AP Bio Unit 7 Review: Natural Selection

Natural selection requires heritable phenotypic variation, competition for limited resources, and differential reproductive success. Individuals with favorable phenotypes leave more offspring, so those alleles increase in frequency over generations. Environments apply selective pressures that can change direction as conditions shift. Selection can be directional (one extreme favored), stabilizing (intermediate favored), or disruptive (both extremes favored). Molecular variation inside cells, such as differences in enzyme activity or protein isoforms, also contributes to fitness differences.

• Evolutionary fitness: Measured by reproductive success, not physical strength. An organism that survives but leaves no offspring has zero fitness.
• Selective pressure: Any biotic factor (predation, disease, competition) or abiotic factor (temperature, drought) that causes differential survival and reproduction.
• Heterozygote advantage: When heterozygotes have higher fitness than either homozygote, as in sickle cell anemia where HbS/HbA individuals have malaria resistance without severe anemia.
• Directional selection: One extreme phenotype is favored, shifting the population mean over time. Example: antibiotic resistance increasing in a bacterial population.
• Phenotypic variation: The raw material for selection. Without variation, all individuals have equal fitness and selection cannot change allele frequencies.

Artificial selection works by the same logic as natural selection, but humans choose which individuals reproduce based on desired traits. Over generations, this shifts allele frequencies and can dramatically change a species. Dog domestication from wolves and the transformation of teosinte into modern maize are classic examples. Artificial selection demonstrates that heritable variation exists and that selecting for specific phenotypes changes populations, which supports the broader mechanism of natural selection.

• Artificial selection: Human-directed breeding that favors specific phenotypes, changing allele frequencies in domesticated species over generations.
• Genetic diversity reduction: Intensive selective breeding can reduce genetic variation in a population, increasing vulnerability to disease or environmental change.

Evolution is not driven by selection alone. Mutation adds new alleles to a population at random. Genetic drift causes random allele frequency changes that are most pronounced in small populations. The bottleneck effect occurs when a population is drastically reduced, leaving survivors with a non-representative sample of the original gene pool. The founder effect occurs when a small group colonizes a new area. Gene flow from migration adds or removes alleles, connecting or separating populations. Changes in allele frequencies across generations are direct evidence that evolution is occurring.

• Genetic drift: Random change in allele frequencies due to chance, not selection. Most powerful in small populations and can fix or eliminate alleles.
• Bottleneck effect: A sharp reduction in population size reduces genetic diversity; surviving alleles may not represent the original population.
• Founder effect: A small group that colonizes a new area carries only a subset of the original gene pool, leading to different allele frequencies in the new population.
• Gene flow: Movement of alleles between populations via migration. Prevents divergence when populations exchange individuals; promotes divergence when it stops.
• Mutation: The ultimate source of new genetic variation. Mutations are random with respect to fitness and provide the raw material on which all other evolutionary forces act.

Hardy-Weinberg equilibrium describes allele and genotype frequencies in a population that is not evolving. The equations are p + q = 1 and p squared + 2pq + q squared = 1, where p is the frequency of one allele and q is the frequency of the other. Five conditions must all be met for equilibrium: large population size, no migration, no new mutations, random mating, and no natural selection. Because these conditions are never fully met in nature, Hardy-Weinberg serves as a null hypothesis. A mismatch between observed and expected frequencies signals that at least one evolutionary force is acting. On the AP exam, you are expected to use the equations to calculate allele and genotype frequencies from given data.

• p + q = 1: The frequencies of all alleles at a locus must sum to 1. If q is given, p = 1 minus q.
• p squared + 2pq + q squared = 1: Expected genotype frequencies under equilibrium. p squared = homozygous dominant, 2pq = heterozygous, q squared = homozygous recessive.
• Null hypothesis: Hardy-Weinberg equilibrium is the baseline expectation of no evolution. Deviations from it indicate that selection, drift, mutation, migration, or nonrandom mating is occurring.
• Genotype frequencies: The proportion of each genotype in a population. Under Hardy-Weinberg, these are predictable from allele frequencies alone.

Evidence for evolution comes from multiple independent fields. Fossils can be dated using the age of surrounding rock strata, carbon-14 decay, and geographic data. Morphological homologies, including vestigial structures like the human coccyx and whale pelvic bones, indicate shared ancestry. Comparisons of DNA nucleotide sequences and protein amino acid sequences show that more closely related species have more similar sequences. All eukaryotes share membrane-bound organelles, linear chromosomes, and genes containing introns, which is structural and molecular evidence for a single common eukaryotic ancestor.

• Fossil dating: Age is estimated from rock layer position, carbon-14 decay rates, and geographic context. Provides a timeline of evolutionary change.
• Morphological homologies: Structural similarities across species that reflect common ancestry, such as the pentadactyl forelimb in mammals.
• Vestigial structures: Reduced or non-functional structures inherited from ancestors in which they were functional, such as whale pelvic bones.
• DNA sequences: More similar DNA sequences between species indicate more recent common ancestry. Used alongside morphological data to infer evolutionary relationships.
• Membrane-bound organelles: Shared by all eukaryotes, including mitochondria and chloroplasts, supporting common eukaryotic ancestry.

Evolution is ongoing. Genomic changes accumulate over time, the fossil record shows continuous change, and resistance to antibiotics, pesticides, herbicides, and chemotherapy drugs demonstrates rapid evolution in real time. MRSA evolving resistance to methicillin and insects evolving resistance to DDT are direct examples of natural selection acting on existing variation. Pathogens also evolve, causing emergent diseases as new variants arise. These examples follow the same logic as natural selection: variation exists, selective pressure is applied, resistant individuals reproduce more, and resistance spreads through the population.

• Antibiotic resistance: Bacteria with resistance alleles survive antibiotic treatment and reproduce, increasing the frequency of resistance in the population. MRSA is a key example.
• Pesticide resistance: Insects with detoxification alleles survive pesticide application and pass those alleles on, shifting population allele frequencies.
• Emergent diseases: Pathogens evolve new variants that can infect new hosts or evade immune responses, demonstrating ongoing molecular evolution.

Phylogenetic trees and cladograms are diagrams that represent hypotheses about evolutionary relationships. Cladograms show branching order based on shared derived characters but do not indicate time or amount of change. Phylogenetic trees add time calibration using fossils or a molecular clock. Nodes represent the most recent common ancestor of the lineages that branch from them. The outgroup is the lineage least related to the others and is used as a reference point. Shared derived characters, called synapomorphies, are the most informative traits for grouping organisms. Molecular data, such as DNA and protein sequences, generally provide more reliable evidence than morphological traits alone because convergent evolution can make unrelated species look similar.

• Cladogram: A branching diagram grouping organisms by shared derived characters. Branch order indicates relative relatedness; branch length has no meaning.
• Phylogenetic tree: Like a cladogram but calibrated for time or amount of change using fossils or a molecular clock.
• Shared derived characteristic: A trait inherited from a common ancestor that is present in two or more lineages. Used to group organisms into clades.
• Outgroup: The lineage that diverged earliest, used as a reference to identify which traits are ancestral and which are derived.
• Molecular clock: Uses the rate of DNA or protein sequence change to estimate when two lineages diverged from a common ancestor.

Speciation occurs when populations become reproductively isolated and can no longer exchange genes. The biological species concept defines a species as a group that can interbreed to produce viable, fertile offspring. Allopatric speciation involves geographic isolation; sympatric speciation occurs in overlapping ranges. Prezygotic barriers prevent mating or fertilization before a zygote forms, including habitat, temporal, behavioral, and mechanical isolation. Postzygotic barriers reduce the fitness of hybrids after fertilization, including hybrid inviability and hybrid sterility. Evolution can proceed slowly through gradualism or in bursts separated by long stable periods in punctuated equilibrium. Adaptive radiation, as seen in Hawaiian Drosophila and Caribbean Anolis lizards, produces rapid speciation when new habitats become available.

• Reproductive isolation: The prevention of gene flow between populations, which is required for speciation to occur.
• Allopatric speciation: Geographic separation prevents gene flow; populations diverge independently and may become separate species.
• Prezygotic isolation: Barriers that prevent mating or fertilization, such as temporal isolation (different breeding seasons) or behavioral isolation (different courtship displays).
• Postzygotic barriers: Barriers acting after fertilization, such as hybrid inviability or hybrid sterility, that reduce hybrid fitness.
• Punctuated equilibrium: Long periods of stasis interrupted by rapid evolutionary change, often following environmental disruption or colonization of new habitats.

Genetic diversity within a population determines how well it can respond to environmental change. Populations with high genetic diversity are more likely to contain individuals that can survive new selective pressures. Populations with low diversity, such as California condors, black-footed ferrets, and prairie chickens, face higher extinction risk because fewer individuals carry alleles suited to new conditions. An allele that is adaptive in one environment can be deleterious in another, which is why maintaining variation matters. Genetic bottlenecks and founder effects reduce diversity and can accelerate extinction risk.

• Genetic diversity: The range of alleles in a population's gene pool. Higher diversity increases the probability that some individuals will survive new environmental pressures.
• Environmental pressure: Any factor that causes differential survival and reproduction. Populations with low diversity may lack individuals capable of surviving a new pressure.
• Extinction risk: Populations with very low genetic diversity, such as those that have passed through a bottleneck, are more vulnerable to disease outbreaks and environmental change.

Earth formed approximately 4.6 billion years ago. Conditions were too hostile for life until about 3.9 billion years ago, and the earliest fossil evidence for life dates to approximately 3.5 billion years ago. The RNA world hypothesis proposes that RNA was the earliest genetic material because it can both store information and catalyze reactions. Three key assumptions of the RNA world hypothesis are: genetic continuity was first assured by RNA replication, base-pairing is necessary for replication, and genetically encoded proteins were not involved as early catalysts. Geological evidence, including stromatolites and isotopic data, supports the timeline for life's origin.

• RNA world hypothesis: Proposes that RNA served as the first genetic material, capable of self-replication and catalysis before DNA and proteins evolved.
• Fossil evidence: Stromatolites and microfossils dating to approximately 3.5 billion years ago provide the earliest physical evidence of life on Earth.
• Geological timeline: Earth formed 4.6 bya; hostile conditions prevented life until about 3.9 bya; earliest life evidence at 3.5 bya gives a plausible window for life's origin.