Evolution Memory Tricks | Biology

🦕 Evolution

Memory Tricks

Proven mnemonics — fast to learn, hard to forget.

Natural Selection

VISH — Variation, Inheritance, Selection pressure, Heritable change

Darwin's 4 postulates — the engine of evolutionary change

Natural selection requires four conditions: Variation exists among individuals. Variation is heritable (passed to offspring). More offspring produced than survive (competition). Individuals with favorable variations survive and reproduce more. Result: favorable heritable traits increase in frequency over generations. Natural selection acts on phenotype but evolution changes genotype frequencies.

Difficulty: Beginner

Fitness

Evolutionary fitness = reproductive success, not physical strength. An organism that survives but doesn't reproduce has zero fitness. Fitness is always relative to environment.

Artificial selection

Humans select which individuals reproduce — domestication of dogs, crops, livestock. Darwin used this as evidence that selection could dramatically change populations. Dog breeds evolved from wolves in ~15,000 years.

Adaptation

Heritable trait that increases fitness in a given environment. Adaptations evolve — they're not designed. Vestigial structures (human tailbone, whale pelvis) = remnants of ancestral adaptations no longer useful.

Common misconceptions

Evolution is not goal-directed — organisms don't "try" to evolve. Individuals don't evolve — populations do. Evolution doesn't always make organisms "better" — just better suited to current environment.

Hardy-Weinberg

p² + 2pq + q² = 1 — "Pure Dominant, Hybrid, Pure Recessive"

p = dominant allele frequency · q = recessive allele frequency · p + q = 1

Hardy-Weinberg equilibrium predicts allele frequencies will stay constant unless evolution is occurring. Equation: p² (homozygous dominant) + 2pq (heterozygous) + q² (homozygous recessive) = 1. Also: p + q = 1. If observed frequencies differ from H-W predictions, evolution is occurring. Requires 5 conditions: large population, random mating, no mutation, no migration, no selection.

Difficulty: Intermediate

Using H-W to find q

If 1/10,000 people have cystic fibrosis (aa), then q² = 0.0001, q = 0.01, p = 0.99. Carrier frequency (2pq) = 2(0.99)(0.01) ≈ 0.02 = 1/50 people are carriers.

Five conditions

1. No mutation. 2. Random mating. 3. No natural selection. 4. Large population (no genetic drift). 5. No gene flow (migration). Violation of any = evolution occurring.

Null hypothesis

H-W equilibrium is the null hypothesis for population genetics — what you'd expect with NO evolution. Comparing observed to H-W expected frequencies tests whether evolution is occurring.

Why useful?

Lets you estimate carrier frequencies for genetic diseases. Useful in conservation (detecting inbreeding in small populations) and forensics (calculating probability of DNA match).

Mechanisms of Evolution

SMNG — Selection, Mutation, Non-random mating, Gene flow

Four forces that change allele frequencies in populations

Evolution = change in allele frequencies in a population. Four mechanisms: Natural Selection (non-random — increases fitness). Genetic Drift (random — stronger in small populations). Mutation (creates new alleles — ultimate source of variation). Gene Flow (migration moves alleles between populations). All four violate Hardy-Weinberg conditions and cause evolution.

Difficulty: Intermediate

Genetic drift

Random changes in allele frequency. Stronger in small populations — chance events have bigger impact. Can eliminate alleles or fix them (frequency = 1.0) by chance alone regardless of fitness.

Bottleneck effect

Population crashes dramatically → survivors are random subset → reduced genetic diversity. Northern elephant seals: hunted to ~20 individuals, now 100,000+ but very low genetic diversity.

Founder effect

Small group colonizes new area → limited genetic variation → allele frequencies differ from source population. Ellis-van Creveld syndrome high in Amish — founder carried the allele. Island colonizations.

Gene flow

Movement of alleles between populations via migration. Tends to make populations more similar. Prevents local adaptation. Barrier to gene flow (mountain range, ocean) can lead to speciation.

Types of Selection

DSD — Directional, Stabilizing, Disruptive

Directional: one extreme · Stabilizing: middle · Disruptive: both extremes

Three modes of natural selection on a trait: Directional selection — one extreme phenotype favored, distribution shifts (antibiotic resistance, industrial melanism). Stabilizing selection — intermediate phenotype favored, variation reduced (human birth weight — too small or too large = higher mortality). Disruptive selection — both extremes favored, middle eliminated (can lead to speciation).

Difficulty: Intermediate

Directional example

Peppered moths: before industrial revolution, white moths camouflaged on light bark. Pollution darkened bark → black moths favored. Post-Clean Air Act → white moths recovering. Classic natural selection demonstration.

Stabilizing example

Human birth weight: optimal ~7-8 lbs. Very small = respiratory/developmental issues. Very large = birth complications. Most common form of selection — maintains existing adaptations.

Sexual selection

Selection based on mating success, not just survival. Intersexual (mate choice — peacock tail). Intrasexual (competition between same sex — elk antlers). Can produce traits that reduce survival but increase reproduction.

Frequency-dependent selection

Fitness depends on how common a phenotype is. Negative frequency-dependence: rare phenotypes have advantage (predators have "search image" for common prey). Maintains diversity in population.

Speciation

Allopatric = Geographic barrier · Sympatric = Same place, different niche

Reproductive isolation = new species · Allopatric most common

Speciation = formation of new species. Allopatric speciation: geographic barrier separates population → isolated populations diverge → reproductive isolation evolves → new species. Most common in animals. Sympatric speciation: speciation without geographic isolation — polyploidy in plants (instant speciation), habitat differentiation. Biological species concept: species = reproductively isolated populations.

Difficulty: Intermediate

Reproductive isolating mechanisms

Prezygotic: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation. Postzygotic: hybrid inviability, hybrid sterility (mule), hybrid breakdown. Prezygotic more efficient — prevents wasted reproductive effort.

Polyploidy

Extra sets of chromosomes — can produce instant speciation in plants. Allopolyploidy: hybrid between two species gets extra chromosome set → fertile. Most crop plants are polyploid (wheat = hexaploid, strawberry = octoploid).

Adaptive radiation

Single ancestral species rapidly diversifies into many species filling different niches. Darwin's finches (14 species from one ancestor), Hawaiian honeycreepers, cichlid fish in African lakes. Usually follows colonization of new environment or mass extinction.

Tempo of speciation

Gradualism: slow, steady change over time. Punctuated equilibrium (Gould & Eldredge): long periods of stasis interrupted by rapid speciation events. Fossil record supports punctuated equilibrium — few transitional forms.

Evidence for Evolution

FAMED — Fossils, Anatomy, Molecules, Embryology, Direct observation

Five independent lines of evidence all point to the same conclusion

Five types of evidence for evolution: Fossils — document evolutionary history and transitional forms. Comparative anatomy — homologous structures (same origin, different function) and vestigial organs. Molecular biology — DNA and protein sequences show evolutionary relationships. Embryology — vertebrate embryos look remarkably similar early in development. Direct observation — antibiotic resistance, dog breeding, Galápagos finch beak changes.

Difficulty: Beginner

Homologous vs analogous structures

Homologous: same evolutionary origin, different function (human arm, bat wing, whale flipper — all modified pentadactyl limb). Analogous: different origin, similar function (convergent evolution — bird wing vs butterfly wing).

Transitional fossils

Tiktaalik: fish-tetrapod transition (fins with wrist bones, neck). Archaeopteryx: dinosaur-bird transition (teeth, claws, feathers). Pakicetus → modern whales: series shows gradual loss of hind limbs over 15 million years.

Molecular evidence

Cytochrome c amino acid sequences: chimps differ from humans by 0, rhesus monkey by 1, dogs by 11, yeast by 51. More differences = more distant relationship. Mirrors morphological phylogenies.

Vestigial structures

Human: coccyx (tailbone), wisdom teeth, goosebumps (arrector pili), palmaris longus muscle, plica semilunaris (third eyelid remnant). Whale: pelvic bones. Python: pelvic spurs. Evidence of common descent.

Phylogeny

LUCA (Last Universal Common Ancestor) — all life from one origin

Tree of life traces all species back to a single ancestral cell ~3.8 billion years ago

All life on Earth shares a common ancestor (LUCA) that lived ~3.8 billion years ago. Evidence: universal genetic code, same L-amino acids, ATP as energy currency, DNA as genetic material. The tree of life shows evolutionary relationships. Molecular phylogenetics uses DNA/protein sequences to build trees. Horizontal gene transfer (especially in prokaryotes) makes the tree more web-like at its base.

Difficulty: Intermediate

Cladistics

Classification based on evolutionary relationships (clades). Shared derived characters (synapomorphies) define clades. Parsimony: choose the tree requiring fewest evolutionary changes. Replaced purely morphological classification.

Molecular clock

DNA accumulates mutations at roughly constant rate. Calibrated with fossil record → estimate divergence times. Human-chimp split: ~6 million years ago. Human-gorilla: ~8 mya. All primates from common ancestor ~85 mya.

Convergent evolution

Independent evolution of similar traits in unrelated lineages — can mislead phylogenies based on morphology. Dolphin and shark: similar body shape, different ancestors. Eyes evolved independently 40+ times. Molecular data helps identify convergence.

Deep time

Earth ~4.5 billion years old. First life ~3.8 bya. First eukaryotes ~2 bya. First multicellular ~600 mya. Cambrian explosion ~540 mya. Dinosaur extinction ~66 mya. Human genus ~2.5 mya. Modern humans ~300,000 ya.

Coevolution

Arms race: predator improves → prey improves → predator improves…

Two species evolve in response to each other — ongoing reciprocal selection

Coevolution occurs when two species exert reciprocal selective pressure on each other. Classic examples: predator-prey arms races (cheetah speed vs gazelle speed), flower-pollinator matching (orchid shapes matching specific bee species), host-parasite coevolution (immune system vs pathogens), plant defenses vs herbivore detoxification. The Red Queen hypothesis: must keep evolving just to stay in the same place.

Difficulty: Advanced

Mimicry

Batesian mimicry: harmless species resembles harmful species (viceroy butterfly mimics monarch). Müllerian mimicry: two or more harmful species resemble each other (multiple poison dart frog species share warning colors — reinforces predator learning).

Red Queen hypothesis

Named after Alice in Wonderland — "must run just to stay in place." Sexual reproduction evolved partly to generate variation for host-parasite arms races. Parasites drive hosts to keep generating new immune variants.

Mutualistic coevolution

Figs and fig wasps: obligate mutualism — fig wasp pollinates fig, fig provides wasp with oviposition site. So specialized each fig species has its own wasp species. 750+ fig species, each with unique wasp.

Antagonistic coevolution

Garter snakes and rough-skinned newts: newt evolved tetrodotoxin (potent neurotoxin), snake evolved TTX (tetrodotoxin) resistance, newt evolved higher TTX, snake evolved more resistance — ongoing arms race documented in Pacific Northwest.

Mass Extinctions

Big Five extinctions — "Ordovician, Devonian, Permian, Triassic, Cretaceous"

5 major mass extinctions in Earth history · 6th underway now

Five mass extinctions have each eliminated 50-96% of species: Ordovician (~445 mya, glaciation), Devonian (~375 mya, multiple causes), Permian (~252 mya, worst ever — 96% of species, volcanic activity), Triassic (~200 mya), Cretaceous-Paleogene (~66 mya, asteroid impact — killed non-avian dinosaurs). After each extinction, surviving lineages radiate to fill empty niches. Current extinction rate suggests a 6th mass extinction is underway.

Difficulty: Intermediate

K-Pg extinction (66 mya)

Chicxulub asteroid impact (Mexico) released energy of 1 billion nuclear bombs → global firestorm, dust cloud blocking sun, acid rain. Non-avian dinosaurs extinct. Mammals (small, burrow-dwelling) survived → radiated into vacant niches.

Permian extinction

Worst extinction — 96% of species lost. Siberian Traps volcanic eruptions released massive CO₂ and SO₂ → global warming, ocean acidification, anoxia. Took 10 million years for recovery.

Evolutionary aftermath

Mass extinctions are evolutionary opportunities. After K-Pg: mammals diversified explosively (Cenozoic = "Age of Mammals"). After Permian: dinosaurs rose to dominance. Extinction clears ecological space for survivors.

Sixth mass extinction

Current extinction rate 100-1000× background rate. HIPPO causes (H=Habitat loss, Invasive species, Pollution, Population, Overharvesting). Unlike past extinctions, caused by a single species. Irreversible on human timescales.

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🎓 Common Exam Questions

Q: What is natural selection? Explain using the VISH framework.

A: VISH = Darwin's four postulates for natural selection: Variation exists among individuals in a population (different beak sizes, coat colors). Inheritance — variation is heritable (passed to offspring via genes). Selection pressure — more offspring are produced than survive, creating competition. Heritable change — individuals with favorable heritable traits survive and reproduce more, so those traits increase in frequency over generations. Key clarifications: natural selection acts on phenotype (expressed traits) but evolution changes genotype frequencies. Selection is not goal-directed; it doesn't make organisms 'better' in an absolute sense — only better suited to current conditions.

Q: How does Hardy-Weinberg equilibrium work and when is it useful?

A: Hardy-Weinberg (H-W) predicts allele and genotype frequencies in a non-evolving population. Equation: p² + 2pq + q² = 1, where p = dominant allele frequency, q = recessive allele frequency, p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive. Also: p + q = 1. Requires five conditions (all violated in real populations): large population, random mating, no mutation, no migration, no selection. Usefulness: if observed frequencies deviate from H-W predictions, evolution is occurring. Used to calculate carrier frequencies for genetic diseases (if 1/10,000 have disease = q² = 0.0001, then q = 0.01, 2pq ≈ 0.02 = 2% carriers).

Q: What are the five lines of evidence for evolution (FAMED)?

A: FAMED: Fossils — document evolutionary history, transitional forms (Tiktaalik = fish to tetrapod), stratigraphic sequence shows progression. Anatomy — homologous structures (same evolutionary origin, different function: human arm/whale flipper/bat wing all = same bones), vestigial organs (human coccyx, whale pelvis), analogous structures (convergent evolution). Molecules — DNA and protein sequences reflect evolutionary relationships; cytochrome c amino acid sequences correlate with fossil-based phylogenies. Embryology — vertebrate embryos strikingly similar early in development (all have pharyngeal pouches, notochord). Direct observation — antibiotic resistance evolution in real time, dog breeding, Galapagos finch beak changes documented by the Grants.

Q: What is the difference between directional, stabilizing, and disruptive selection (DSD)?

A: Three modes: Directional selection — one extreme phenotype is favored, the distribution shifts in that direction. Example: antibiotic resistance (bacteria with any resistance survive), industrial melanism (dark moths favored in polluted forests). Stabilizing selection — intermediate phenotype is favored, variation is reduced. Example: human birth weight (too small = premature problems, too large = delivery problems — intermediate weight has highest survival). Disruptive selection — both extremes are favored, the middle is eliminated. Example: beak sizes in African seedcracker birds (large beaks for hard seeds, small for soft seeds, medium beaks disadvantaged). Can eventually lead to speciation.

Q: Describe the five mass extinctions and explain what is unusual about the current extinction crisis.

A: The Big Five: Ordovician (~445 mya) — glaciation, ~86% species lost. Devonian (~375 mya) — ~75% species lost, multiple causes. Permian (~252 mya) — worst ever, ~96% species lost, massive volcanism (Siberian Traps). Triassic (~200 mya) — ~76% species lost, volcanism. Cretaceous-Paleogene/K-Pg (~66 mya) — asteroid impact, ~76% species including non-avian dinosaurs. After each, surviving lineages radiated to fill empty niches. Current crisis: extinction rate 100-1000x background rate, caused by a single species (humans) via HIPPO factors — unprecedented in Earth history and irreversible on human timescales.

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