📚Study Guide: Natural Selection
Unit 7: Natural Selection
Natural selection is the unifying theory of biology, explaining how populations adapt to their environments over evolutionary time. This unit covers the mechanisms of evolution--natural selection, genetic drift, gene flow, and mutation--and the evidence supporting evolutionary theory. Students must understand Hardy-Weinberg equilibrium as a null hypothesis; if a population is in Hardy-Weinberg equilibrium, it is NOT evolving. Deviations from expected allele and genotype frequencies indicate evolutionary forces at work. Phylogenetic trees, which depict evolutionary relationships among species, require careful interpretation. Students must understand how shared derived characters (synapomorphies) are used to construct cladograms and how molecular data (DNA/protein sequences) can confirm or revise evolutionary hypotheses. Speciation, the formation of new species, occurs when gene flow between populations is interrupted by geographic isolation (allopatric speciation) or other reproductive barriers (sympatric speciation). The AP exam heavily tests students' ability to interpret phylogenetic trees, calculate allele frequencies, and explain how natural selection acts on phenotypic variation. Understanding convergent evolution, adaptive radiation, and the distinction between microevolution (change within a population) and macroevolution (speciation and higher-level patterns) is essential for top scores.
Key Concepts
- Mechanisms of Evolution: Natural selection (differential survival/reproduction based on heritable traits), genetic drift (random changes in allele frequencies, especially strong in small populations--founder effect and bottleneck effect), gene flow (movement of alleles between populations via migration), and mutation (ultimate source of new genetic variation).
- Hardy-Weinberg Equilibrium: p + q = 1 (allele frequencies); p^2 + 2pq + q^2 = 1 (genotype frequencies). Conditions for equilibrium: no mutations, random mating, no gene flow, large population size, and no natural selection. If these conditions are violated, evolution occurs.
- Types of Selection: Directional selection favors one extreme phenotype. Stabilizing selection favors intermediate phenotypes. Disruptive (diversifying) selection favors both extremes. Frequency-dependent selection depends on the rarity or commonness of a phenotype.
- Speciation: Reproductive isolation prevents gene flow. Prezygotic barriers include temporal, habitat, behavioral, mechanical, and gametic isolation. Postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Allopatric speciation occurs via geographic isolation; sympatric speciation occurs in the same geographic area (e.g., polyploidy in plants).
- Phylogenetic Trees: Branching diagrams showing evolutionary relationships based on shared characteristics. Nodes represent common ancestors. Outgroups are used to root the tree and determine character polarity. Monophyletic (clade), paraphyletic, and polyphyletic groups are distinguished by common ancestry.
Vocabulary
- Fitness: The reproductive success of an individual relative to others in the population; not about physical strength but about passing alleles to the next generation.
- Gene Pool: The total collection of alleles in a population at a given time.
- Bottleneck Effect: A sharp reduction in population size due to environmental events, leading to loss of genetic variation and potential changes in allele frequencies.
- Reproductive Isolation: Mechanisms that prevent members of different species from producing viable, fertile offspring.
- Convergent Evolution: The independent evolution of similar features in species of different lineages due to similar environmental pressures (e.g., wings in bats and birds).
- Adaptive Radiation: Rapid diversification of an ancestral species into many new forms, particularly when a change in the environment makes new resources available (e.g., Darwin's finches).
Processes and Diagrams to Know
- Phylogenetic Tree Interpretation: Be able to identify sister taxa, most recent common ancestors, and determine which tree best fits a data set of shared derived characters.
- Cladogram Construction: Use a character matrix to build a cladogram by grouping organisms based on shared derived traits.
- Selection Graphs: Be able to draw and label graphs for directional, stabilizing, and disruptive selection, showing the original population distribution and the shifted distribution after selection.
Experimental Designs
- Peppered Moth Simulation: Modeling how industrial melanism demonstrates directional selection as soot-covered trees favored dark morphs.
- Hardy-Weinberg Calculation: Using class data (e.g., PTC tasting ability) to calculate observed vs. expected genotype frequencies and test for evolutionary change.
Common Mistakes
- Confusing Evolution and Natural Selection: Evolution is the change in allele frequencies over time. Natural selection is ONE mechanism causing evolution. Genetic drift and gene flow also cause evolution.
- Assuming Evolution Always Leads to Complexity: Evolution optimizes fitness in a specific environment. If simplicity increases fitness, organisms may evolve to become simpler (e.g., parasites losing unnecessary metabolic pathways).
- Misidentifying Ancestral vs. Derived Characters: On cladograms, traits present in the outgroup are ancestral; traits unique to a clade are derived.
- Applying Hardy-Weinberg to Small Populations: Hardy-Weinberg assumes an infinitely large population. Small populations violate this assumption due to genetic drift.
AP Exam Strategies
- Always State Allele Frequencies: In Hardy-Weinberg problems, explicitly state p and q before calculating genotype frequencies. Show all work.
- Trace Evolutionary Change on Trees: When analyzing phylogenetic trees, trace character changes along branches and identify where key adaptations evolved.
- Use "Increased Fitness" Language: In natural selection explanations, explicitly connect the trait to survival and reproduction: "Individuals with trait X had higher fitness because..."
- Distinguish Microevolution and Macroevolution: Microevolution refers to changes in allele frequencies within a population. Macroevolution refers to patterns above the species level, such as adaptive radiation and mass extinctions.
Real-World Applications
- Antibiotic Resistance: Overuse of antibiotics creates strong directional selection favoring resistant bacterial strains, a major public health crisis requiring evolutionary thinking to solve.
- Pesticide Resistance: Similarly, insects evolve resistance to pesticides through natural selection, necessitating integrated pest management strategies.
- Conservation Biology: Understanding bottleneck effects and genetic drift helps conservationists maintain genetic diversity in endangered species like the Florida panther or cheetah.