Unit 7: Natural Selection — Quick Review

Evidence for evolution

• Fossils, biogeography, comparative anatomy (homologies), embryology, molecular (DNA/protein similarity).
• Homologous = shared ancestry. Analogous = shared function, independent origin (convergent).
• Vestigial structures — reduced remnants of ancestral function.

Natural selection in four lines

1. Heritable variation exists.
2. More offspring produced than environment supports.
3. Better-adapted individuals reproduce more.
4. Traits accumulate over generations.

Modes of selection

• Stabilizing → intermediate wins (human birth weight).
• Directional → one extreme wins (peppered moths).
• Disruptive → both extremes win, middle loses.
• Sexual → mating success (peacock tails).
• Frequency-dependent → rare variant advantage.

Hardy-Weinberg

$p + q = 1 \qquad p^2 + 2pq + q^2 = 1$

Five assumptions (no evolution):

1. No mutation · 2. Random mating · 3. No selection · 4. No gene flow · 5. Large population

Each violation = a driver of evolution.

Five drivers

Mechanism	Signature
Mutation	New alleles appear
Selection	Non-random fitness
Drift	Random; strong in small populations
Gene flow	Alleles move between populations
Non-random mating	Mate choice (assortative)

Bottleneck = population crash → reduced diversity (cheetahs). Founder effect = small colonizing group → non-representative alleles (Amish).

Speciation

• Allopatric — geographic isolation (most common).
• Sympatric — no geographic barrier (polyploidy in plants; habitat shift).
• Prezygotic barriers: temporal, habitat, behavioral, mechanical, gametic.
• Postzygotic barriers: hybrid inviability, sterility, breakdown.

Phylogeny quick rules

• A clade = common ancestor + ALL descendants.
• Recency of common ancestor = closeness of relationship.
• Trees can be rotated; tip order doesn't imply relatedness.
• Molecular clocks estimate divergence times from neutral mutations.

Macroevolution

• Adaptive radiation — rapid diversification (Galápagos finches).
• Convergent — unrelated lineages, similar traits.
• Coevolution — two species mutually shape each other (pollinators).
• Punctuated equilibrium vs. gradualism.
• Five mass extinctions; likely sixth in progress.

Origin of life highlights

• Miller-Urey: amino acids from abiotic conditions.
• RNA world hypothesis — RNA preceded DNA.
• Endosymbiosis explains mitochondria + chloroplasts.

💡 Exam Tip: When asked about allele-frequency change, always (1) identify which assumption is violated and (2) name the mechanism.

Key Terms

• Fitness — relative reproductive success.
• Adaptation — heritable trait that improves fitness.
• Gene pool — all alleles in a population.
• Clade / monophyletic group — common ancestor + all descendants.
• Synapomorphy — shared derived trait.
• Genetic drift — random allele-frequency change.

Must-Know for the Exam

• Solve an H-W problem starting from $q^2$.
• State which H-W assumption is violated for each of the five drivers.
• Distinguish stabilizing, directional, and disruptive selection from a graph.
• Describe allopatric vs. sympatric speciation with examples.
• Classify convergent vs. divergent evolution.
• Interpret relatedness on a cladogram by node depth.
• Name at least three lines of evidence for evolution with specific examples.

💡 Exam Tip: If you can't remember which mode of selection a graph shows, look at where the new mean sits relative to the old distribution — centered (stabilizing), shifted (directional), or bimodal (disruptive).