Origin of life, Miller's experiment, Darwin's theory, evidences, adaptive radiation, Hardy–Weinberg principle, evolution of life forms and origin of man — with SVG diagrams, a Hardy–Weinberg calculator, MCQs, A-R and PYQs.
| Theory | Key idea |
|---|---|
| Panspermia | Units of life (spores) came from outer space and were transferred to different planets (early Greek thinkers; still favoured by some astronomers). |
| Spontaneous generation | Life arose from decaying, rotting matter (straw, mud). Louis Pasteur disproved it: in pre-sterilised flasks no life arose; life comes only from pre-existing life. |
| Chemical evolution (Oparin–Haldane) | First life came from pre-existing non-living organic molecules (RNA, protein etc.) formed from inorganic constituents under early-earth conditions: high temperature, volcanic storms, reducing atmosphere (CH₄, NH₃). |
| Special creation | All species created as such; diversity unchanged; earth ~4000 years old. Challenged in the 19th century. |
S. L. Miller created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C and obtained amino acids. Similar experiments gave sugars, nitrogen bases, pigments and fats. Meteorites also contain similar compounds.
| Evidence | Key points |
|---|---|
| Paleontological (fossils) | Fossils = remains of hard parts of life forms in sedimentary rocks. Different layers hold fossils of different ages (e.g. dinosaurs, now extinct). Age found by radioactive dating. Life forms are restricted to certain geological time-spans. |
| Embryological | Ernst Heckel: embryos of all vertebrates have vestigial gill slits (functional only in fish). Karl Ernst von Baer disproved it: embryos never pass through adult stages of other animals. |
| Comparative anatomy & morphology | Homology (divergent evolution; common ancestry) vs analogy (convergent evolution) — see below. |
| Biochemical | Similar proteins and genes performing a given function in diverse organisms indicate common ancestry. |
| Artificial selection | Man bred new varieties (dogs, crops) within hundreds of years; nature could do the same over millions. |
| Natural selection in action | Industrial melanism in moths; resistance to herbicides, pesticides, antibiotics and drugs (evolution by anthropogenic action) arising in months or years. |
| Point | Homologous | Analogous |
|---|---|---|
| Structure | Same anatomical plan / origin | Different anatomy / origin |
| Function | Different | Similar |
| Evolution | Divergent | Convergent |
| Indicates | Common ancestry | Similar habitat/selection pressure |
| Examples | Forelimbs of whale, bat, cheetah, human (humerus, radius, ulna, carpals, metacarpals, phalanges); thorn of Bougainvillea and tendril of Cucurbita; vertebrate hearts, brains | Wings of butterfly and bird; eye of octopus and mammals; flippers of penguin and dolphin; sweet potato (root) and potato (stem) |
1850s (before industrialisation): more white-winged moths on lichen-covered trees. 1920 (after industrialisation): more dark-winged (melanised) moths — trunks darkened by smoke and soot. Moths that camouflage survive predation. Lichens are indicators of industrial pollution (do not grow in polluted air). No variant is completely wiped out. Evolution is stochastic (chance events and chance mutation), not directed.
Adaptive radiation is the evolution of different species in a given geographical area, starting from a common ancestor and radiating into different habitats.
Evolution by use and disuse of organs; giraffes elongated necks while foraging and passed on this acquired character. Nobody believes it any more.
Mendel's 'factors' were ignored by Darwin. Hugo de Vries (evening primrose) proposed mutations — large differences arising suddenly — as the cause of evolution.
| Point | Darwin | de Vries |
|---|---|---|
| Variations | Small, heritable, directional | Mutations — large, random, directionless |
| Speciation | Gradual | Single-step large mutation = saltation |
Allele frequencies in a population are stable and constant from generation to generation; the gene pool remains constant — genetic equilibrium. Sum of all allele frequencies = 1.
Enter the frequency of the recessive phenotype (q², between 0 and 1).
| Stage | Time | Features |
|---|---|---|
| Dryopithecus & Ramapithecus | ~15 mya | Hairy, walked like gorillas/chimpanzees. Dryopithecus more ape-like; Ramapithecus more man-like. |
| Man-like primates | 3–4 mya | Fossils in Ethiopia and Tanzania; not taller than 4 feet, walked upright. |
| Australopithecines | ~2 mya | East African grasslands; hunted with stone weapons but essentially ate fruit. |
| Homo habilis | — | First human-like being (hominid); brain 650–800 cc; probably did not eat meat. |
| Homo erectus | ~1.5 mya | Fossils in Java (1891); brain ~900 cc; probably ate meat. |
| Neanderthal man | 1,00,000–40,000 years ago | Brain 1400 cc; near East & central Asia; used hides to protect the body; buried their dead. |
| Homo sapiens | 75,000–10,000 yrs (ice age) | Arose in Africa, moved across continents, developed into distinct races. |
Q1. In a population, 4% individuals show a recessive trait (aa). Find the frequency of the dominant allele and of carriers (Aa), assuming Hardy–Weinberg equilibrium.
q² = 0.04 → q = √0.04 = 0.2.
p + q = 1 → p = 1 − 0.2 = 0.8.
2pq = 2 × 0.8 × 0.2 = 0.32 (32%). Also p² = 0.64 (AA).
Q2. Explain industrial melanism as an example of natural selection.
1850s: more white-winged moths; 1920 (after industrialisation): more dark-winged moths in the same area.
Soot darkened tree trunks; predators spotted contrasting moths and picked them out. Dark moths camouflaged and survived.
Better-adapted variants survive and increase in the population; no variant is wiped out; rural (unpolluted) areas retained few melanic moths.
Q3. Why can a bacterial population show evolution within days while a fish population takes millions of years?
The rate of appearance of new forms depends on the life cycle/life span. Microbes divide fast and become millions within hours, so selected variants outgrow others in days; fish and fowl have life spans of years, so change takes millions of years.
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Q. Describe Miller's experiment. What did it prove?
S. L. Miller (1953) passed electric discharge through a closed flask with CH₄, H₂, NH₃ and water vapour at 800°C and obtained amino acids. It supported chemical evolution — organic molecules can form from inorganic ones under early-earth conditions.
Q. Distinguish between homologous and analogous organs with examples.
Homologous: same structure, different function, divergent evolution (forelimbs of whale, bat, human). Analogous: different structure, similar function, convergent evolution (wings of butterfly and bird).
Q. What is adaptive radiation? Give two examples.
Evolution of different species from a common ancestor in a geographical area, radiating into different habitats. Examples: Darwin's finches and Australian marsupials.
Q. State Hardy–Weinberg principle. List the factors that affect it.
Allele frequencies in a population remain constant across generations (p² + 2pq + q² = 1). Disturbed by gene flow, genetic drift, mutation, genetic recombination and natural selection.
Q. Write a short note on the evolution of man.
Dryopithecus/Ramapithecus (15 mya) → Australopithecines (2 mya) → Homo habilis (650–800 cc) → Homo erectus (~1.5 mya, 900 cc) → Neanderthal (1400 cc) → Homo sapiens; cave art ~18,000 and agriculture ~10,000 years ago.