Chapter 6, Evolution Class XII NCERT and CBSE

Evolution, class XII NCERT

1. Chapter 6 Evolution Pooja Singh PGT Biolo gy

2. Homologous organs

3. Analogous organs

4. Natural selection Before Industrial Revolution Tree trunks were covered in light-colored lichens. White-winged moths could easily camouflage, so they survived more Dark-winged moths were easily seen by birds and eaten more.

5. During Industrial Revolution: Pollution killed lichens and made tree trunks dark with soot. Now, dark-winged moths ( melanised ) were better camouflaged. White moths became more visible, so birds ate them more. Result: More dark moths survived and reproduced.

6. Types of Natural Selection Stabilising Selection Favours the average/medium trait in a population. Selects against extreme traits . Helps maintain constancy in a population. Example- Human birth weight : Very low or very high birth weight has higher risk of death. Babies with average weight have better survival.

7. Directional Selection Favours one extreme trait over the others. Population gradually shifts in one direction . Example : Peppered moths during industrial revolution : Dark-coloured moths were favoured as trees turned dark with soot. White moths became rare.

8. Disruptive Selection Favours both extremes , but selects against the average trait. Leads to two or more different forms in the population. Birds with either very small or very large beaks : Small beaks eat soft seeds Large beaks crack hard seeds Medium beaks are less efficient at both

9. Adaptive Radiation Darwin visited the Galápagos Islands during his voyage on HMS Beagle. He observed many species of finches, all slightly different, but likely descended from a common ancestor. Finches adapted to the available food sources by developing different types of beaks. Thick, strong beaks – for crushing seeds Long, pointed beaks – for catching insects. One species split into many as they adapted to different food sources – this is called adaptive radiation. Over time, they became different species, each specialized for their environment.  

10. Saltation Saltation means sudden, large changes in an organism that lead to new traits or even new species. Unlike Darwin’s idea of slow and gradual evolution, saltation suggests big changes can happen in a single step. This idea was supported by Hugo de Vries, who observed mutations in evening primrose plants.

11. HARDY-WEINBERG PRINCIPLE Allele frequencies in a population remain constant from generation to generation unless disturbed by external factors For a population to stay in genetic equilibrium , it must have: No mutation No migration (no gene flow) Large population size Random mating No natural selection  

12. Mathematical Expression: For a gene with 2 alleles : Let p = frequency of allele A Let q = frequency of allele a (So, p + q = 1 ) Then, the genotype frequencies are: AA = p² Aa = 2pq aa = q² So, p² + 2pq + q² = 1

13. Founder effect The Founder Effect is a type of genetic drift . It happens when a small group of individuals from a large population starts a new population in a new place. Because this group is small , it carries only a small part of the original gene pool .

14. A BRIEF ACCOUNT OF EVOLUTION Diversification of Animal Life Invertebrates emerged around 500 million years ago Jawless fish appeared by ~350 mya, later leading to lobe‑finned fishes (e.g., coelacanths) that gave rise to amphibians Amphibians evolved into reptiles , whose eggs could resist drying Some reptilian lineages re-entered water, evolving into marine reptiles (~200 mya) Dinosaurs dominated the Mesozoic, with giants like Tyrannosaurus rex , and went extinct ~65 mya Early mammals , small and shrew-like, also appeared during the Jurassic

15. Human Evolution Primates – Our Ancestors Human beings belong to the order Primates (which includes monkeys and apes). Earliest primates appeared about 15 million years ago (mya) .

16. Dryopithecus & Ramapithecus (15 – 10 mya) Lived in Africa and Asia . Dryopithecus : Mostly ape-like Walked on all fours

17. Ramapithecus : More man-like Probably walked upright Ramapithecus is considered closer to human ancestry.

18. Australopithecus (About 2 mya) Found in East Africa . Also called “Southern ape” . Walked upright , had a small brain (about 500 cc) . Used stone tools and ate fruits and possibly meat

19. Homo habilis (“Handy man”) (2.0 – 1.5 mya) First true human. Brain size: 650 – 800 cc . Did not eat meat . Used stone tools .

20. Homo erectus (1.8 mya) Brain size: ~900 cc . First to use fire and hunt animals. Walked fully upright. Lived in Asia and Africa

21. Neanderthal Man (100,000 – 40,000 years ago) Lived in Europe and western Asia . Brain size: ~1400 cc . Buried the dead (sign of culture). Used tools and animal skins .

22. Homo sapiens – Modern Humans (Origin ~200,000 years ago) Evolved in Africa . Migrated to other parts of the world around 60,000 years ago . Brain size: ~1450 cc . Developed language, art, agriculture, and civilization