Biology · Chapter 09

🐒 Evolution

Darwin's theory, natural selection, evidence.

💡 How Life Changes Over Time

Evolution is the gradual change in characteristics of species over many generations. It explains the incredible diversity of life — from bacteria to blue whales.

The theory was proposed by Charles Darwin in 1859 (book: "On the Origin of Species") based on his voyage on HMS Beagle and observations of finches in Galápagos Islands.

Darwin's theory of Natural Selection — 4 ideas:

1. Variation — individuals in a population differ slightly
2. Inheritance — traits pass from parent to child
3. Competition — more offspring than environment can support
4. Survival of the fittest — those with helpful traits survive and reproduce more → next generation has more of those traits

Over millions of years, small changes accumulate → new species emerge.

🦒 Classic example — peppered moth

Before the Industrial Revolution in England, peppered moths were mostly light-colored — well camouflaged on light tree bark. Dark moths were rare and got eaten by birds.

After factories blackened the trees with soot, dark moths blended in better and survived more. Within ~50 years, dark moths became the dominant form. After pollution laws cleaned the air, light moths came back. Evolution observed in human lifetime.

🦴 Evidence for evolution (NEET high-yield)

• Fossils — remains of ancient organisms show progression
• Homologous organs — same structure, different function (human arm, whale flipper, bat wing all have same bone pattern)
• Analogous organs — different structure, same function (bird wing vs insect wing)
• Vestigial organs — useless leftovers (appendix in humans, hindlimb bones in whales)
• DNA similarities — humans and chimps share 98.8% DNA

⏳ The timeline

• Earth formed: ~4.5 billion years ago
• First life (bacteria): ~3.8 billion years ago
• First multicellular life: ~600 million years ago
• Dinosaurs: 230-65 million years ago
• First mammals: ~200 million years ago
• First humans (Homo): ~2.5 million years ago
• Modern humans (Homo sapiens): ~300,000 years ago

🎬

Natural Selection in Action

Animation

Watch how the moth population shifts after the environment changes. This is natural selection in action.

🔬

Drive Natural Selection

Interactive

Toggle the environment and watch generations of moths adapt.

Practice (NCERT): What are homologous organs? Give an example.

Homologous organs are organs that have similar internal structure and origin but may perform different functions in different organisms. They suggest common ancestry — strong evidence for evolution.

Examples:
• Human arm (grasping) — Whale flipper (swimming) — Bat wing (flying)
All three have the same basic bones: humerus, radius, ulna, carpals, metacarpals, phalanges. They evolved from a common ancestor and adapted differently.

Practice (NEET): What is the difference between homologous and analogous organs? Give one example each.

Homologous organs: same structure, different function. Evolved from common ancestor (divergent evolution). Example: arm of human, flipper of whale, wing of bat — all have same bones.

Analogous organs: different structure, same function. Evolved independently in different ancestors (convergent evolution). Example: wing of bird (modified forelimb with feathers) vs wing of insect (skin folds, no bones). Both fly, but completely different origin.

Homologous → evidence for evolution from common ancestor. Analogous → evidence that similar environments can shape similar solutions.