Evolution Class 12 Notes

Multiple independent lines of evidence converge to support the theory of organic evolution.

• Embryological evidence: vertebrate embryos show gill slits, tail and notochord at early stages, reflecting shared ancestry.
• Fossils: preserved remains or impressions of organisms in rocks; their sequence in sedimentary layers shows a gradual change from simple to complex forms.
• Vestigial organs: rudimentary, non-functional remnants of once-useful structures — e.g. the human appendix, wisdom teeth, nictitating membrane and the coccyx (vestigial tail).

Key observations by Darwin during the Beagle voyage included: overproduction of offspring, struggle for existence, limited food and space, and competition leading to differential survival.

• Mutation is the ultimate source of genetic variation; natural selection acts on the variation already present.
• Genetic recombination during meiosis (crossing over, independent assortment) adds to variation.
• Migration, genetic drift and non-random mating also shift allele frequencies in small populations.

Five agents break Hardy-Weinberg equilibrium and drive evolution: (1) Gene flow — migration of alleles between populations; (2) Genetic drift — random change in allele frequencies, especially in small populations; the founder effect occurs when a few individuals colonise a new area; (3) Mutation — introduces new alleles; (4) Natural selection — differential survival and reproduction favour certain genotypes; (5) Non-random mating — changes genotype frequencies.

• Geographic (allopatric) isolation: a physical barrier (river, mountain, glacier) separates populations, which diverge independently over time.
• Reproductive (sympatric) isolation: populations occupy the same area but are isolated by differences in mating season, behaviour, gamete incompatibility or chromosomal differences.
• Coevolution: two or more species reciprocally influence each other's evolution — e.g. the mutualism between flowering plants and their specific pollinators, or predator-prey arms races.

Human evolution traces the lineage from a common ancestor shared with great apes (orangutan, gorilla, chimpanzee) to modern Homo sapiens. The fossil record shows a trend of increasing brain size, bipedalism, tool use and complex social behaviour.

• Dryopithecus — ape-like ancestor (~15 mya), arboreal, semi-erect posture.
• Ramapithecus — more man-like (~14 mya), probable bipedal habits.
• Australopithecus — small brain (~400–500 cc), clearly bipedal, found in Africa (~2 mya).
• Homo habilis — first tool maker (~1.5 mya), brain 600–700 cc.
• Homo erectus — larger brain (~900 cc), used fire, spread beyond Africa.
• Homo neanderthalensis — large brain (1400 cc), buried their dead, robust build.
• Homo sapiens — modern humans (~74,000 years ago), brain 1350–1450 cc, language, art and complex culture.

Example: In a population of 100 individuals, 36 show the recessive phenotype. Using the Hardy-Weinberg principle, calculate the allele frequencies and the number of carriers.

Solution: Recessive genotype q² = 36/100 = 0.36, so q = 0.6 and p = 1 − 0.6 = 0.4. Carrier frequency 2pq = 2 × 0.4 × 0.6 = 0.48. Number of carriers = 0.48 × 100 = 48 individuals. Homozygous dominant = p² × 100 = 0.16 × 100 = 16 individuals.

Example: Wings of butterflies and wings of birds are analogous organs. Explain why this is evidence for convergent evolution rather than divergent evolution.

Solution: Butterfly wings and bird wings serve the same function (flight) but have completely different structural origins — butterfly wings are chitinous membranes, while bird wings are modified forelimbs with bones, muscles and feathers. Since the common ancestor of insects and vertebrates did not have wings, both lineages independently evolved flight in response to similar selective pressures. This is convergent evolution, not divergent evolution from a winged ancestor.

Exercises (NCERT)

1. 1
   Explain the theory of spontaneous generation.
2. 2
   State the theory of biogenesis. How does Pasteur's experiment support it?
3. 3
   What was the Miller-Urey experiment? Why is it significant?
4. 4
   What are the different types of evidence for evolution?
5. 5
   What are homologous and analogous organs? Give examples.
6. 6
   Explain Darwin's theory of natural selection.
7. 7
   What is adaptive radiation? Give an example.
8. 8
   What is the modern synthetic theory of evolution?
9. 9
   State the Hardy-Weinberg principle.
10. 10
    What are the factors that affect the Hardy-Weinberg equilibrium?
11. 11
    Explain the types of natural selection.
12. 12
    What is speciation? How does it occur?
13. 13
    Differentiate between allopatric and sympatric speciation.
14. 14
    What is coevolution? Give an example.
15. 15
    Describe the evolution of humans.
16. 16
    What is the significance of fossils in evolution?
17. 17
    Explain the concept of adaptive radiation with reference to Darwin's finches.
18. 18
    What is a connecting link? Give an example.

Answers at a glance

1. 1
   The theory of spontaneous generation held that life arose from non-living matter; it was disproved by Pasteur.
2. 2
   The theory of biogenesis states that life arises from pre-existing life; Pasteur's swan-neck flask experiment showed that sterilised broth remained sterile unless exposed to air carrying microbes.
3. 3
   The Miller-Urey experiment synthesised amino acids from simple gases (CH4, NH3, H2, water vapour) and electric discharge, supporting chemical evolution.
4. 4
   Evidences: palaeontological (fossils), anatomical (homologous/analogous organs), embryological, biochemical and adaptive radiation.
5. 5
   Homologous organs have the same structure but different functions (e.g. forelimbs of mammals); analogous organs have different structures but the same function (e.g. wings of butterfly and bird).
6. 6
   Darwin's theory: variation exists; organisms produce more offspring than survive; there is a struggle for existence; the fittest survive (natural selection) and pass on favourable traits.
7. 7
   Adaptive radiation is the diversification of a single ancestral form into many forms; e.g. Darwin's finches.
8. 8
   The modern synthetic theory combines natural selection with genetics, mutation and population genetics.
9. 9
   The Hardy-Weinberg principle: in a large, randomly mating population with no evolutionary forces, allele and genotype frequencies stay constant (p^2 + 2pq + q^2 = 1).
10. 10
    Factors: mutation, gene flow, genetic drift, non-random mating and natural selection.
11. 11
    Natural selection types: stabilising, directional and disruptive.
12. 12
    Speciation is the formation of new species, usually through reproductive isolation.
13. 13
    Allopatric speciation occurs when populations are geographically separated; sympatric speciation occurs within the same area.
14. 14
    Coevolution is the mutual evolutionary influence of two species; e.g. a flower and its pollinator.
15. 15
    Human evolution passed through Dryopithecus, Ramapithecus, Australopithecus, Homo habilis, Homo erectus, Homo neanderthalensis and Homo sapiens.
16. 16
    Fossils give direct evidence of past organisms and show the sequence of evolution.
17. 17
    Darwin's finches on the Galapagos showed adaptive radiation of beak types suited to different foods.
18. 18
    A connecting link shows features of two groups; e.g. Archaeopteryx links reptiles and birds.

MCQ Questions

2 Mark Questions

1. 1
   Differentiate between homologous and analogous organs.
2. 2
   What was the Miller-Urey experiment?
3. 3
   State the Hardy-Weinberg principle.
4. 4
   What are the factors affecting the Hardy-Weinberg equilibrium?
5. 5
   What is adaptive radiation?

3 Mark Questions

1. 1
   Explain Darwin's theory of natural selection.
2. 2
   Describe the types of natural selection.
3. 3
   What is speciation? How does it occur?
4. 4
   Explain the concept of adaptive radiation with an example.
5. 5
   Describe the evolution of humans.

5 Mark Questions

1. 1
   Describe the various evidences of evolution.
2. 2
   Explain the Hardy-Weinberg principle and the factors that affect it.
3. 3
   Describe the origin and evolution of life on Earth.