Natural Selection
The most heavily tested unit in AP Biology. Unit 7 covers Darwin's theory of natural selection, the molecular and population-level mechanisms of evolution, Hardy-Weinberg equilibrium calculations, phylogenetics, and speciation. Evolution is the unifying theme of all biology — concepts from every prior unit connect here.
Introduction to Natural Selection
Evolution is a change in the genetic composition of a population over time. Natural selection is the primary mechanism of evolution: individuals with heritable traits better suited to their environment tend to survive longer and leave more offspring, passing those traits to the next generation. Over many generations, advantageous traits become more common in the population.
Darwin's Four Postulates of Natural Selection
Individuals within a population differ in their phenotypic traits. Some of this variation is heritable (encoded in DNA). Without variation, natural selection cannot act — all individuals would be equally fit.
Some of the phenotypic variation is heritable — it can be passed from parents to offspring through genes. Non-heritable variation (e.g., scars from injuries, environmentally induced changes) cannot be selected for and does not drive evolution.
Populations produce more offspring than the environment can support (Malthusian growth). Resources are limited → competition for food, mates, and space. Not all offspring survive to reproduce.
Individuals with traits better suited to current environmental conditions survive longer and reproduce more (have higher fitness). Fitness is measured by reproductive success — the number of viable, fertile offspring produced relative to others in the population.
NEVER write: "The organism evolved to survive" or "The giraffe stretched its neck to reach leaves, and this trait was passed on." This is Lamarckism — the incorrect idea that acquired traits are inherited.
ALWAYS write: "Individuals with longer necks had higher fitness in this environment and reproduced more, so the frequency of the long-neck allele increased in the population over generations."
Key distinctions: individuals are selected (they survive or die), but populations evolve (allele frequencies change). Organisms do not "want" or "try" to evolve. Natural selection acts on existing variation — it does not create it.
Natural Selection — Types and Mechanisms
Natural selection acts on phenotypic variation in a population. Because phenotype is largely determined by genotype, selection on phenotype changes allele frequencies over generations. The direction and intensity of selection depend on the environment.
Three Modes of Natural Selection
| Mode | What Is Favored | Effect on Distribution | Example |
|---|---|---|---|
| Directional Selection | One extreme phenotype; the other extreme is selected against | Bell curve shifts toward the favored extreme; mean phenotype changes | Antibiotic resistance: bacteria with resistance survive; mean MIC (minimum inhibitory concentration) increases. Beak size in Darwin's finches during drought (larger beaks favored). |
| Stabilizing Selection | Intermediate phenotype; both extremes selected against | Bell curve narrows; variance decreases; mean stays the same | Human birth weight (very small and very large babies have higher mortality — intermediate weight is optimal). Clutch size in birds. |
| Disruptive Selection | Both extreme phenotypes; intermediate selected against | Bell curve splits into two peaks; variance increases; can lead to speciation | Black-bellied seedcracker finches: large and small beak sizes survive better than medium (two distinct food sources). African cichlids. |
Sexual Selection
A form of natural selection where traits evolve because they increase an individual's ability to acquire mates rather than to survive. Two mechanisms:
- Intrasexual selection (male-male competition): Competition between members of the same sex for access to mates. Selection for size, strength, antlers, fighting ability. Winner gets to mate.
- Intersexual selection (female choice): Members of one sex (usually female) preferentially choose mates based on specific traits. Creates "runaway" selection for elaborate displays (peacock tail, bird-of-paradise plumage). The trait may decrease survival but increase reproductive success — net effect is positive if reproductive benefit > survival cost.
Fitness at the Molecular Level
Variation in molecules within cells underlies phenotypic variation. A mutation that changes a protein's amino acid sequence may alter enzyme activity, receptor binding, structural integrity, or regulatory function — changing an organism's phenotype and therefore its fitness. Populations with greater molecular diversity (more allelic variants of key genes) have a broader "toolkit" for responding to environmental change.
Fitness is ALWAYS relative to an environment. A trait that increases fitness in one environment may be neutral or deleterious in another. Sickle cell heterozygotes (HbS/HbA) have higher fitness in malaria-endemic regions (some protection against malaria) than in non-endemic regions (where the sickle allele has only costs).
Evolution requires heritable variation. If all individuals are identical (no genetic variation), or if all variation is non-heritable (environmental), natural selection cannot change allele frequencies. Mutations and sexual recombination are the primary sources of heritable variation.
A population of beetles shows a range of shell color from light brown to dark brown. After several generations in a forest with dark bark, the population shifts toward darker coloration, with light brown beetles nearly disappearing. Which mode of natural selection best describes this pattern?
- (A) Directional selection, because one phenotypic extreme (dark color) is favored and the other (light color) is selected against.
- (B) Stabilizing selection, because an intermediate phenotype is favored.
- (C) Disruptive selection, because both extremes are favored over the intermediate.
- (D) Sexual selection, because darker males are preferred by females for mating.
Artificial Selection
Artificial selection is the intentional selection of specific heritable traits in organisms by humans for breeding purposes. It follows the same mechanism as natural selection — differential reproduction based on phenotype — but the "selector" is human preference rather than the natural environment. Darwin used artificial selection as direct evidence that selection could dramatically change a population.
How Artificial Selection Works
Humans identify individuals with desired traits and allow only those individuals to reproduce. Over generations, the frequency of alleles producing those traits increases dramatically in the population. The same genetic principles apply as in natural selection — heritability, variation, and differential reproduction.
All domestic dog breeds (Canis lupus familiaris) descended from a common wolf ancestor over ~15,000 years of artificial selection. Humans selected for temperament, size, coat type, herding ability, etc. — producing >400 distinct breeds with vastly different phenotypes from identical wolf DNA.
Teosinte (wild corn ancestor) was transformed into modern Zea mays over ~9,000 years by selecting for larger kernels, softer cob, reduced branching. Modern wheat, rice, soybeans — all dramatically different from their wild ancestors due to millennia of artificial selection.
Cattle selected for milk yield (dairy breeds like Holstein) vs. muscle mass (beef breeds like Angus). Chickens selected for egg production vs. meat production. Selection pressures imposed by humans can drive rapid change over just tens of generations.
Drosophila melanogaster (fruit flies) and Brassica rapa (Wisconsin Fast Plants) are common model organisms for artificial selection experiments — rapid generations and visible traits allow selection responses to be observed within weeks. See also INV 1 (Artificial Selection lab).
Artificial selection = evidence that selection CAN work. Darwin used domestic animals and crops as his most accessible examples that selection could transform populations. AP FRQs may ask you to explain how artificial selection supports the mechanism of natural selection, or to predict the outcome of a selection experiment.
Artificial selection can produce RAPID change. Because humans can apply intense directional selection every generation, artificial selection can transform a population far faster than natural selection typically operates. This demonstrates that evolution via selection is mechanistically feasible on biologically short timescales.
Population Genetics — Mechanisms of Evolution
Evolution is a change in allele frequencies in a population over time. Multiple mechanisms can alter population genetic structure — not just natural selection. Understanding all mechanisms is essential for AP Biology.
Mechanisms That Affect Population Genetic Structure
| Mechanism | Description | Directional? | Key Features |
|---|---|---|---|
| Natural Selection | Differential survival and reproduction based on heritable phenotypic variation in a given environment | Yes — toward higher fitness phenotypes | The only adaptive mechanism; increases frequency of beneficial alleles; requires heritable variation and environmental "filter" |
| Mutation | Random changes in DNA sequence; the ultimate source of all new genetic variation | No — random direction | Mutation rates are very low per gene per generation; provides raw material for all other mechanisms to act on; can be beneficial, neutral, or deleterious |
| Genetic Drift | Random changes in allele frequencies due to chance sampling in small populations | No — random direction | More powerful in small populations; can fix or eliminate alleles by chance regardless of fitness; reduces genetic variation within populations |
| Gene Flow | Movement of individuals (or gametes) between populations, transferring alleles | Depends on direction of movement | Tends to homogenize allele frequencies between populations; prevents genetic divergence; opposes speciation |
| Non-random Mating | Individuals choose mates based on phenotype (sexual selection, assortative mating, inbreeding) | Indirect / variable | Primarily changes genotype frequencies, not allele frequencies directly. Inbreeding increases homozygosity without changing allele frequencies. Assortative mating can eventually affect allele frequencies indirectly. Listed here because it can contribute to evolutionary change, but it differs from the other four mechanisms which directly alter allele frequencies. |
Genetic Drift — Types and Consequences
In any finite population, random sampling of gametes means offspring do not perfectly represent parental allele frequencies. In small populations, chance fluctuations can dramatically change allele frequencies over a few generations — or even eliminate an allele entirely (fixation of the alternative allele). Large populations are buffered against drift.
A sudden, drastic reduction in population size (due to disaster, disease, hunting) reduces the gene pool. The surviving population is a random subset — not necessarily representative of the original. Results in: (1) loss of genetic variation, (2) random change in allele frequencies, (3) increased homozygosity, (4) inbreeding depression. Example: Cheetahs (genetic bottleneck ~10,000 years ago) are nearly identical genetically — vulnerable to disease.
A small group of individuals ("founders") colonizes a new area, isolated from the original population. The founders carry only a fraction of the original gene pool. Allele frequencies in the new population may differ dramatically from the source population due to random sampling. Example: Old Order Amish communities (high frequency of rare genetic disorders due to founder effect from small 18th-century founder groups).
Movement of individuals between populations transfers alleles. Immigration adds new alleles; emigration removes alleles. Gene flow reduces genetic differences between populations — it is a homogenizing force that counteracts divergence. Without gene flow, isolated populations will diverge (a prerequisite for speciation). Gene flow between mainland and island populations can prevent island endemics from evolving.
A volcanic eruption on an island kills 95% of a lizard population. The surviving 5% reproduce and the population recovers. Compared to the original population, the recovered population is most likely to show
- (A) increased genetic diversity due to natural selection for volcano-resistant traits
- (B) reduced genetic diversity and different allele frequencies due to the bottleneck effect
- (C) the same allele frequencies as before because the survivors were the most fit individuals
- (D) increased mutation rates as a response to the population decline
Hardy-Weinberg Equilibrium
The Hardy-Weinberg Principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary forces — the population is in Hardy-Weinberg Equilibrium (HWE). This is a null model: it describes what would happen if evolution were NOT occurring. In reality, HWE conditions are never perfectly met, making it a useful baseline for detecting evolution.
The Five Conditions for Hardy-Weinberg Equilibrium
- 1. Large population size — eliminates genetic drift (random sampling errors)
- 2. No migration — no gene flow in or out of the population
- 3. No new mutations — no new alleles introduced
- 4. Random mating — all genotype combinations equally likely to mate
- 5. No natural selection — all genotypes have equal fitness
If any of the first four conditions (large population, no migration, no mutation, no selection) are violated, allele frequencies will change — evolution is occurring. Violations of random mating primarily shift genotype frequencies and can contribute to evolutionary divergence indirectly. HWE is the null hypothesis for evolution: if a population deviates significantly from HWE predictions, at least one evolutionary force is acting.
The Hardy-Weinberg Equations
Genotype frequencies: p² + 2pq + q² = 1
p = frequency of dominant allele (A) | q = frequency of recessive allele (a)
p² = frequency of homozygous dominant (AA) | 2pq = frequency of heterozygotes (Aa) | q² = frequency of homozygous recessive (aa)
Hardy-Weinberg Calculation — Step-by-Step Strategy
In a population of 500 rabbits, 45 are albino (homozygous recessive, aa). Calculate allele and genotype frequencies, and determine the expected number of each genotype.
Step 1: Find q² (frequency of homozygous recessive, the only genotype you can directly observe from phenotype): q² = 45/500 = 0.09
Step 2: Find q: q = √0.09 = 0.30
Step 3: Find p: p = 1 − q = 1 − 0.30 = 0.70
Step 4: Calculate genotype frequencies: p² = (0.70)² = 0.49 (AA); 2pq = 2(0.70)(0.30) = 0.42 (Aa); q² = 0.09 (aa)
Step 5: Expected numbers: AA = 0.49 × 500 = 245; Aa = 0.42 × 500 = 210; aa = 0.09 × 500 = 45. Check: 245 + 210 + 45 = 500 ✓
Key insight: The MOST important step is calculating q from q² — you need the phenotype data for the recessive homozygote because only that genotype can be unambiguously identified by phenotype.
In a population of 1,000 individuals, the frequency of the recessive allele (q) for a trait is 0.4. Assuming Hardy-Weinberg equilibrium, how many individuals in the population are expected to be heterozygous carriers (Aa)?
- (A) 160 individuals
- (B) 320 individuals
- (C) 480 individuals
- (D) 400 individuals
Evidence of Evolution
Evolution is supported by evidence from multiple independent scientific disciplines. This convergence of evidence from fields that have no reason to conspire together is the hallmark of a robust scientific theory.
Lines of Evidence for Evolution
Fossils document the history of life — showing gradual change in organisms over time, transitions between major groups (fish → tetrapods, dinosaurs → birds), and the sequence in which major taxa appeared. Dating methods: (1) stratigraphic position (depth/age of rock layer), (2) radiometric dating (carbon-14 for recent; uranium-lead for ancient), (3) index fossils of known age. The fossil record is incomplete but strongly supports evolution.
Homologous structures: Same underlying anatomy, different function — inherited from a common ancestor modified by evolution. Human arm, bat wing, whale flipper, dog leg all share the same bone arrangement (humerus, radius, ulna, carpals). Vestigial structures: Reduced, functionless remnants of structures that were functional in ancestors — whale pelvic bones, human appendix, snake leg bones. These only make sense in an evolutionary context.
DNA sequence similarity correlates with evolutionary relatedness. The more similar two species' genomes, the more recently they shared a common ancestor. Humans and chimpanzees share ~98.7% DNA identity. Cytochrome c (an electron transport protein) is nearly identical across all aerobic organisms. The universal genetic code (same codons → same amino acids in all life) is strong evidence for universal common ancestry.
Species distribution patterns reflect evolutionary history and continental drift. Marsupials are concentrated in Australia (isolated continent) because they evolved there after separation from other continents. Similar environments on different continents have ecologically similar but genetically distinct species (convergent evolution). Island species most closely resemble nearest mainland species — colonization then divergence.
Evolution has been directly observed: antibiotic resistance in bacteria (Staphylococcus aureus → MRSA); pesticide resistance in insects; industrial melanism in peppered moths (Biston betularia); beak size changes in Darwin's finches (Grant studies in the Galápagos); E. coli evolving novel citrate metabolism in the Lenski long-term evolution experiment (begun 1988, >75,000 generations — a landmark example of experimental evolution and direct observation of a new metabolic trait arising). Evolution is not "just a theory" — it is an observable phenomenon.
Embryos of distantly related vertebrates (fish, birds, humans) are strikingly similar in early stages — all have pharyngeal slits, tails, and similar body plans that only diverge later. This similarity reflects shared developmental genes (Hox genes, other transcription factors) inherited from a common ancestor.
Describe THREE distinct lines of evidence from different scientific disciplines that support the theory of evolution by natural selection. For each line of evidence, explain what it shows and how it supports evolution.
Evidence 2 — Molecular/Genetic Evidence (Biochemistry): DNA sequence comparisons across species show patterns of similarity that match morphological and fossil-based evolutionary trees. Closely related species (e.g., humans and chimpanzees, ~98.7% DNA identity) have more similar sequences than distantly related species. Shared non-coding sequences, endogenous retroviruses, and pseudogenes at identical genomic locations in related species can only be explained by common ancestry — they cannot have arisen independently at the same position.
Evidence 3 — Anatomical Homologies (Comparative Anatomy): Homologous structures — different organisms sharing the same basic anatomical structure — indicate common ancestry. The pentadactyl limb (five-fingered forelimb) is found in humans, bats, whales, and horses, all with the same set of bones (humerus, radius, ulna, carpals, metacarpals, phalanges) despite very different functions. This shared structure makes no engineering sense unless inherited and modified from a common ancestor. Vestigial structures (whale pelvic bones, human coccyx) further indicate ancestral traits that have lost their original function.
Common Ancestry
All life on Earth shares a common ancestor — this is one of the most well-supported conclusions in all of science. Evidence for universal common ancestry comes from the observation that all living organisms share certain fundamental molecular and cellular features that could only have arisen once and been inherited by all subsequent life.
Evidence for Common Ancestry of All Eukaryotes
- Membrane-bound organelles: All eukaryotes have a nucleus, mitochondria, ER, and Golgi — the same basic organelle set, inherited from the first eukaryotic ancestor (~1.8 billion years ago). Mitochondria in all eukaryotes descended from the same α-proteobacterial endosymbiont event (Unit 2.10).
- Linear chromosomes: All eukaryotes have multiple linear chromosomes packaged with histone proteins — a synapomorphy (shared derived feature) of eukaryotes, absent in prokaryotes.
- Genes with introns: Introns and RNA splicing are characteristic features of many eukaryotic genes — this is a synapomorphy of eukaryotes largely absent in prokaryotes. The spliceosome machinery is conserved across eukaryotes, indicating a single evolutionary origin. (Note: not every eukaryotic gene contains introns, but the presence of spliceosomal introns is a defining eukaryotic feature.)
Deeper Common Ancestry — All of Life
All three domains of life (Bacteria, Archaea, Eukarya) share: (1) the same genetic code, (2) DNA as the genetic material, (3) the same amino acids in proteins, (4) ATP as the universal energy currency, (5) ribosomes as the site of translation, (6) plasma membranes. The probability of all these features arising independently is essentially zero — they must have been inherited from a single universal common ancestor (LUCA — Last Universal Common Ancestor) approximately 3.5–4 billion years ago.
Continuing Evolution
Evolution is not a historical process that ended — it is ongoing. All species continue to evolve in response to changing environments, biological interactions, and random processes. Several contemporary examples demonstrate evolution in real time.
Evidence That Evolution Is Still Occurring
| Example | Mechanism | Observable Change |
|---|---|---|
| Antibiotic resistance | Directional natural selection for resistance alleles; horizontal gene transfer in bacteria (conjugation) | MRSA (methicillin-resistant Staphylococcus aureus), multi-drug resistant tuberculosis, resistant gonorrhea — all evolved within decades of antibiotic use |
| Pesticide/herbicide resistance | Directional selection for resistance mutations in agricultural pests | DDT-resistant mosquitoes, glyphosate-resistant weeds — evolved within years; insecticide resistance now documented in hundreds of pest species |
| Chemotherapy resistance in cancer | Somatic evolution within a tumor — cancer cells with resistance mutations survive drug treatment and repopulate the tumor | Initial response followed by relapse and resistance is a predictable evolutionary outcome in cancer treatment |
| Emerging infectious diseases | Pathogen evolution (mutation + selection) allows crossing species barriers; antigenic variation allows escape from immune memory | SARS-CoV-2 evolution (Alpha, Delta, Omicron variants); influenza strain evolution requiring annual vaccine updates; HIV drug resistance |
| Genomic changes over time | Accumulation of neutral mutations provides a "molecular clock" for dating divergence events | Genome sequencing across populations and species documents the ongoing accumulation of genetic change |
The correct evolutionary explanation for antibiotic resistance: Before antibiotics were used, a population of bacteria contained rare individuals with random mutations conferring resistance (pre-existing variation). When antibiotics were introduced, susceptible bacteria were killed (selection pressure). Resistant individuals survived and reproduced, passing resistance alleles to offspring. Over generations, resistant bacteria became dominant. The antibiotic did NOT cause the mutation — it selected for pre-existing mutations.
Incorrect (Lamarckian) explanation to avoid: "The bacteria adapted to the antibiotic by developing resistance." This implies directed mutation — which does not happen.
Phylogeny — Evolutionary Trees and Cladograms
A phylogenetic tree (or cladogram) is a branching diagram that represents the hypothesized evolutionary relationships among species or groups. The tree depicts common ancestors at branch points (nodes) and shows how lineages have diverged over time. Phylogenetic trees are hypotheses — they are constantly revised as new evidence (especially molecular data) accumulates.
Key Components of a Phylogenetic Tree
- Node (branch point): Represents the most recent common ancestor of all taxa branching from that point. The node does NOT represent a specific named species — it is an ancestral population.
- Taxon (tip/leaf): The terminal ends of branches — represent living or extinct species/groups being studied.
- Branch length: In phylogenetic trees (not cladograms), branch length is proportional to evolutionary change (number of mutations or time elapsed). In cladograms, branch lengths are not meaningful.
- Outgroup: A taxon (species/group) that is less closely related to all other taxa in the analysis. Used to root the tree and determine which characters are ancestral vs. derived.
- Sister taxa: Two groups that share an immediate common ancestor — they are each other's closest relatives on the tree.
Cladograms vs. Phylogenetic Trees
| Feature | Cladogram | Phylogenetic Tree |
|---|---|---|
| Branch lengths | Not meaningful — all branches same visual length | Proportional to amount of evolutionary change or time |
| Time scale | Does NOT show time or rate of change | Can be calibrated by molecular clock or fossil dates |
| What they share | Both show branching relationships (topology), identify nodes as common ancestors, use same evidence (molecular, morphological, fossil) | |
| Reading most recent common ancestor | Trace back from two taxa to the first (most recent) shared node — that node = their most recent common ancestor (MRCA) | |
Building and Reading Cladograms
Shared derived characters (synapomorphies) are traits present in the common ancestor of a clade and inherited by all descendants — these define clades. Ancestral characters (plesiomorphies) are traits present in the outgroup and earlier ancestors. To build a cladogram: identify the shared derived characters, group taxa that share each character, nest the groups from most inclusive to least.
Rule 1 — Closer = more related: The fewer nodes you must cross to connect two taxa, the more closely related they are (more recent common ancestor).
Rule 2 — Do NOT read left-to-right order as relatedness: Rotating branches around a node does not change evolutionary relationships. Species at the tip furthest right is NOT "most evolved" — the tree has no direction of advancement.
Rule 3 — Node = MRCA: To find the most recent common ancestor of any two taxa, trace both lineages back to the first node where they connect. That node is their MRCA. All taxa "below" that node (in that clade) share that ancestor.
A cladogram shows the following relationships: Lamprey (outgroup), then a node connecting all vertebrates, then a node separating jawed vs. jawless fish, then within jawed vertebrates a node separating bony fish from tetrapods, then within tetrapods a node separating amphibians from amniotes (reptiles, birds, mammals). Based on this cladogram, which pair of organisms is MOST closely related?
- (A) Lamprey and shark
- (B) Shark and frog
- (C) Lizard and bird
- (D) Frog and shark
Speciation
A new species forms when two populations become reproductively isolated — unable to interbreed and produce viable, fertile offspring. The Biological Species Concept defines a species as a group of organisms that can interbreed and produce viable, fertile offspring under natural conditions, and that are reproductively isolated from other such groups.
How Speciation Occurs
Geographic barrier physically separates a population. Separated subpopulations evolve independently under different selection pressures and through genetic drift. Over time, they accumulate enough genetic differences that they can no longer interbreed → become separate species. Most common mode of speciation. Example: Grand Canyon (separated squirrel populations → Kaibab vs. Abert's squirrels).
Speciation within a single geographic area without physical separation. Can occur through: (1) polyploidy (chromosome duplication creates immediate reproductive isolation — common in plants), (2) habitat differentiation (different populations exploit different resources in the same area → assortative mating), (3) sexual selection (strong mate preferences drive divergence). Example: Apple maggot fly (Rhagoletis) — shifted from hawthorne to apple trees, diverged without geographic separation.
Evolution occurs slowly and continuously over millions of years through the gradual accumulation of small genetic changes. The fossil record would show continuous transitional forms. Darwin's original view. Some clades (e.g., horseshoe crabs) show very slow change over long periods.
Populations remain morphologically stable (stasis) for long periods, then undergo rapid evolutionary change in short bursts — often associated with environmental disruption, colonization of new habitats, or mass extinction events. The fossil record often shows sudden appearances of new forms. Gould and Eldredge (1972). Explains "gaps" in the fossil record.
Reproductive Isolating Mechanisms
Reproductive isolation mechanisms prevent gene flow between incipient species. They are divided into pre-zygotic (prevent fertilization) and post-zygotic (prevent viable offspring even if fertilization occurs).
| Category | Mechanism | Description | Example |
|---|---|---|---|
| Pre-zygotic (before fertilization) | Habitat isolation | Species use different microhabitats in same area | Two oak species in same forest — one lives in wet soil, one in dry soil |
| Temporal isolation | Species breed at different times (season, time of day) | Skunk cabbage (spring) vs. related species (summer); frogs with different peak calling seasons | |
| Behavioral isolation | Different courtship signals, calls, or mating displays | Firefly species with different flash patterns; bird species with different songs; cricket calls | |
| Mechanical isolation | Structural incompatibility of reproductive organs or flowers | Sage species with different flower structures that attract different pollinators | |
| Gametic isolation | Gametes incompatible — sperm cannot fertilize eggs of other species | Sea urchin sperm carry species-specific proteins that bind only to same-species eggs | |
| Post-zygotic (after fertilization) | Reduced hybrid viability | Hybrid embryo does not develop properly; dies before reproductive age | Tiger × lion hybrids often die early; some salamander hybrids die as embryos |
| Reduced hybrid fertility | Hybrid survives but is sterile | Mule (horse × donkey) — viable but infertile because chromosomes cannot pair in meiosis | |
| Hybrid breakdown | F1 hybrids are fertile, but F2 or later generations are not viable or fertile | Some Drosophila crosses — F1 is fertile but F2 has reduced fitness |
Evolutionary Patterns
A common ancestral population evolves into multiple, phenotypically distinct species through adaptation to different environments. Produces homologous structures. Example: Darwin's finches on Galápagos — diversified from a common finch ancestor into species with different beak shapes for different food sources.
Unrelated species independently evolve similar traits in response to similar environmental pressures. Produces analogous structures — similar function, different evolutionary origin. Example: Dolphin (mammal), ichthyosaur (reptile), and shark (fish) all evolved streamlined body shapes for aquatic locomotion independently.
Rapid diversification of a single ancestral lineage into multiple species filling available ecological niches — often after mass extinction or colonization of a new environment. Example: Hawaiian Drosophila (>1000 species from one ancestor), Anolis lizards in Caribbean, mammals after dinosaur extinction (K-Pg boundary).
Two species reciprocally evolve in response to each other's evolutionary changes. Often involves tight ecological interactions: predator-prey, host-parasite, mutualist pairs. Example: flowering plants and their pollinators (orchid and bee; yucca moth and yucca plant); prey species developing poison and predator developing resistance.
Two closely related frog species live in the same geographic area but breed at different times of year — one in early spring and one in midsummer. This is an example of which type of reproductive isolating mechanism?
- (A) Mechanical isolation
- (B) Post-zygotic reduced hybrid viability
- (C) Temporal isolation
- (D) Habitat isolation
Variations in Populations and Genetic Diversity
The genetic diversity within a population directly affects its ability to withstand environmental change and avoid extinction. Genetic diversity is a buffer against extinction — it ensures that some individuals are likely to survive a novel challenge.
Why Genetic Diversity Matters
- Populations with high genetic diversity contain a wide variety of alleles. When the environment changes (new pathogen, climate shift, new predator), some individuals are likely to carry alleles providing resistance or tolerance → those survive and reproduce → population persists.
- Populations with low genetic diversity (from bottlenecks, founder effects, inbreeding) are more vulnerable. If a novel pathogen emerges, nearly all individuals may be susceptible → catastrophic population decline or extinction. Example: Irish potato famine — near-monoculture of a single potato variety → devastating impact when Phytophthora infestans struck.
- Alleles that are deleterious in one environment can be neutral or beneficial in another. Maintaining allelic diversity (including "deleterious" alleles) provides insurance against future environmental changes. Sickle cell allele: deleterious when homozygous (sickle cell disease) but beneficial in heterozygotes in malaria-endemic regions (heterozygote advantage = balanced polymorphism maintains both alleles).
Conservation biology applications: California condors, black-footed ferrets, prairie chickens — all reduced to tiny populations → extreme bottlenecks → severe inbreeding depression and loss of genetic variation. Recovery programs must manage for genetic diversity.
Heterozygote advantage maintains variation: When heterozygotes have higher fitness than either homozygote (overdominance/balancing selection), both alleles are maintained in the population. This is called balanced polymorphism. The most famous example is sickle cell anemia in malaria zones — explains why a disease-causing allele is maintained at high frequency.
Origins of Life on Earth
The origin of life on Earth is supported by geological, chemical, and molecular evidence. While the exact mechanism of the first self-replicating molecule is uncertain, the RNA World Hypothesis is the leading scientific model and is directly testable by AP Biology standards.
Timeline of Early Earth and Life
| Date (bya) | Event | Significance |
|---|---|---|
| ~4.6 bya | Earth forms from accretion of solar nebula | Initial Earth too hot, bombarded by meteorites — no stable liquid water |
| ~4.0–3.9 bya | Late Heavy Bombardment ends; Earth cools; oceans form | First stable liquid water; environment becomes potentially hospitable to life |
| ~3.5 bya | Oldest fossil evidence of life (stromatolites — cyanobacterial mats in Australia) | Life originated between 3.9 and 3.5 bya — a plausible range of ~400 million years |
| ~2.7 bya | Cyanobacteria begin oxygenic photosynthesis | O₂ begins accumulating in atmosphere ("Great Oxygenation Event" ~2.4 bya) |
| ~1.8 bya | First eukaryotes appear | Endosymbiosis of mitochondria; nuclear envelope |
| ~0.6 bya | First multicellular animals (Ediacaran fauna) | Cambrian explosion follows (~0.54 bya) — rapid diversification of animal phyla |
The RNA World Hypothesis
The RNA World Hypothesis proposes that RNA — not DNA or protein — was the first molecule capable of both storing genetic information AND catalyzing chemical reactions (like replication). This would resolve the "chicken-and-egg" problem of the origin of life: proteins (enzymes) are needed to replicate DNA, but DNA is needed to make proteins.
1. RNA was the first genetic material: RNA can serve as an information-carrying molecule (base pairing rules, linear sequence) similar to DNA. RNA nucleotides could have been synthesized abiotically from simpler organic molecules (supported by Miller-Urey experiment and meteorite chemistry).
2. RNA as a hypothetical self-replicating catalyst: RNA is hypothesized to have been capable of both storing information (via base sequence) and catalyzing chemical reactions (ribozyme activity), making an early RNA-based system plausible without requiring protein enzymes. This RNA-based replication is thought to have provided the first genetic continuity — though the exact mechanism remains an active area of research.
3. No protein catalysts initially: Ribozymes (catalytic RNA molecules) could have performed early metabolic reactions. Evidence: the ribosome's catalytic core (peptidyl transferase activity) is RNA, not protein — suggesting the ribosome is a molecular fossil of the RNA world. RNA catalysis has been demonstrated in the lab.
Over time, RNA was replaced by DNA (more stable, more accurate) as the genetic material, and proteins took over most catalytic functions (more chemically versatile than RNA). RNA remains as the intermediary between DNA and protein in all modern cells — perhaps because the transition was incomplete, or because RNA's roles are still optimal for transcription/translation.
The RNA World Hypothesis proposes that RNA was the original genetic material before DNA. Which of the following observations provides the strongest support for this hypothesis?
- (A) RNA is synthesized from DNA during transcription in all modern cells.
- (B) RNA is less chemically stable than DNA, making it a poor candidate for a genetic material.
- (C) The catalytic core of the ribosome is made of rRNA, not protein — suggesting that RNA once performed both information storage and catalytic functions.
- (D) DNA contains thymine instead of uracil, making DNA more stable as a genetic material than RNA.
Mixed Practice Questions
In a population of 2,000 individuals, the frequency of phenylketonuria (PKU), an autosomal recessive disorder, is 1 in 10,000. Assuming Hardy-Weinberg equilibrium: (a) What is the frequency of the recessive allele (q)? (b) What is the frequency of the dominant allele (p)? (c) How many individuals in the population are expected to be heterozygous carriers?
Frequency of homozygous recessive (affected individuals) = q² = 1/10,000 = 0.0001
q = √0.0001 = 0.01
(b) Frequency of dominant allele p:
p = 1 − q = 1 − 0.01 = 0.99
(c) Expected number of heterozygous carriers:
Frequency of carriers = 2pq = 2(0.99)(0.01) = 0.0198 ≈ 2%
Expected number = 0.0198 × 2,000 = 39.6 ≈ 40 individuals
Key insight: Even though only 1 in 10,000 individuals has PKU (q² = 0.0001), approximately 1 in 50 (2%) are carriers. Carriers are vastly more common than affected individuals when q is small — this is why many genetic diseases are more common in heterozygous carriers than in homozygous affected individuals.
A population of bacteria is treated with an antibiotic. Initially, 99.9% of bacteria are killed. However, over several generations, the survivors reproduce and the population recovers — but now almost all bacteria are resistant to the antibiotic. Using the principles of natural selection, explain this observation. Be sure to address: (a) the source of variation, (b) the selection pressure, (c) the change in allele frequencies, and (d) why this does NOT support Lamarckism.
(b) Selection pressure: When the antibiotic was applied, it acted as a powerful directional selection pressure. Bacteria without resistance mutations were killed (low fitness in this environment). Bacteria with resistance mutations survived and reproduced (high fitness). The antibiotic created a severe filter that eliminated susceptible phenotypes.
(c) Change in allele frequencies: Before treatment, the resistance allele was rare in the population (e.g., frequency ~0.001). After treatment, susceptible individuals (carrying sensitive alleles) died. The resistant survivors reproduced, and their offspring inherited the resistance allele. Over multiple generations, the frequency of the resistance allele increased dramatically (approaching 1.0 = fixation) while the sensitive allele became extremely rare or was eliminated — evolution occurred.
(d) Why this does NOT support Lamarckism: Lamarckism would predict that individual bacteria "sensed" the antibiotic, "adapted" by generating targeted resistance mutations, and then passed this acquired resistance to offspring. This did NOT happen. The resistance mutations were random, pre-existing in the population before antibiotic exposure — the antibiotic did not induce or direct them. The antibiotic selected for pre-existing variation; it did not create new, purposefully directed mutations. This is a fundamental distinction: natural selection acts on existing variation; it does not direct mutation.
High-Frequency Errors to Avoid
- 🦒Lamarckian language: "organisms evolved to survive"Individual organisms do NOT evolve. Populations evolve. Organisms do not "try to adapt" or develop traits they "need." Natural selection acts on existing heritable variation — mutations are random, not directed. Always write from the population perspective: "the frequency of resistant alleles increased in the population over generations."
- 🎲Saying genetic drift only affects allele frequencies in large populationsGenetic drift is most powerful in SMALL populations — random sampling errors have proportionally larger effects when sample size is small. In large populations, random errors are averaged out. Drift is the dominant evolutionary force in small, isolated populations (founder effect, bottleneck).
- 📐Using phenotype frequencies instead of genotype frequencies in HWE calculationsThe critical starting point for HWE calculations is q² = frequency of the RECESSIVE PHENOTYPE (homozygous recessive), not the dominant phenotype frequency. "1 in 10,000 are affected" → q² = 0.0001 → q = 0.01. Never start from the dominant phenotype (3/4 in F2 is a Mendelian ratio, not a population frequency).
- 🌿Confusing analogous and homologous structuresHOMOLOGOUS structures = same underlying anatomy, different function, inherited from common ancestor (bird wing and human arm — same bones). ANALOGOUS (convergent) structures = same function, different evolutionary origin, superficially similar (bird wing and butterfly wing — both for flying but completely different anatomy). Homologous = evidence for common ancestry; analogous = evidence for convergent evolution.
- 🦠Saying gene flow promotes speciationGene flow PREVENTS speciation by keeping populations genetically similar, preventing divergence. The interruption of gene flow (via geographic isolation or reproductive barriers) is what allows speciation to occur. Gene flow is a homogenizing force, not a diversifying one.
- 🔬Thinking cladograms show the most "evolved" species at the tipsAll tips of a cladogram represent equally "evolved" species — none is more "advanced" than others. Evolution is not a ladder with humans at the top. The tree shows branching patterns and relatedness, not a progression. Do not read left-to-right tip order as a rank of "advancement."
- ⚖️Saying HWE means a population is NOT evolvingHWE is a NULL MODEL — it describes the expected genotype/allele frequencies if evolution is NOT occurring. If a population DEVIATES from HWE predictions, evolution IS happening. HWE conditions (large population, no selection, no drift, no mutation, no gene flow, random mating) are never fully met in nature — real populations always evolve to some degree.
Unit 7 — Key Takeaways
4 postulates: variation, heritability, overproduction, differential reproduction. Fitness = reproductive success. Directional / stabilizing / disruptive selection. Never write Lamarckian language — populations evolve, individuals are selected.
Human-directed selection of heritable traits. Same mechanism as natural selection. Demonstrates selection can rapidly transform populations. All dog breeds from wolf ancestor.
5 evolutionary forces: selection, mutation, drift, gene flow, non-random mating. Drift: most powerful in small populations. Bottleneck + founder effects reduce genetic variation. Gene flow = homogenizing force.
p + q = 1; p² + 2pq + q² = 1. 5 conditions for equilibrium (null hypothesis). Start with q² from recessive phenotype → q → p → 2pq (carriers). Deviation from HWE = evolution occurring.
Fossils (radiometric dating), molecular/DNA similarity, homologous structures, vestigial organs, biogeography, direct observation (antibiotic resistance). Universal features of life (genetic code, ATP, ribosomes) = common ancestry of all life.
Antibiotic resistance = pre-existing variation selected by antibiotic (NOT induced by antibiotic). Pesticide/chemotherapy resistance, emerging pathogens — all demonstrate ongoing evolution.
Cladograms: nodes = most recent common ancestors; outgroup = least related; shared derived characters define clades. Closer = more recent common ancestor. Cladogram ≠ phylogenetic tree (no time scale). Molecular data preferred over morphology.
Biological species concept. Allopatric (geographic barrier) vs. sympatric (same area). Pre-zygotic (temporal, behavioral, mechanical, gametic, habitat) vs. post-zygotic (reduced viability/fertility, hybrid breakdown) isolation. Divergent vs. convergent vs. adaptive radiation.
High genetic diversity = resilience to environmental change. Low diversity = extinction risk. Balanced polymorphism (sickle cell). RNA World: RNA was first genetic material + catalyst; ribosome rRNA catalytic core = evidence.
Unit 7 = 13–20% of the AP Biology Exam — the single highest-weighted unit, with ~48 MCQs in the progress check. The absolute top topics are: Hardy-Weinberg calculations (start from q², always show work), correct Darwinian language (populations evolve, not individuals; no Lamarckism), pre- vs. post-zygotic isolation mechanisms, cladogram reading and most recent common ancestor identification, evidence for evolution from multiple disciplines, antibiotic resistance as a natural selection example, and bottleneck vs. founder effect distinctions. For FRQs, always address variation, selection pressure, change in allele frequencies, and heritability when explaining any natural selection scenario.