Study guide
This chapter is educational content only and does not guarantee any exam outcome. It follows Units 7 and 8 of the AP Biology framework, covering the evidence for evolution, the mechanisms of natural selection and speciation, and how populations and communities interact within ecosystems. This content leans heavily on quantitative reasoning, so expect Hardy-Weinberg calculations, population growth curves, mark-recapture estimates, and trophic-efficiency problems to appear alongside conceptual questions.
Evidence for Evolution and Mechanisms of Natural Selection
Evolution, descent with modification over generations, is supported by multiple independent lines of evidence: the fossil record shows a chronological progression of forms and transitional fossils; comparative anatomy reveals homologous structures (similar structures from a common ancestor, like the forelimb bones shared by whales, bats, and humans, despite different functions) and vestigial structures (reduced remnants of ancestral features); comparative embryology shows shared developmental patterns among related species; and molecular biology reveals that species sharing a more recent common ancestor tend to have more similar DNA and protein sequences, including the near-universal genetic code itself. Natural selection, the primary mechanism of adaptive evolution, requires heritable variation within a population, differential survival and reproduction based on that variation, and an environment that favors certain variants, producing a population increasingly suited to that environment over generations. Selection can act in different patterns on a continuous trait: directional selection favors one extreme phenotype, shifting the population mean; stabilizing selection favors the intermediate phenotype, reducing variation; and disruptive selection favors both extremes over the intermediate, potentially splitting the population's phenotype distribution. Consider an invented field study: researchers track beak depth in a finch population before and after a multi-year drought that eliminated small, soft seeds and left mainly large, hard seeds; mean beak depth increases measurably from before to after the drought, and the variance around the mean narrows, consistent with directional selection favoring birds able to crack the remaining large seeds. Other mechanisms alongside natural selection also change allele frequencies, including genetic drift (random chance fluctuations, pronounced in small populations, including bottleneck and founder effects), gene flow (movement of alleles between populations via migration), and mutation (the ultimate source of new genetic variation).
Hardy-Weinberg Equilibrium
The Hardy-Weinberg model describes a theoretical population in which allele frequencies remain constant across generations, serving as a null hypothesis against which real evolutionary change is measured. It assumes no mutation, no gene flow, infinite population size (no genetic drift), random mating, and no natural selection; when any assumption is violated, allele or genotype frequencies shift from the model's predictions, indicating evolution is occurring. The model uses two equations: p + q = 1 (where p and q are the frequencies of the two alleles at a locus) and p^2 + 2pq + q^2 = 1 (where p^2 is the frequency of the homozygous dominant genotype, 2pq the heterozygous genotype, and q^2 the homozygous recessive genotype). A typical data problem: in a population of 1000 individuals, an invented recessive condition appears in 90 people. Since q^2 = 90/1000 = 0.09, q = 0.3, so p = 1 - 0.3 = 0.7. The predicted number of heterozygous carriers is 2pq times the population size, or 2 times 0.7 times 0.3 times 1000, which is 420 individuals, and the predicted homozygous dominant count is p^2 times 1000, or 0.49 times 1000, which is 490 individuals; these three genotype counts (490, 420, and 90) should sum to the full population of 1000 as a check. If a follow-up survey years later finds q has shifted to 0.35, that shift itself is evidence that one or more Hardy-Weinberg assumptions no longer holds in that population, and the specific scenario described (a new predator, a habitat split, immigration) usually points to which assumption was violated.
Speciation and Phylogenetics
A species is generally defined, under the biological species concept, as a population whose members can interbreed and produce fertile offspring in nature; reproductive isolation, the inability to do so, is what allows populations to diverge into separate species. Reproductive barriers are classified as prezygotic (preventing mating or fertilization, including habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation) or postzygotic (allowing fertilization but reducing hybrid viability or fertility). Speciation can occur allopatrically, when a physical barrier (a new river, a mountain range, a founder population on an island) separates populations that then diverge independently, or sympatrically, without geographic separation, often through mechanisms like polyploidy in plants or strong disruptive selection paired with behavioral changes. Phylogenetic trees represent hypothesized evolutionary relationships among taxa, built primarily from shared derived characters (traits present in a group and its common ancestor but not in more distant relatives) and increasingly from molecular sequence data; a node in the tree represents a common ancestor, and the branches leading from it represent independently evolving lineages, so two taxa are more closely related if they share a more recent common ancestor node, regardless of how the tree happens to be drawn or rotated on the page. Cladograms built from molecular data (comparing DNA or protein sequences) can sometimes reveal relationships not obvious from anatomy alone, since analogous structures (which arose independently through convergent evolution in similar environments, like the wings of insects and birds) can superficially resemble homologous structures despite reflecting no shared recent ancestry.
Population Growth Models and Community Ecology
Population ecology models growth using two idealized patterns. Exponential growth occurs when resources are unlimited, producing a J-shaped curve as the population's growth rate remains proportional to its current size. Logistic growth incorporates a carrying capacity, K, the maximum population size an environment can sustain given its resources, producing an S-shaped curve where growth rate slows as the population approaches K due to density-dependent factors like competition, disease, and predation; density-independent factors, such as a wildfire or severe storm, reduce population size regardless of density. Mark-recapture is a common field method for estimating population size: researchers capture and mark a sample of individuals, release them, then later capture a second sample and count how many are marked. An invented study captures and tags 80 fish from a lake, releases them, and a week later captures 100 fish, of which 20 are tagged; using the Lincoln-Peterson formula, total population estimate equals (first sample size times second sample size) divided by number recaptured marked, or (80 times 100) divided by 20, giving an estimated population of 400 fish. Community ecology examines interactions like competition, predation, and symbiosis (including mutualism, commensalism, and parasitism), along with keystone species, whose removal disproportionately affects community structure relative to their abundance. Energy flow through ecosystems is inefficient: on average only about 10% of energy at one trophic level transfers to the next (the rest is lost as heat or used in metabolism), so a food chain with 10,000 kilocalories of producers supports roughly 1,000 kilocalories of primary consumers and about 100 kilocalories of secondary consumers, a pattern illustrated by the energy pyramid and explaining why food chains rarely exceed four or five trophic levels and why energy pyramids are always narrower at higher trophic levels, unlike some biomass or number pyramids which can occasionally invert.
Key terms
- Homologous structure
- — A structure shared by different species because they inherited it from a common ancestor, even if it now serves different functions.
- Directional selection
- — A pattern of natural selection favoring one phenotypic extreme, shifting the population's trait distribution over time.
- Genetic drift
- — Random, chance-driven changes in allele frequencies, with disproportionate impact in small populations, including bottleneck and founder effects.
- Hardy-Weinberg equilibrium
- — A null model predicting constant allele and genotype frequencies across generations when mating is random and mutation, migration, drift, and selection are absent.
- Reproductive isolation
- — Any barrier, prezygotic or postzygotic, that prevents two populations from successfully interbreeding and producing fertile offspring.
- Allopatric speciation
- — Speciation that occurs when a geographic barrier separates populations, which then diverge independently until reproductive isolation arises.
- Convergent evolution
- — The independent evolution of similar traits (analogous structures) in unrelated lineages facing similar environmental pressures.
- Carrying capacity (K)
- — The maximum population size an environment can sustain long-term, given available resources, limiting logistic growth.
- Mark-recapture
- — A field method estimating population size by comparing the proportion of marked individuals in a second sample to the number originally marked and released.
- Trophic efficiency
- — The proportion of energy transferred from one trophic level to the next, averaging about 10%, with the remainder lost as heat or used in metabolism.
- Keystone species
- — A species whose impact on community structure and stability is disproportionately large relative to its abundance.
Exam tips
- In Hardy-Weinberg problems, always start from the homozygous recessive frequency (q^2), since it is the one genotype frequency you can usually read directly from a stated phenotype frequency.
- Before concluding a population is or is not evolving, check which specific Hardy-Weinberg assumption the question's scenario violates (small population size suggests drift, a described migration suggests gene flow, and so on).
- For mark-recapture, set up the proportion (marked in second sample / second sample size) equals (total marked released / total population) before solving — this avoids formula-memorization errors under time pressure.
- On phylogenetic tree questions, relatedness is read from shared nodes (branch points), not from the left-to-right order taxa happen to be listed in at the tips.
- For energy pyramid or trophic-efficiency problems, apply the roughly 10% rule level by level rather than trying to jump directly from producers to a distant consumer level in one step.