Evolution — Long Notes

Evolution is the process by which the diversity of life has arisen — through descent with modification across vast time-scales. In modern terms, evolution = change in gene (allele) frequencies of populations over generations.

1. The Origin of the Universe and Life

• The Big Bang ~13.7 billion years ago produced hydrogen and helium; galaxies, stars, and planets formed over billions of years.
• Earth formed ~4.5 bya. The early atmosphere was reducing — mainly NH₃, CH₄, H₂O, H₂, no free O₂.
• Life first appeared ~3.5 bya. All modern life descends from single-celled ancestors.

1.1 Oparin–Haldane Hypothesis

Life originated through chemical evolution — small organic molecules formed from inorganic precursors in the primitive ocean ("primordial soup").

1.2 Miller & Urey (1953)

Simulated primitive conditions in a closed flask:

• CH₄ + NH₃ + H₂ + water vapour heated + electric spark (mimicking lightning).
• After a week, amino acids and other organic compounds appeared in the trap.
• Experimental support for abiotic synthesis.

Life's first cells were probably heterotrophic anaerobes. Cyanobacteria later evolved oxygenic photosynthesis, gradually oxidising the atmosphere over 2 by (~2 bya "Great Oxidation Event").

2. Evidence for Evolution

2.1 Palaeontological Evidence

• Fossils in successive rock layers reveal an ordered progression of life forms.
• Dating: relative (rock strata) and absolute (radioactive decay, C-14, U-Pb, K-Ar).
• Famous transitional fossils: Archaeopteryx (reptile–bird link), Tiktaalik (fish–amphibian), horse evolution series (Eohippus → Mesohippus → Merychippus → Pliohippus → Equus).

2.2 Comparative Anatomy

• Homologous organs: same evolutionary origin, different functions.
• Example: human arm, whale flipper, bat wing, horse forelimb — all have the same underlying pentadactyl (5-digit) plan. → Divergent evolution.
• Analogous organs: different origin, same function.
• Example: wings of a butterfly (insect) vs a bird (vertebrate); flippers of penguin vs dolphin. → Convergent evolution.
• Vestigial organs — remnants of ancestral structures with lost function: human appendix, wisdom teeth, coccyx (tailbone), muscles moving the ears, nictitating membrane.

2.3 Embryological Evidence

• Early embryos of vertebrates look strikingly similar (gill slits, tail) → common ancestry.
• Haeckel's "ontogeny recapitulates phylogeny" is now regarded as an over-simplification but the basic observations still support evolution.

2.4 Molecular Evidence

• DNA/protein sequence similarities across species mirror the tree of life. Cytochrome-c, ribosomal RNA sequences, histones.
• 99% DNA identity between humans and chimpanzees.

2.5 Biogeographic Evidence

• Related species often occupy adjacent geographic regions (Darwin's finches, marsupials of Australia).
• Islands host unique fauna descended from mainland species.

2.6 Direct Observation (Evolution in real time)

• **Peppered moth (Biston betularia)**: pre-industrial England — light-coloured moths dominant; industrial soot → dark moths favoured (industrial melanism). Post-1950 clean air → light-coloured moths returned.
• Antibiotic resistance in bacteria — humans watch evolution happen within decades.
• Herbicide-resistant weeds and insecticide-resistant pests.

3. Theories of Evolution

3.1 Lamarck (1809) — Use and Disuse

• Organisms change traits during life (giraffe stretching neck) and pass these to offspring — inheritance of acquired characters.
• Rejected because acquired somatic changes don't alter the germline.

3.2 Darwin (1859) — Natural Selection

• Voyage of HMS Beagle, especially the Galapagos Islands, exposed him to finches with different beak shapes suited to their food.
• Published On the Origin of Species (1859).
• Core ideas:

1. Individuals vary within a population.
2. All species produce more offspring than survive — struggle for existence.
3. Variations that improve survival + reproduction are favoured — "survival of the fittest".
4. Favourable traits accumulate over generations → new species.

• Alfred Russel Wallace independently arrived at similar conclusions.

3.3 The Modern Synthesis (Neo-Darwinism)

• Fuses Darwin + Mendel + population genetics + molecular biology.
• Sources of heritable variation: mutation (new alleles), recombination during meiosis, and gene flow between populations.
• Natural selection acts on this variation to drive evolution.

4. Hardy–Weinberg Principle

In a large, randomly mating population, allele frequencies stay constant across generations if no evolutionary forces act.

Mathematics:

• Let allele frequencies be p (A) and q (a) with p + q = 1.
• Genotype frequencies: p² (AA) + 2pq (Aa) + q² (aa) = 1.

Assumptions (5): no mutation, no migration, no genetic drift, no selection, random mating.

Deviation from H-W ⇒ evolution is occurring.

5. Forces Driving Evolution

1. Mutations — sudden random heritable changes; the ultimate source of new alleles.
2. Recombination — reshuffling during meiosis creates new combinations.
3. Genetic drift — random fluctuations in allele frequencies, especially strong in small populations. Two special cases:

• Bottleneck effect — sudden population crash (e.g. epidemic, catastrophe).
• Founder effect — small subset of a population colonises a new area.

1. Natural selection — differential survival and reproduction based on phenotype.
2. Gene flow (migration) — individuals moving between populations carry alleles with them, homogenising differences.

6. Types of Natural Selection

Consider a bell-curve distribution of a trait:

• Stabilising selection — extremes are eliminated, average is favoured; example: human birth weight (very small or very large babies have higher mortality).
• Directional selection — one extreme is favoured; the population shifts toward it; example: peppered moth industrial melanism, DDT-resistant mosquitoes.
• Disruptive selection — both extremes are favoured, the mean is selected against, potentially leading to two subpopulations; example: Galapagos finches with either small or large beaks after drought altered seed availability.

7. Speciation

Formation of new species requires reproductive isolation between populations so they cannot interbreed.

• Allopatric speciation — geographic separation (mountains, rivers, islands). Example: Darwin's finches.
• Sympatric speciation — new species form in the same region, without physical barriers. Common in plants via polyploidy (whole-genome duplication).

8. Adaptive Radiation

Evolution of diverse forms from a common ancestral stock, each adapted to a different niche.

• Darwin's finches on the Galapagos — different beak types for different foods.
• Australian marsupials — mice-like, mole-like, cat-like, and wolf-like forms all evolved from a single marsupial ancestor.

9. Human Evolution — A Snapshot

Human lineage diverged from other apes ~15 mya. Key stages:

Fossil	Approx. age	Brain (cc)	Notable
Dryopithecus	15 mya	—	Ape-like ancestor
Ramapithecus	15 mya	—	Sometimes grouped with above
Australopithecus	4 mya	~500	Bipedal, small brain, Africa
Homo habilis	2 mya	650-800	First tools (Oldowan)
Homo erectus	1.5 mya	~900	Used fire, migrated out of Africa
Homo neanderthalensis	100-40 kya	1400	Europe/Asia, buried dead
Homo sapiens	~200 kya	~1350	Modern human; Africa → global

Modern humans developed language, art, agriculture (~10,000 ya), and civilisation.

Key take-aways

1. Life originated by chemical evolution on early Earth — supported by Miller–Urey.
2. Evidence from fossils, anatomy, embryology, molecular biology, biogeography, and direct observation converges on descent with modification.
3. Darwin's natural selection + Mendel's genetics + population thinking = the Modern Synthesis.
4. Populations evolve via mutation, recombination, drift, gene flow, and selection; deviation from Hardy–Weinberg equilibrium signals evolution.
5. Speciation (allopatric, sympatric) and adaptive radiation explain biological diversity.
6. Human evolution shows increasing brain size, tool use, and migration culminating in H. sapiens.