Unit 7: Natural Selection
AP Biology — 108 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Natural selection stands as the cornerstone mechanism of evolutionary change, operating on heritable phenotypic variation within populations to shift allele frequencies across generations. This unit investigates how random mutations, genetic recombination, and gene flow generate the molecular diversity upon which selective pressures act, favoring traits that enhance survival and reproductive success in specific environments. Students will explore the full continuum of evolutionary evidence—from the fossil record and comparative anatomy to molecular phylogenetics and shared genomic sequences—revealing the common ancestry uniting all life. The Hardy-Weinberg equilibrium model provides a critical quantitative null hypothesis: when allele frequencies remain stable across generations, no evolution is occurring; deviations flag the action of selection, drift, migration, or non-random mating. Speciation mechanisms, including allopatric divergence driven by geographic isolation and sympatric pathways such as polyploidy, explain how reproductive isolation crystallizes new species. Coevolutionary dynamics, exemplified by evolutionary arms races between predators and prey or hosts and pathogens, illustrate the perpetual, reciprocal adaptation that shapes ecological communities.
Molecular Deep-Dive
Natural selection operates fundamentally as a filter on molecular variation. At the DNA level, point mutations, frameshifts, gene duplications, and chromosomal rearrangements generate novel alleles; these arise randomly with respect to organismal need. The central dogma translates these sequence changes into altered mRNA transcripts and, ultimately, modified polypeptide chains. A single amino acid substitution can reshape protein tertiary structure, modulating enzyme kinetics, receptor-ligand binding affinity, or structural stability. Natural selection acts on the resulting phenotypic differences—whether in coat pigmentation, hemoglobin oxygen affinity, or metabolic efficiency—not on the genotype directly.
Consider the β-globin gene: a thymine-to-adenine transversion converts codon 6 from GAG (glutamic acid) to GTG (valine). Under low-oxygen conditions, the valine-substituted hemoglobin S polymerizes, distorting erythrocytes into sickle shapes. In regions where Plasmodium falciparum malaria is endemic, heterozygotes (HbA/HbS) maintain sufficient functional hemoglobin while experiencing reduced parasite replication, a classic case of balancing selection maintaining both alleles in the population. This molecular mechanism illustrates how selection preserves deleterious alleles when heterozygote fitness exceeds both homozygotes.
Hardy-Weinberg equilibrium provides the mathematical baseline for detecting microevolution. In a diploid, sexually reproducing population with two alleles (A and a) at a single locus, let p equal the frequency of allele A and q equal the frequency of allele a (p + q = 1). Under equilibrium—no mutation, no gene flow, infinite population size, random mating, and no selection—genotype frequencies stabilize at p² (AA), 2pq (Aa), and q² (aa), summing to 1. When observed genotype counts deviate from these expected frequencies, statisticians use chi-square analysis to infer evolutionary forces. For example, an excess of heterozygotes may indicate heterozygote advantage, while a deficit suggests inbreeding or assortative mating.
Phylogenetic reconstruction depends on molecular sequence data, particularly conserved genes such as ribosomal RNA and slowly evolving protein-coding loci. Shared derived characters (synapomorphies) identified through cladistic analysis define monophyletic clades. Molecular clocks, calibrated using fossil constraints, exploit the relatively constant accumulation of neutral substitutions to estimate divergence times. Homologous genes diverge through speciation (orthologs) or gene duplication events (paralogs), and distinguishing between them is critical for accurate tree construction. Convergent molecular evolution can produce analogous sequences in unrelated lineages; recognizing homoplasy prevents erroneous grouping.
Speciation mechanisms reflect the interplay of molecular divergence and reproductive isolation. Allopatric speciation initiates when a geographic barrier physically separates a population, halting gene flow. In isolated subpopulations, independent mutations accumulate, and differing selective regimes drive divergent evolution of prezygotic barriers (e.g., temporal or behavioral isolation) or postzygotic barriers (e.g., hybrid inviability or sterility). Sympatric speciation, though rarer, proceeds without geographic separation; in plants, whole-genome duplication (polyploidy) instantly creates a reproductively isolated lineage, as seen in the speciation of wheat and other domesticated crops. In animals, disruptive selection on ecological traits, coupled with assortative mating, can generate reproductive isolation within a single locale.
Evidence for evolution spans multiple independent lines. The fossil record chronicles transitional forms—Tiktaalik exhibits both fish-like scales and tetrapod-like limb bones, bridging aquatic and terrestrial vertebrates. Comparative embryology reveals conserved developmental programs, such as pharyngeal arches in vertebrate embryos that become gills in fish and ear bones in mammals. At the molecular level, shared pseudogenes, endogenous retroviral insertions, and universal codon usage across all domains of life constitute compelling signatures of common descent. These molecular fossils, inherited from ancestors, persist because they are selectively neutral and thus accumulate stably over evolutionary time.
The evolutionary arms race exemplifies coevolutionary escalation. Predator-prey, host-parasite, and competitive interactions generate reciprocal selective pressure. At the molecular level, this manifests as rapid evolution of offensive and defensive genes. Venomous organisms diversify toxin gene families through duplication and neofunctionalization, while prey co-evolve resistant ion channels and receptor variants. Pathogens evolve antigenic surface proteins to evade host immune recognition, driving continuous diversification of MHC loci in host populations. This molecular tit-for-tat produces sustained directional selection on both interactors, escalating trait extremity without fixed endpoint.
Variation within populations is maintained by several mechanisms. Mutation constantly introduces novel alleles at low frequencies. Balancing selection, including heterozygote advantage and frequency-dependent selection, actively preserves polymorphism. Gene flow from neighboring populations can reintroduce alleles lost through drift. Finally, environmental heterogeneity—varying selection pressures across microhabitats—maintains different adaptive peaks within a single population, preventing fixation of any single allele. Understanding how these forces interact to shape allele frequency distributions is central to population genetics and the study of evolution.
AP Exam Trap (FRQ)
Interactive Glossary
| Term | Definition |
|---|---|
| ------ | ------------ |
| Natural Selection | The differential survival and reproduction of individuals due to differences in heritable phenotypic traits. This mechanism increases the frequency of advantageous alleles in a population over successive generations. |
| Allele Frequency | The proportion of a specific allele relative to all alleles at a given locus within a population. Changes in this measure across generations indicate that evolutionary processes are acting on the population. |
| Hardy-Weinberg Equilibrium | A null model stating that allele and genotype frequencies remain constant across generations in the absence of evolutionary forces. The equations p + q = 1 and p² + 2pq + q² = 1 describe the expected genotype distribution mathematically. |
| Speciation | The evolutionary process by which one species splits into two or more reproductively isolated lineages. It typically requires the accumulation of prezygotic or postzygotic barriers that prevent successful interbreeding. |
| Allopatric Speciation | The formation of new species following geographic separation of a population by a physical barrier. Isolated subpopulations evolve independently through genetic drift and divergent natural selection. |
| Sympatric Speciation | The emergence of a new species within the same geographic area as the parent population. In plants, this commonly occurs through polyploidy, while in animals it may involve ecological niche specialization and assortative mating. |
| Phylogenetic Tree | A branching diagram that depicts hypothesized evolutionary relationships among taxa based on shared derived characters. Nodes represent common ancestors, and branch tips represent extant or extinct species. |
| Homologous Structures | Anatomical features in different species that share a common ancestral origin despite potentially serving different functions. The forelimbs of mammals, birds, and reptiles exemplify this pattern of shared structural blueprint. |
| Analogous Structures | Traits in different species that perform similar functions but evolved independently through convergent evolution. These similarities arise from comparable selective pressures rather than shared ancestry. |
| Adaptive Radiation | The rapid diversification of a single ancestral species into multiple ecologically distinct forms. This typically occurs when organisms colonize new environments with numerous unoccupied ecological niches. |
| Genetic Drift | A random change in allele frequencies due to chance events, with effects most pronounced in small populations. The bottleneck and founder effects are two important special cases of this mechanism. |
| Gene Flow | The movement of alleles between populations through the migration of individuals or gametes. This process tends to reduce genetic divergence between populations and can counteract the effects of local adaptation. |
| Directional Selection | A mode of natural selection that favors individuals at one phenotypic extreme, shifting the population mean over generations. Antibiotic resistance in bacteria commonly emerges through this selective pattern. |
| Stabilizing Selection | A mode of natural selection that favors intermediate phenotypes and selects against both extreme variants. This reduces phenotypic variation and maintains the population mean near its current value. |
| Disruptive Selection | A mode of natural selection that simultaneously favors both phenotypic extremes over intermediate forms. This process can lead to the maintenance of polymorphism or potentially drive sympatric speciation. |
| Convergent Evolution | The independent evolution of similar traits in distantly related lineages occupying similar ecological roles. Sharks and dolphins share streamlined body shapes despite their distant evolutionary relationship. |
| Coevolution | Reciprocal evolutionary changes in two or more interacting species, such as predators and prey or hosts and parasites. Each species exerts selective pressure on the other, driving continuous adaptation. |
| Vestigial Structures | Anatomical features that have lost most or all of their original function through evolution but persist in reduced form. Whale pelvic bones and human tailbones serve as classic examples of these evolutionary remnants. |
| Bottleneck Effect | A sharp reduction in population size due to environmental catastrophe, leading to loss of genetic diversity. The surviving population carries only a subset of the original population's alleles, potentially altering allele frequencies dramatically. |
| Founder Effect | A form of genetic drift occurring when a small number of individuals establish a new population. The new colony's gene pool reflects only the alleles carried by the founders, which may differ substantially from the source population. |
| Fitness | The relative reproductive success of an organism compared to others in the same population. It is quantified by the number of viable, fertile offspring an individual contributes to the next generation. |
| Prezygotic Barrier | A reproductive isolating mechanism that prevents fertilization from occurring between members of different species. Examples include temporal, habitat, behavioral, mechanical, and gametic isolation mechanisms. |
| Postzygotic Barrier | A reproductive isolating mechanism that reduces the viability or fertility of hybrid offspring after fertilization has occurred. Hybrid inviability, hybrid sterility, and hybrid breakdown represent three categories of this barrier. |
| Molecular Clock | A method for estimating the time since two species diverged from a common ancestor using mutation accumulation rates. This technique assumes that neutral mutations accumulate at a relatively constant rate over evolutionary time. |
| Balanced Polymorphism | The stable maintenance of two or more alleles at a locus in a population due to selective advantages. Heterozygote advantage and frequency-dependent selection are common mechanisms preserving this genetic diversity. |
Quantitative Skill-Set
Mastering population genetics calculations is essential for the AP Biology exam. The Hardy-Weinberg equilibrium provides the foundational quantitative framework:
Core Equations:
Given p or q, calculate all frequencies:
If the frequency of the dominant allele (A) is p = 0.7, then q = 1 − 0.7 = 0.3. Expected genotype frequencies are: AA = (0.7)² = 0.49, Aa = 2(0.7)(0.3) = 0.42, and aa = (0.3)² = 0.09. These sum to 1.0, confirming equilibrium.
Working backward from phenotype data:
If you observe that 16% of a population displays a recessive phenotype (aa), then q² = 0.16, so q = 0.4, and p = 1 − 0.4 = 0.6. The heterozygote frequency is 2pq = 2(0.6)(0.4) = 0.48, meaning 48% of individuals carry one copy of each allele.
Detecting evolution:
Compare observed genotype counts to Hardy-Weinberg expected values using chi-square analysis. A significant chi-square value (exceeding the critical value at p = 0.05) rejects the null hypothesis of equilibrium, indicating that evolutionary forces—selection, drift, gene flow, non-random mating, or mutation—are altering allele frequencies.
Allele frequency change across generations:
Natural selection changes allele frequencies predictably. If genotype AA has a fitness (w) of 1.0, Aa has 0.8, and aa has 0.4, the mean fitness of the population is calculated as w̄ = p²(wAA) + 2pq(wAa) + q²(wₐa). The new frequency of allele A after one generation of selection is p' = (p² × wAA + pq × wAa) / w̄. This quantifies the rate at which advantageous alleles increase in frequency.
Study Moves
Exam Linkage
AP Biology free-response questions deploy specific task verbs that signal the depth of response required. "Explain" demands a mechanistic chain: describe the cause, the molecular or cellular process, and the resulting effect. For natural selection questions, this means linking a specific environmental pressure → differential survival based on a named phenotypic trait → change in allele frequency across generations. "Justify" requires supporting a claim with quantitative evidence or specific data from the prompt; cite Hardy-Weinberg calculations or phylogenetic tree topology. "Predict" asks you to extrapolate outcomes given new conditions—state what will happen and why, using evolutionary logic. "Describe" requires detailing characteristics without explaining causation. "Calculate" demands showing all work with units; Hardy-Weinberg problems must display each step: determining q² from recessive phenotype frequency, taking the square root, computing p, then deriving all genotype frequencies.
Graders reward mechanistic precision: earn points by naming the specific selective pressure, identifying the heritable trait under selection, explaining differential reproductive success, and demonstrating how allele frequencies shift. Avoid teleological language—organisms do not "want" or "try" to evolve; mutations do not "arise because" they are needed. State explicitly that variation is random and selection is non-random. On phylogeny questions, distinguish homologous from analogous traits and explain that shared derived characters define clades. Quantitative responses must show formula substitution and final answers; estimated or unstated calculations lose credit. Master these verb-specific strategies, and pair each claim with precise biological reasoning to maximize FRQ scores.