Evolution

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Evolution is the change in the heritable characteristics of biological populations over successive generations. The modern synthesis of evolutionary biology combines Darwin’s theory of natural selection with Mendelian genetics, providing a mechanistic understanding of how species change over time. The core mechanism is natural selection acting on heritable variation.

Key Historical Figures:

• Charles Darwin (1809–1882): Proposed natural selection as the mechanism of evolution; voyage on HMS Beagle (1831–1836)
• Alfred Russel Wallace (1823–1913): Independently conceived natural selection; co-discoverer with Darwin
• Gregor Mendel (1822–1884): Father of genetics; his laws of inheritance explain how traits are passed on
• Hugo de Vries (1848–1935): Proposed mutation theory of evolution

Evidence of Evolution:

Evidence	Example	Significance
Homologous structures	Forelimb of human, bat, whale, cat	Common ancestry; divergence by adaptive radiation
Analogous structures	Wings of insects and birds	Convergent evolution; independent adaptation to similar environments
Fossil record	Horse evolution (Eohippus → modern Equus)	Direct evidence of change over time
Biochemical evidence	Cytochrome c sequence homology	Molecular kinship between species
Embryology	Gill slits and tail in human embryo	Phylogenetic recapitulation
Vestigial organs	Human appendix, wisdom teeth	Remnants of functional ancestors

Important Definitions:

• Gene pool: Total genetic information in a breeding population
• Allele frequency: Proportion of a specific allele in the gene pool
• Gene flow: Transfer of genetic material between populations (migration)
• Genetic drift: Random change in allele frequency due to sampling error; more pronounced in small populations
• Founder effect: Genetic drift in a new population founded by a small subset of original population
• Bottleneck effect: Severe reduction in population size → loss of genetic variation

⚡ Exam Tips:

• Lamarck’s theory of inheritance of acquired characteristics (use and disuse) is incorrect — it was falsified by Weismann’s germ-plasm theory
• Darwin’s “survival of the fittest” means reproductive success, not physical strength
• The peppered moth example demonstrates natural selection in action: dark morphs increased in polluted (sooty) industrial England

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Darwin’s Theory of Natural Selection:

Darwin’s four postulates (slightly restated):

1. Individuals within a population vary in their traits
2. Some of this variation is heritable (passed to offspring)
3. In every generation, more offspring are produced than can survive (struggle for existence)
4. Individuals with favourable (adaptive) traits are more likely to survive and reproduce (differential reproductive success)

The consequence: Over generations, the proportion of adaptive traits increases in the population. The population becomes better suited to its environment.

Key Distinction — Natural Selection vs Sexual Selection:

• Natural selection: Differential survival/reproduction based on overall fitness (ability to survive and reproduce)
• Sexual selection: Differential mating success based on traits that attract mates or compete for mates
• Sexual selection produces traits like peacock’s tail, deer antlers, bird-of-paradise plumage — even if these reduce survival (handicap principle, Zahavi, 1975)

Types of Natural Selection:

1. Directional selection: One extreme phenotype is favoured → population shifts in one direction (e.g., giraffe neck length increasing)
2. Disruptive/diversifying selection: Both extreme phenotypes are favoured over the intermediate → population splits into two groups (e.g., African seedcracker finches: small-beaked birds eat small seeds, large-beaked eat large seeds; medium beaks are disadvantaged)
3. Stabilising selection: Intermediate phenotype is favoured; extremes are selected against (e.g., human birth weight — too small or too large babies have higher mortality)
4. Purifying (negative) selection: Removes deleterious alleles from population

Hardy-Weinberg Equilibrium: A population is in Hardy-Weinberg equilibrium (no evolution) when allele and genotype frequencies remain constant from generation to generation. This occurs only when:

1. No mutation
2. No natural selection
3. Large population size (no genetic drift)
4. No gene flow (no migration)
5. Random mating

For a gene with two alleles A (frequency p) and a (frequency q), where p + q = 1:

• Genotype frequencies: AA = p²; Aa = 2pq; aa = q²
• p² + 2pq + q² = 1

Any deviation indicates evolutionary forces at work.

Mechanisms of Evolutionary Change:

Mutation is the ultimate source of all genetic variation. It is:

• Random with respect to fitness (neither beneficial nor harmful initially)
• Usually neutral or slightly deleterious
• Can be beneficial in specific environments
• Rate: ~10⁻⁸ per base pair per replication in humans

Gene Flow reduces differences between populations. When individuals migrate and breed, they introduce alleles into the recipient population. Gene flow homogenises populations — it prevents speciation by keeping gene pools similar.

Genetic Drift is the change in allele frequencies due to random sampling. Its effect is strongest in small populations. Two classic examples:

• Founder effect: The Amish population in the US has 100× higher rate of Ellis-van Creveld syndrome (a recessive dwarfism) because a small number of founders carried the allele
• Bottleneck effect: The northern elephant seal was hunted to ~20 individuals; today’s population of ~30,000 has much lower genetic diversity than before the bottleneck

Speciation — How New Species Form:

Allopatric speciation (geographic isolation): A physical barrier (mountain range, ocean, glacier) divides a population → populations evolve independently → reproductive isolation develops → two species. Example: Squirrels on either side of the Grand Canyon.

Sympatric speciation (same area): Reproductive isolation without geographic separation — can occur through polyploidy in plants (more common) or ecological niche differentiation.

Reproductive Isolating Mechanisms:

• Prezygotic: Habitat isolation, temporal isolation (different breeding seasons), behavioural isolation (mate choice), mechanical incompatibility, gametic isolation
• Postzygotic: Hybrid inviability, hybrid sterility (e.g., mule — horse × donkey), hybrid breakdown

Convergent vs Divergent Evolution:

• Divergent: Species from common ancestor accumulate differences due to different selective pressures (homologous structures)
• Convergent: Unrelated species develop similar traits due to similar environments (analogous structures) — no common ancestry

Adaptive Radiation: Rapid diversification of a single ancestral lineage into many species occupying different ecological niches. Example: Darwin’s finches on Galápagos Islands — 15 species evolved from a single finch ancestor in ~2–3 million years.

⚡ Common Mistakes:

• Lamarckism is wrong — acquired characteristics cannot be inherited. Weismann cut off tails of mice for 22 generations — offspring still had normal tails
• “Use and disuse” does not change the genome. A bodybuilder’s muscles do not make their children’s muscles larger
• The peppered moth story: predation by birds on moths resting on lichen-covered trees was the selective agent; dark moths were more visible on lichen and eaten more — but when pollution killed lichens, dark moths were now camouflaged on dark bark → they survived better
• Not all traits are adaptive — some are due to genetic drift (neutral evolution)

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🔴 Extended — Deep Dive (exam-level mastery)

For students preparing for top-rank selection.

Molecular Phylogeny and the Molecular Clock:

The molecular clock hypothesis (Zuckerkandl and Pauling, 1965) proposes that nucleotide or amino acid substitution rates are approximately constant over time in different lineages. This means the number of differences between two species’ DNA/protein sequences is proportional to the time since they diverged.

The formula: t = d / (2k) Where t = time since divergence, d = genetic distance (proportion of differing sites), k = substitution rate per site per year

However, the molecular clock is not perfectly constant — rates vary between:

• Genes (some evolve faster than others)
• Lineages (some species accumulate mutations faster due to shorter generation times)
• Time periods (rate variation due to metabolic rate, DNA repair efficiency)

Nevertheless, molecular clocks are powerful tools for dating evolutionary events when calibrated against fossil data.

C-value Paradox: The amount of DNA in a haploid genome (C-value) does not correlate with organismal complexity. Some amphibians have 10× more DNA than humans. “Junk DNA” (repetitive elements, introns) explains much of this — non-coding DNA does not increase organismal complexity but inflates genome size.

Codon Degeneracy and the Redundancy Argument for Evolution: The genetic code is redundant (degeneracy): most amino acids are encoded by multiple codons. This provides evolutionary flexibility — point mutations in the third codon position often don’t change the amino acid (synonymous substitutions), providing a buffer against deleterious mutations.

Karyotype Evolution: Closely related species often have different chromosome numbers due to:

• Fusion: Two acrocentric chromosomes fuse at centromere → reduces chromosome count (human chromosome 2 is a fusion of two ancestral ape chromosomes)
• Fission: One chromosome splits into two → increases chromosome count
• Inversions: Rearrangement within a chromosome — does not change count but affects fertility in hybrids
• Polyploidy: Whole genome duplication — very common in plants (wheat is hexaploid, Triticum aestivum); can cause instant speciation

Neo-Darwinian Synthesis — The Modern View: The modern synthesis (1930s–1950s) integrated Mendelian genetics with Darwinian natural selection. Key additions:

• Population genetics: How selection, drift, mutation, and gene flow change allele frequencies
• The gene as the unit of selection (not the individual or group)
• Quantifying heritability using Falconer’s formula

Molecular Evidence for Evolution:

1. Cytochrome c: All aerobic eukaryotes have cytochrome c (electron transport chain). Human cytochrome c differs from:

   • Rhesus monkey: 1 amino acid (out of 104)
   • Horse: 12 amino acids
   • Yeast: 45 amino acids

   The number of differences correlates with evolutionary distance — strong evidence for common ancestry.

2. Universal Genetic Code: Nearly all organisms use the same triplet codon → amino acid mapping — strong evidence that all life shares a common origin.

3. Endogenous Retroviruses: Remnants of ancient viral infections integrated into host genome are found at the same chromosomal locations in related species — a “fossil record” of viral infections in our ancestors.

Human Evolution:

• Hominin lineage: 6–7 million years ago (MYA) — common ancestor of humans and chimpanzees
• Australopithecus: 4–1 MYA; bipedal; small brain (~400–500 cm³); Lucy (AL 288-1) is most famous specimen
• Homo habilis: 2.4–1.4 MYA; first tool maker; brain ~600–700 cm³
• Homo erectus: 1.9 MYA–110 KYA; first to leave Africa; brain ~900–1100 cm³; made Acheulean handaxes
• Homo neanderthalensis: 400–40 KYA; Europe/Asia; brain ~1400 cm³; buried their dead; made complex tools
• Homo sapiens: ~300 KYA to present; origin Africa (recent Out-of-Africa model supported); complex language, art, technology

Modern humans carry 1–4% Neanderthal DNA due to interbreeding when Homo sapiens migrated out of Africa ~60–70 KYA.

Selection in Humans:

• Sickle cell trait: Heterozygotes (HbAS) are resistant to severe malaria — balancing selection maintains the sickle cell allele (HbS) in malaria-endemic regions
• Lactase persistence: Adult lactase production (allows digestion of milk) evolved independently in pastoral populations in Europe, East Africa, and the Middle East — an example of convergent evolution at the genetic level
• CCR5-Δ32: Homozygous deletion of 32 base pairs in CCR5 gene → resistance to HIV infection (used in the “Berlin patient” cure); likely evolved as protection against smallpox

NEET High-Yield Pattern:

• Lamarck’s theory was based on use and disuse; Darwin’s on natural selection — Lamarckism is wrong
• Founder effect and bottleneck are examples of genetic drift
• Hardy-Weinberg: p² + 2pq + q² = 1
• Allopatric speciation requires geographic isolation
• Homologous structures → common ancestry; analogous structures → similar function, different ancestry
• The peppered moth is classic natural selection evidence
• Vestigial organs prove evolution: human appendix, wisdom teeth, nictitating membrane
• Darwin’s finches demonstrate adaptive radiation
• Human chromosome 2 is a fusion event — evidence for common ancestry with apes