Evolution

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Evolution — Quick Facts

Key Definitions:

• Evolution: The change in heritable characteristics of biological populations over successive generations
• Natural Selection: The process whereby organisms better adapted to their environment tend to survive and produce more offspring (Charles Darwin, 1859)
• Fitness: The ability of an organism to survive and reproduce in its specific environment
• Adaptation: A heritable trait that helps an organism survive and reproduce in its environment

Core Principles:

• All life shares common ancestry (Universal Common Descent)
• Species change over time through mutation, recombination, and natural selection
• The unit of selection is the gene
• Evolution occurs at the population level, not the individual level

⚡ Exam Tips for MDCAT:

• Darwin’s theory is the foundation — remember all four postulates: variation, inheritance, high reproductive rate, survival of the fittest
• The famous finch example from Galápagos Islands is frequently tested
• Distinguish between Lamarck’s “inheritance of acquired characteristics” and Darwin’s natural selection
• MDCAT 2023 had 2 questions directly on natural selection mechanisms

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Evolution — Study Guide

Darwin’s Theory of Natural Selection:

Charles Darwin proposed that evolution occurs through natural selection. The four requirements are:

1. Variation: Individuals in a population differ from one another
2. Inheritance: Some traits are heritable and passed to offspring
3. Differential survival and reproduction: Individuals with favourable traits survive longer and reproduce more
4. Limited resources: Resources are limited, leading to competition

Evidence for Evolution:

Evidence Type	Example	Significance
Fossil Record	Archaeopteryx (bird-like dinosaur)	Shows transitional forms
Comparative Anatomy	Pentadactyl limb in mammals, birds, reptiles	Common ancestry
Molecular Biology	Cytochrome c sequence homology	Universal genetic code
Biogeography	Marsupials in Australia	Continental distribution
Direct Observation	Antibiotic resistance in bacteria	Real-time evolution
Homologous Structures	Forelimb of human, bat, whale	Same embryonic origin
Analogous Structures	Wings of insect vs. bird	Convergent evolution

Types of Natural Selection:

• Directional Selection: One extreme phenotype is favoured (e.g., giraffe neck length)
• Stabilising Selection: Intermediate phenotype favoured (e.g., human birth weight)
• Disruptive Selection: Both extreme phenotypes favoured (e.g., shell thickness in limpets)

Common Student Mistakes:

• Confusing “survival of the fittest” with “strongest survives” — fitness means reproductive success
• Thinking evolution is purposeful — it has no direction or goal
• Believing individual organisms evolve — populations evolve
• Confusing genetic drift with natural selection

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Evolution — Comprehensive Notes

Population Genetics and the Hardy-Weinberg Principle:

The Hardy-Weinberg equilibrium states that allele and genotype frequencies remain constant in an idealised, non-evolving population:

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

Where $p$ = frequency of dominant allele, $q$ = frequency of recessive allele.

The conditions required for HWE:

1. No mutation
2. No gene flow (migration)
3. Random mating
4. No genetic drift (large population)
5. No selection

Microevolution occurs when any of these conditions are violated.

Mechanisms of Evolution:

1. Gene Flow (Migration): Movement of alleles between populations. Reduces genetic differences between populations.

2. Genetic Drift: Random change in allele frequencies, especially impactful in small populations. Two types:

   • Bottleneck Effect: Population drastically reduced (natural disaster), surviving alleles become overrepresented
   • Founder Effect: New population established by small group, allele frequencies reflect founders

3. Mutation: Ultimate source of new alleles. Rate approximately $10^{-5}$ per gene per generation.

4. Non-random Mating:

   • Inbreeding: Increases homozygosity, can expose deleterious recessive alleles
   • Assortative mating: Preferentially choosing similar phenotypes

Speciation:

• Allopatric Speciation: Geographic isolation leads to speciation (e.g., Grand Canyon squirrel species)
• Sympatric Speciation: Speciation without geographic isolation (e.g., cichlid fish in African lakes)
• Reproductive Isolation Mechanisms:
  • Prezygotic: Habitat, temporal, behavioural, mechanical, gametic isolation
  • Postzygotic: Hybrid inviability, hybrid sterility, hybrid breakdown

Molecular Evolution:

• Molecular Clock Hypothesis: Substitution rate in DNA is roughly constant over time
• Neutral Theory: Most molecular evolution is driven by random drift of neutral mutations
• Synonymous vs. Non-synonymous substitutions: Silent vs. amino acid-changing mutations

Human Evolution Timeline:

• Sahelanthropus tchadensis (~7-6 mya) — earliest hominin
• Australopithecus afarensis (~3.9-2.9 mya) — “Lucy” specimen
• Homo habilis (~2.4-1.4 mya) — first tool maker
• Homo erectus (~1.9 mya - 110 kya) — first fire user, spread out of Africa
• Homo neanderthalensis (~400-40 kya) — Europe/Asia
• Homo sapiens (~300 kya-present) — modern humans

⚡ MDCAT High-Yield Patterns:

• Questions often ask to identify correct statements about natural selection
• Diagrams of finch beaks are frequently used
• Understanding the difference between analogous and homologous structures is crucial
• Remember that evolution is not linear — it’s a branching process (tree of life, not ladder)

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