AP Biology Unit 7: Natural Selection

Free Unit Notes

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I. Introduction to Natural Selection (Topic 7.1)

A. Evolution and inherited variation

1. Evolution is a change in a population’s inherited characteristics across generations, often measured as a change in allele frequencies.

2. An allele is a version of a gene. Its frequency is its proportion of all copies of that gene in a population.

3. Individuals survive and reproduce, but populations evolve. An individual does not acquire needed inherited traits by trying to adapt.

B. Conditions for natural selection

1. Individuals vary, some variation is heritable, and those differences affect survival or reproduction.

2. Organisms often produce more offspring than available resources can support. Competition creates differences in survival and reproductive success.

3. Individuals with favorable inherited traits tend to leave more offspring, making those traits more common over generations.

C. Fitness and environmental change

1. Evolutionary fitness means relative reproductive success: the contribution of inherited alleles to future generations.

2. Biotic factors, such as predators, and abiotic factors, such as temperature, affect which traits improve fitness.

3. Environmental changes can change the direction and rate of selection. A useful trait in one environment may be harmful in another.

II. Natural Selection (Topic 7.2)

A. Phenotypes and adaptations

1. Natural selection acts directly on phenotypes, the observable traits influenced by genotype and environment.

2. Selection changes allele frequencies when differences in reproductive success are linked to heritable variation.

3. An adaptation is an inherited characteristic that improves fitness in a particular environment. Adaptations involve tradeoffs and do not produce perfect organisms.

4. Differences in the types or amounts of cellular molecules can affect fitness. For example, an enzyme that breaks down an antibiotic can help a bacterium survive treatment.

B. Patterns of selection

1. Directional selection favors one extreme phenotype and shifts the trait distribution toward that extreme.

2. Stabilizing selection favors intermediate phenotypes and reduces variation around the average.

3. Disruptive selection favors both extremes over intermediate phenotypes and can increase differences within a population.

4. To interpret a selection graph, compare the frequencies of phenotypes before and after selection. The curves describe populations, not individuals changing traits.

C. Selection and resistance

1. Mutations arise without regard to an organism’s needs. An antibiotic does not produce a needed resistance mutation on purpose.

2. Antibiotics kill susceptible bacteria more effectively than resistant bacteria. Resistant survivors reproduce, so resistance can become more common.

3. Selection is nonrandom with respect to reproductive success, even though the mutations supplying new variation are random with respect to usefulness.

III. Artificial Selection (Topic 7.3)

A. Human choice and inherited traits

1. Artificial selection occurs when humans choose which organisms reproduce based on desired heritable traits.

2. Breeding crops for larger seeds or dogs for particular sizes changes trait and allele frequencies across generations.

3. Selection requires heritable variation. Environmental differences alone do not ensure that selected traits pass to offspring.

B. Effects on diversity

1. Using only a few breeders can reduce genetic diversity and increase inbreeding, which can expose harmful recessive alleles.

2. Artificial selection chooses parents. Genetic engineering directly changes DNA.

IV. Population Genetics (Topic 7.4)

A. Gene pools and mutation

1. A gene pool includes all alleles in a population. Changes in allele frequencies provide evidence of evolution.

2. Mutation changes DNA and creates new alleles. Its effects can be beneficial, harmful, or neutral in a particular environment.

3. Sexual reproduction reshuffles existing alleles; recombination alone does not create new alleles.

B. Genetic drift

1. Genetic drift is a chance change in allele frequencies. Its effects are strongest in small populations and do not depend on an allele improving fitness.

2. An allele can become fixed, reaching frequency 1, or be lost, reaching frequency 0, through chance alone.

3. The bottleneck effect occurs when a sharp reduction in population size leaves survivors with an unrepresentative sample of the original alleles.

4. The founder effect occurs when a small group establishes a population with allele frequencies different from its source population.

5. Drift can reduce diversity and cause isolated populations to diverge. Repeated small-population simulations can produce different outcomes by chance.

C. Gene flow and population change

1. Gene flow moves alleles between populations when migrants or gametes contribute to reproduction.

2. Gene flow generally makes populations more similar and can prevent divergence into separate species when it is strong enough.

3. Natural selection, drift, mutation, and gene flow can act at the same time. A change in allele frequency alone does not identify its cause.

V. Hardy-Weinberg Equilibrium (Topic 7.5)

A. Model and conditions

1. Hardy-Weinberg equilibrium models a population whose allele and genotype frequencies remain constant across generations at a particular locus.

2. Its conditions are a very large population, random mating, no mutation, no gene flow, and no natural selection at that locus.

3. Violating these conditions can change allele frequencies, genotype frequencies, or both. Nonrandom mating alone can change genotype proportions without changing allele frequencies.

B. Allele and genotype equations

1. For two alleles, p + q = 1. Here, p and q are the frequencies of the two alleles.

2. Under equilibrium, p² + 2pq + q² = 1. The expected genotype frequencies are p² for AA, 2pq for Aa, and q² for aa.

3. Multiply an expected genotype frequency by population size to predict the number of individuals with that genotype.

4. For N diploid individuals, p = (2 × number of AA + number of Aa) ÷ (2N). Direct allele counting does not require equilibrium.

C. Applying and evaluating the model

1. If 9% show a fully recessive phenotype under equilibrium, q² = 0.09, q = 0.30, and p = 0.70. The expected heterozygote frequency is 2pq = 0.42, or 42%.

2. A dominant phenotype can include AA and Aa individuals. Its frequency cannot be treated as p² unless the genotype is known.

3. Compare observed genotype counts with expected counts. A mismatch can indicate that the model does not fit, but it does not identify which condition failed.

4. A chi-square test uses χ² = Σ[(observed − expected)² ÷ expected]. Evaluate the result using appropriate degrees of freedom and a significance threshold.

VI. Evidence of Evolution (Topic 7.6)

A. Fossils and geological evidence

1. The fossil record documents organisms from the past and changes in lineages over time. Transitional fossils combine features that help connect ancestral and later groups.

2. Relative dating places fossils in chronological order. In undisturbed sedimentary layers, deeper layers are generally older.

3. Radiometric dating estimates numerical ages from predictable radioactive decay in suitable materials. Fossilization is rare, so the record is incomplete.

B. Structural and geographic evidence

1. Homologous structures share an inherited underlying pattern, even when their functions differ. Human arms and whale flippers share a forelimb bone pattern.

2. Vestigial structures are inherited features with reduced or changed functions compared with their ancestral forms.

3. Analogous structures perform similar functions but evolved independently, as in bird and insect wings. They reflect convergent evolution rather than close ancestry.

4. Biogeography, the geographic distribution of organisms, supports descent with modification when distributions match isolation and geological history.

C. Molecular and cellular evidence

1. Similarities in DNA, RNA, and protein sequences support common ancestry. More closely related lineages generally share more sequence similarities.

2. Shared metabolic pathways, DNA and RNA use, and the nearly universal genetic code support common ancestry across life.

3. Agreement among molecular, structural, geological, and fossil evidence strengthens evolutionary explanations.

VII. Common Ancestry (Topic 7.7)

A. Shared eukaryotic features

1. Eukaryotes share membrane-bound organelles, generally linear nuclear chromosomes, and genes containing introns. These features support descent from common ancestors.

2. Shared ancestry means that living lineages descended from earlier ancestral populations. One living species need not have descended from another living species.

B. Endosymbiotic evidence

1. Endosymbiotic theory explains mitochondria and chloroplasts as descendants of prokaryotes that became permanent partners within ancestral cells.

2. Their circular DNA, bacterial-like ribosomes, division resembling binary fission, and double membranes support this explanation.

VIII. Continuing Evolution (Topic 7.8)

A. Evidence of ongoing change

1. Genomic changes, changes in the fossil record, and observed shifts in allele frequencies show that evolution continues.

2. Resistance can evolve to antibiotics, pesticides, and herbicides. Selection among cancer cells can also produce resistance to chemotherapy drugs.

3. Mutations and selection can produce pathogen variants with different transmission or immune-evasion traits, contributing to emerging diseases.

B. Changing selective pressures

1. Short generation times can allow evolutionary changes to become visible quickly across many generations.

2. A resistance allele may reduce fitness without the drug, but removing the drug does not guarantee that resistance will disappear.

3. To investigate continuing evolution, compare inherited variants or reproductive success across generations while controlling relevant environmental conditions.

IX. Phylogeny (Topic 7.9)

A. Evidence and tree construction

1. Phylogeny is evolutionary history. Phylogenetic trees and cladograms are testable hypotheses about relationships among lineages.

2. Trees can use anatomical traits from living organisms or fossils and similarities in DNA or protein sequences. Molecular data often provide more reliable detail than appearance alone.

3. Shared derived characters are traits inherited from a common ancestor that distinguish a group from earlier lineages. Gains and losses of traits help infer branches.

4. An outgroup is outside the focal group and helps identify ancestral and derived character states.

B. Reading relationships

1. A node represents a common ancestor. Lineages sharing a more recent common ancestor are more closely related.

2. Sister taxa share an immediate common ancestor. A clade contains an ancestor and all its descendants.

3. Rotating branches around a node does not change relationships. The spacing or order of tip labels does not determine relatedness.

4. A cladogram shows branching order without measuring time or amount of change. Interpret tree branch lengths only when a scale specifies their meaning.

5. Fossils or molecular clocks can calibrate trees. A molecular clock estimates divergence from accumulated sequence changes using a calibrated rate.

6. New evidence can revise a tree. Similar traits that evolved independently can mislead relationship estimates based only on appearance.

X. Speciation (Topic 7.10)

A. Species and reproductive isolation

1. Speciation forms new species as populations become reproductively isolated and diverge genetically.

2. The biological species concept defines a species as organisms that can interbreed in nature and produce viable, fertile offspring. It is difficult to apply to fossils and asexual organisms.

3. Reduced gene flow allows mutation, drift, and selection to produce differences between populations.

B. Geographic patterns and barriers

1. Allopatric speciation follows geographic isolation. A physical barrier can reduce gene flow, allowing reproductive isolation to evolve.

2. Sympatric speciation occurs with geographic overlap. In plants, polyploidy, having additional complete chromosome sets, can create reproductive isolation.

3. Prezygotic barriers prevent mating or fertilization through differences in habitat, timing, behavior, reproductive structures, or gamete compatibility.

4. Postzygotic barriers reduce hybrid survival or reproduction after fertilization. Examples include sterile hybrids and reduced fitness in later hybrid generations.

C. Evolutionary patterns and rates

1. Gradualism involves slow evolutionary change over long periods. Punctuated equilibrium involves long periods of little change interrupted by relatively rapid change and speciation.

2. Divergent evolution occurs when related lineages become increasingly different as they adapt to different conditions.

3. Adaptive radiation is relatively rapid diversification from a common ancestor into multiple species using different ecological opportunities.

4. Convergent evolution produces similar adaptations in separate lineages exposed to similar selective pressures.

XI. Variations in Populations (Topic 7.11)

A. Sources and inheritance of variation

1. Mutation creates new alleles. Crossing over, independent assortment, and random fertilization create new allele combinations.

2. Phenotypic differences can also result from environmental effects. A difference must be heritable for selection on it to produce an inherited evolutionary response.

B. Diversity and environmental pressures

1. Genetically diverse populations are more likely to contain variants that support survival and reproduction after an environmental change.

2. Low diversity can increase vulnerability to disease, decline, or extinction. Diversity improves the chance of a successful response but does not guarantee it.

3. Alleles that improve fitness under one condition can lower it under another. Fitness depends on the current selective pressures.

4. Population size can recover after a bottleneck while genetic diversity remains low because lost alleles do not automatically return.

XII. Origins of Life on Earth (Topic 7.12)

A. Geological evidence and chemical models

1. Earth formed about 4.6 billion years ago. The AP model places more suitable conditions by about 3.9 billion years ago and early fossil evidence of life by about 3.5 billion years ago.

2. These dates constrain origin-of-life models; they do not establish the exact time or sequence in which life arose.

3. Proposed steps include the abiotic formation of organic molecules, assembly of larger molecules, enclosure in membrane-like compartments, and emergence of self-replicating systems.

4. Experiments show that some organic building blocks can form from nonliving precursors under particular conditions. This does not demonstrate the complete creation of life.

B. RNA-world hypothesis

1. The RNA-world hypothesis proposes that RNA served as early genetic material before modern DNA-protein systems became established.

2. RNA replication could have maintained genetic continuity, with complementary base pairing supporting copying of sequence information.

3. Early catalysis would not have required genetically encoded proteins. Ribozymes, RNA molecules that catalyze reactions, support this possibility.

4. Lipid compartments could have concentrated reactants and separated internal chemistry from the environment.

5. Natural selection could act once replicating systems had heritable variation and differences in reproductive success. The exact origin of those systems remains under investigation.

Curriculum verification: checked against the current official College Board AP Biology Course and Exam Description, Topics 7.1–7.12. apcentral.collegeboard.org

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