Evolution & Natural Selection

• Evolution — change in inherited characteristics of populations over generations.
• Natural selection — organisms better adapted leave more offspring (Darwin–Wallace).
• Evidence — fossils, homologous organs, embryology, molecular similarities.
• Speciation — new species when populations isolated and diverge.
• Human evolution — tree from primates (NCERT diagram level).

1. Variation — raw material for natural selection; arises in populations.
2. Survival of the fittest — better adapted survive and reproduce more.
3. Acquired vs inherited — only inherited traits passed to offspring matter for evolution.
4. Homologous organs — same origin, different function (forelimbs of human, whale, bat).
5. Analogous organs — same function, different origin (wing of bird vs insect).
6. Fossils — preserved remains; show transitional forms; age by strata.
7. Molecular evidence — DNA/protein similarities between related species.
8. Geographical isolation — can lead to speciation (different selection pressures).
9. Human evolution — Australopithecus → Homo habilis → H. erectus → H. sapiens (NCERT sequence).
10. Not goal-directed — evolution is not progress toward perfection; adaptation to environment.

Example of natural selection — antibiotic resistance in bacteria

Step 1 — Bacterial population has random variation in antibiotic resistance.
Step 2 — Antibiotic kills most susceptible bacteria.
Step 3 — Resistant survivors reproduce → population mostly resistant.
Step 4 — Not acquired during life inherited — resistant strains selected.
Conclusion: natural selection favours traits improving survival in given environment.

• Lamarckism — acquired characters inherited (wrong for evolution mechanism).
• Misconception: evolution means individual adapts in lifetime (population changes).
• Confusing homologous with analogous organs.
• Thinking evolution always means more complex (not necessarily).
• Using human evolution as linear ladder with no branches (simplified tree in NCERT).

• Define natural selection.
• Give one example of homologous organs.
• How do fossils support evolution?
• Why is antibiotic overuse a selection pressure?

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