Q.Write an essay on evidence for evolution of life on earth. OR Describe lac-operon with diagram.

Evolution is supported by fossils, comparative anatomy (homologous/analogous/vestigial organs), embryology, molecular/biochemical similarity and direct observation (peppered moth, resistance).

1. Palaeontological (fossil) evidence. Fossils are remains/impressions of past organisms preserved in rock layers (strata). Their study shows that life forms have changed over geological time, with simpler forms in older strata and complex ones in younger ones. Connecting-link fossils (e.g. Archaeopteryx, between reptiles and birds) and the horse fossil series show a clear line of descent.

2. Evidence from comparative anatomy & morphology.

• Homologous organs: structures with the same basic plan and origin but different functions — e.g. the forelimbs of whale, bat, cheetah and human have the same bones (humerus, radius-ulna, carpals…). They indicate common ancestry → divergent evolution (adaptive radiation). Plant example: thorn and tendril of Bougainvillea/Cucurbita.
• Analogous organs: structures with different origins but the same function — e.g. wings of butterfly and bird, eyes of octopus and mammal, flippers of penguin and dolphin. They result from convergent evolution (similar habits/environment).
• Vestigial organs: reduced, functionless remnants that were functional in ancestors — e.g. vermiform appendix, nictitating membrane, wisdom teeth, ear muscles in humans — proof of descent from ancestors in which they worked.

3. Embryological evidence. Early embryos of vertebrates (fish, reptile, bird, human) are strikingly similar (e.g. all show gill slits), suggesting common ancestry (though Haeckel's strict recapitulation is now modified).

4. Biochemical / molecular evidence. All organisms share the same biomolecules (DNA, RNA, proteins) and a nearly universal genetic code. The greater the similarity in DNA/protein sequences (e.g. cytochrome-c, haemoglobin) between two species, the closer their evolutionary relationship.

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