In this chapter: origin of life; Miller's experiment; evidences for evolution (fossils, homology, analogy, biochemistry, industrial melanism); adaptive radiation; Darwin and natural selection; de Vries and mutation; Hardy-Weinberg principle; types of natural selection; a brief history of life; origin and evolution of humans.Origin of life
- The universe is thought to be about 20 billion years old (Big Bang theory; this is the NCERT figure, and current estimates are about 13.8 billion years); the Earth about 4.5 billion years. Life appeared about 500 million years after the Earth formed, i.e. almost four billion years ago.
- Special creation and spontaneous generation were early ideas. Louis Pasteur showed that life comes only from pre-existing life: in pre-sterilised flasks, killed yeast gave rise to no life, while flasks open to air developed new organisms.
- Oparin (Russia) and Haldane (England) proposed that the first life came from pre-existing non-living organic molecules (e.g. RNA, protein), after a period of chemical evolution. Early Earth had high temperature, volcanic storms and a reducing atmosphere with CH4, NH3 etc.
- S.L. Miller (1953) created similar conditions in a closed flask with CH4, H2, NH3 and water vapour at 800 °C, with electric discharge, and obtained amino acids. Others later obtained sugars, nitrogen bases, pigments and fats. Meteorites contain similar compounds.
- The first non-cellular forms of life may have arisen about 3 billion years ago; the first cellular forms about 2000 million years ago, probably single cells in water.
Evidences for evolution
Paleontological (fossils)
Fossils are remains of hard parts of life forms preserved in rocks. Sedimentary layers of different ages contain different life forms, some extinct (like dinosaurs). Their age is found by radioactive dating. They show that life forms have changed over time.
Comparative anatomy
| Homologous organs | Analogous organs | |
|---|---|---|
| Meaning | Same basic structure, different functions | Different structure, similar function |
| Type of evolution | Divergent | Convergent |
| Examples | Forelimbs of whale, bat, cheetah and human (same bones: humerus, radius, ulna, carpals, metacarpals, phalanges); hearts and brains of vertebrates; thorn of Bougainvillea and tendril of Cucurbita | Wings of butterfly and bird; eye of octopus and mammals; flippers of penguin and dolphin; sweet potato (root) and potato (stem) |
Biochemical evidence
Similar proteins and genes performing the same function in very different organisms point to common ancestry.
Natural selection in action: industrial melanism
In England, before industrialisation (around 1850), white-winged moths were more common than dark-winged (melanised) moths. After industrialisation (by 1920), dark-winged moths became more common. Smoke and soot killed the whitish lichens on tree trunks and darkened the bark, so white moths became visible to predators while dark moths were camouflaged. In rural areas without industry, melanic moths stayed rare. Lichens are indicators of industrial pollution.
Similar selection happens much faster with human action: resistance to herbicides and pesticides, and microbes resistant to antibiotics.
Adaptive radiation
- Evolution of different species in a geographical area, starting from one point and radiating into other habitats.
- Darwin's finches (Galapagos Islands): from original seed-eating ancestors, many forms with altered beaks arose, including insectivorous and vegetarian finches.
- Australian marsupials: many different marsupials evolved from an ancestral stock within the Australian continent.
- When more than one adaptive radiation occurs in an isolated area, similar forms can arise independently: convergent evolution. Placental mammals in Australia resemble corresponding marsupials, e.g. the placental wolf and the Tasmanian wolf (marsupial).
Theories of how evolution happens
Darwin: natural selection
- Charles Darwin's theory rests on two key concepts: branching descent and natural selection. Alfred Wallace, working in the Malay Archipelago, reached similar conclusions at about the same time.
- Fitness means reproductive fitness: those better fitted to the environment leave more progeny.
- The rate of appearance of new forms depends on life span. A bacterial population with built-in variation, placed in a new medium, can produce a new variant population within days; the same process in fish or fowl takes millions of years.
de Vries: mutation
Hugo de Vries, working on evening primrose, proposed that evolution is caused by mutations: large differences arising suddenly. He called single-step large mutation saltation. Darwin's variations were small and directional, and evolution gradual; de Vries' mutations were random and directionless.
Hardy-Weinberg principle
- In a population, allele frequencies remain constant from generation to generation: the gene pool is in genetic equilibrium.
- The sum of allelic frequencies is 1. With two alleles of frequency p and q: p2 + 2pq + q2 = 1, the binomial expansion of (p + q)2. p2 = frequency of AA, 2pq = Aa, q2 = aa.
- When measured frequencies differ from expected ones, the difference shows evolutionary change.
- Five factors disturb the equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection.
- Gene flow: movement of part of a population to another population changes allele frequencies in both.
- Genetic drift: change in allele frequency by chance. When a small drifted population becomes very different and forms a new species, the original drifted population is called the founders, and the effect the founder effect.
Types of natural selection
| Type | Result |
|---|---|
| Stabilising | More individuals acquire the mean character value |
| Directional | More individuals acquire a value other than the mean (the curve shifts one way) |
| Disruptive | More individuals acquire peripheral values at both ends of the distribution |
A brief history of life
| Time (million years ago) | Event |
|---|---|
| ~2000 | First cellular life; some cells release O2 |
| ~500 | Invertebrates formed and active |
| ~350 | Jawless fish; lobe-finned fish move between water and land |
| ~320 | Seaweeds and a few plants |
| ~200 | Some land reptiles return to water (e.g. Ichthyosaurs) |
| ~65 | Dinosaurs disappear suddenly |
- The first organisms to invade land were plants.
- Lobefins evolved into the first amphibians. In 1938, a Coelacanth, thought extinct, was caught in South Africa.
- Amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up. Giant ferns (pteridophytes) of that era formed coal deposits. Reptiles then dominated the Earth for about 200 million years.
- Tyrannosaurus rex was about 20 feet tall with dagger-like teeth.
- The first mammals were shrew-like. Continental drift let Australian pouched mammals survive (no competition), while South American mammals were overridden by North American fauna when the continents joined.
- Some mammals live wholly in water: whales, dolphins, seals and sea cows.
Origin and evolution of humans
| Form | When | Key facts |
|---|---|---|
| Dryopithecus and Ramapithecus | ~15 mya | Hairy primates that walked like gorillas and chimpanzees. Ramapithecus more man-like; Dryopithecus more ape-like |
| Man-like primates | 3 to 4 mya | Fossils from Ethiopia and Tanzania; walked upright in eastern Africa; probably not taller than 4 feet |
| Australopithecines | ~2 mya | East African grasslands; hunted with stone weapons but essentially ate fruit |
| Homo habilis | ~2 mya | First human-like being (hominid); brain 650 to 800 cc; probably did not eat meat |
| Homo erectus | ~1.5 mya | Fossils found in Java (1891); brain about 900 cc; probably ate meat |
| Neanderthal man | 1,00,000 to 40,000 years ago | Brain 1400 cc; near East and central Asia; used hides for protection and buried their dead |
| Homo sapiens | Modern humans arose during the ice age, 75,000 to 10,000 years ago | Arose in Africa and spread across continents. Cave art about 18,000 years ago (e.g. Bhimbetka rock shelters, Raisen district, Madhya Pradesh). Agriculture about 10,000 years ago |
Common traps: (1) Homologous = divergent; analogous = convergent. (2) Sweet potato and potato are analogous. (3) Miller's gases: CH4, H2, NH3, water vapour (no O2). (4) de Vries worked on evening primrose. (5) Homo habilis 650 to 800 cc; H. erectus 900 cc; Neanderthal 1400 cc.NEET focus
- Oparin-Haldane, Miller's apparatus and products.
- Homology vs analogy examples; industrial melanism; adaptive radiation examples.
- Darwin vs de Vries; Hardy-Weinberg equation and the five disturbing factors; founder effect.
- Three types of natural selection.
- Human evolution table: time, brain size, place.
Practice questions
The thorn of Bougainvillea and the tendril of Cucurbita are examples of:
- Analogous organs
- Homologous organs
- Vestigial organs
- Convergent evolution
Show answer
B. Divergent evolution.
Which gases did Miller use in his experiment?
- CH4, H2, NH3, water vapour
- O2, CO2, N2, water vapour
- CH4, O2, NH3
- CO2, H2, N2
Show answer
A. A reducing atmosphere.
In a population in Hardy-Weinberg equilibrium, the frequency of heterozygotes is:
- p2
- q2
- 2pq
- p + q
Show answer
C.
When more individuals acquire peripheral character values at both ends of the distribution, the selection is:
- Stabilising
- Directional
- Disruptive
- Artificial
Show answer
C.
Saltation (single-step large mutation) was proposed by:
- Darwin
- Lamarck
- Hugo de Vries
- Wallace
Show answer
C.
The brain capacity of Homo erectus was about:
- 650 cc
- 900 cc
- 1400 cc
- 1650 cc
Show answer
B.
The Tasmanian wolf and the placental wolf illustrate:
- Divergent evolution
- Convergent evolution
- Homology
- Genetic drift
Show answer
B.