Class 12 Biology Notes

Unit VII: Genetics and Evolution · Chapter 6

Evolution

Earth's history in one timeline

Time ago Event
~20 billion years Big Bang: the universe begins
~4.5 billion years Earth forms
~4 billion years First life appears
~2000 million years First cellular life forms
~500 million years Invertebrates
~350 million years Jawless fish; lobefins lead to amphibians
~200 million years Reptiles and dinosaurs
~65 million years Dinosaurs disappear; mammals spread
~75,000–10,000 years Modern humans (Homo sapiens) spread

Origin of life

  • Spontaneous generation (life from rotting matter) was an old idea. Louis Pasteur disproved it: life comes only from pre-existing life.
  • Oparin and Haldane said the first life came from non-living organic molecules (like RNA and proteins) through chemical evolution.

Miller's experiment (1953)

Flask with CH₄, H₂, NH₃ and water vapour → heated to 800 °C with electric sparks → amino acids formed.

Evidence for evolution

1. Fossils (palaeontology)

  • Fossils are remains of old organisms preserved in rocks. Different rock layers have different life forms, showing how life changed over time.

2. Comparative anatomy

Homologous organs Analogous organs
Origin / structure Same Different
Function Different Same
Type of evolution Divergent (one ancestor → different uses) Convergent (different ancestors → same use)
Examples Forelimbs of whale, bat, cheetah, human; thorn of Bougainvillea and tendril of Cucurbita; vertebrate hearts and brains Wings of butterfly and bird; eyes of octopus and mammals; flippers of penguin and dolphin; sweet potato (root) and potato (stem)

3. Natural selection seen in action

  • Industrial melanism (England): before factories, light (white-winged) moths were more common on light, lichen-covered trees. After industrialisation, smoke killed the lichens and darkened the tree trunks, so dark moths were better hidden from birds and became more common.
  • Resistance: use of herbicides, pesticides and antibiotics has selected resistant weeds, insects and bacteria within a short time.

Adaptive radiation

  • Darwin's finches on the Galápagos Islands: from seed-eating ancestors, finches evolved different beaks to eat seeds, insects and more.
  • Australian marsupials (kangaroo, koala, Tasmanian wolf and more) evolved from one ancestor on the same continent.
  • When adaptive radiation happens in different places and gives similar-looking animals (e.g. placental wolf and Tasmanian wolf, a marsupial), it is convergent evolution.

Theories of evolution

Lamarck Darwin Hugo de Vries
Use and disuse of organs; acquired characters are inherited Natural selection: "survival of the fittest" Mutation theory
Giraffes stretched their necks to reach leaves, and passed on long necks Giraffes with longer necks survived better and had more babies Big, sudden changes (saltation) create new species
Rejected Accepted (with modern genetics) Worked on evening primrose (Oenothera lamarckiana)

Darwin's natural selection, step by step

  1. Overproduction: organisms produce many offspring.
  2. Limited resources → struggle for existence.
  3. Variation: no two individuals are the same.
  4. Survival of the fittest: better-suited individuals survive.
  5. They reproduce more and pass on useful traits.
  6. Over many generations → new species.
  • Fitness in Darwin's sense means reproductive fitness: leaving more offspring, not being the strongest!
  • Alfred Wallace, working in the Malay Archipelago, reached the same conclusion.

Hardy–Weinberg principle

  • If p = frequency of allele A and q = frequency of allele a:
    • p + q = 1
    • p² + 2pq + q² = 1 (p² = AA, 2pq = Aa, q² = aa)

5 factors that disturb the balance:

  1. Gene migration (gene flow)
  2. Genetic drift (chance changes)
  3. Mutation
  4. Genetic recombination
  5. Natural selection
  • Founder effect: a small group moves to a new place. By chance, its allele frequencies are different from the original population, so the new group can become a new species.

Three types of natural selection

Type Which individuals win? Result
Stabilising The average ones More individuals near the average
Directional One extreme The average shifts to one side
Disruptive Both extremes Two peaks form

A quick story of plant and animal evolution

  • Plants: early land plants (like Zosterophyllum, Psilophyton) → ferns and seed ferns → gymnosperms → angiosperms (flowering plants).
  • Animals: jawless fish (~350 mya) → lobefins (like the coelacanth, the ancestor of amphibians) → amphibians → reptiles → dinosaurs, birds and mammals.

Human evolution

Hominid Time Key features
Dryopithecus and Ramapithecus ~15 mya Hairy, walked like gorillas; Ramapithecus was more man-like
Australopithecus ~2 mya East African grasslands; stone weapons; ate fruit
Homo habilis ~2 mya First human-like being; brain 650–800 cc; probably did not eat meat
Homo erectus ~1.5 mya Brain ~900 cc; probably ate meat
Neanderthal man 1,00,000–40,000 years ago Brain ~1400 cc; buried the dead
Homo sapiens Arose in Africa Cave art ~18,000 years ago; farming ~10,000 years ago

30-second revision

  • Miller: CH₄, H₂, NH₃, water vapour + sparks → amino acids.
  • Homologous = same origin, divergent; analogous = same function, convergent.
  • Industrial melanism: dark moths survived better after pollution.
  • Darwin's finches and Australian marsupials = adaptive radiation.
  • Fitness = reproductive fitness.
  • Hardy–Weinberg: p² + 2pq + q² = 1; 5 disturbing factors (MGMRN).
  • Brain size: habilis 650–800 → erectus 900 → Neanderthal 1400 cc.