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Biology Class 12 CBSE

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Chapter 6 – Evolution

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Origin of Life

  • The Universe: Approximately 20 billion years old. The Big Bang theory explains a singular huge explosion that led to the expansion of the universe.
  • Earth’s Origin: Formed about 4.5 billion years ago. Early Earth had no atmosphere; it was covered by water vapor, methane, carbon dioxide, and ammonia.
  • Life’s Arrival: Appeared 500 million years after the formation of Earth (about 4 billion years ago).
  • Panspermia: An early Greek theory suggesting units of life (spores) were transferred to different planets, including Earth.
  • Spontaneous Generation: The dismissed theory that life arose from rotting and decaying matter like straw or mud.
  • Louis Pasteur’s Experiment: He demonstrated that life comes only from pre-existing life using a pre-sterilized swan-necked flask.
  • Oparin-Haldane Theory: Proposed that life originated from non-living organic molecules (RNA, proteins) through “chemical evolution.”
  • Miller-Urey Experiment (1953): Created similar conditions to early Earth (800°C, electric discharge, CH4, H2, NH3, and water vapor) and observed the formation of amino acids.

Evidences for Evolution

  • Paleontology: The study of fossils (remains of hard parts of life-forms found in rocks). Different aged rock sediments contain different fossils.
  • Homology (Divergent Evolution): Organs with the same structure but different functions. Example: Forelimbs of whales, bats, cheetahs, and humans (all have humerus, radius, ulna, carpals, etc.).
  • Analogy (Convergent Evolution): Organs with different structures but similar functions. Example: Wings of butterfly and birds; Flippers of Penguins and Dolphins.
  • Biochemical Evidence: Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry.
  • Industrial Melanism: In England, the peppered moth (Biston betularia) showed that dark-colored moths survived better in polluted areas where lichens died and tree trunks became dark.
  • Anthropogenic Evolution: Evolution caused by human action, such as the selection of resistant varieties of bacteria due to the overuse of antibiotics or pesticides.

Adaptive Radiation

  • Definition: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats).
  • Darwin’s Finches: On Galapagos Islands, Darwin observed small black birds with different beak types (insectivorous, vegetarian, etc.) all evolved from a single seed-eating ancestor.
  • Australian Marsupials: A variety of marsupials, each different from the other, evolved from an ancestral stock within the isolated Australian continent.

Theories of Evolution

  • Lamarckism: Proposed that evolution occurs by the “use and disuse” of organs. Example: Giraffes stretching their necks to reach high leaves.
  • Darwinian Theory: Based on two key concepts: Branching Descent and Natural Selection. Fitness is the end result of the ability to adapt and get selected by nature.
  • Mutation Theory (Hugo de Vries): Proposed that mutation (large, sudden change) is the cause of evolution.
    • Darwin’s Variations: Small, directional, and gradual.
    • De Vries’ Mutations: Random, directionless, and “Saltation” (single-step large mutation).

Hardy-Weinberg Principle

  • Concept: Allele frequencies in a population are stable and constant from generation to generation (Genetic Equilibrium).
  • The Equation: p2 + 2pq + q2 = 1.
    • p: Frequency of allele A.
    • q: Frequency of allele a.
    • p2: Frequency of AA individuals.
    • q2: Frequency of aa individuals.
    • 2pq: Frequency of Aa individuals.
  •  Factors Affecting Equilibrium: Gene migration, Genetic drift, Mutation, Genetic recombination, and Natural Selection.
  • Founder Effect: When a small group of individuals migrates to a new area, they become “founders,” and the new population shows a different genotype from the original.

Mechanism of Natural Selection

  • Stabilizing Selection: More individuals acquire the mean (average) character value.
  • Directional Selection: More individuals acquire a value other than the mean (the peak shifts).
  • Disruptive Selection: Individuals at both extremes are favored (two peaks form).

A Brief History of Evolution

  • 500 mya: Invertebrates were formed.
  • 350 mya: Jawless fish evolved.
  • 320 mya: Seaweeds and few plants existed.
  • Coelacanth (1938): A lobe-finned fish caught in South Africa; thought to be the ancestor of modern amphibians.
  • 65 mya: Dinosaurs suddenly disappeared from Earth.
  • Tyrannosaurus rex: The biggest dinosaur (20 feet high, dagger-like teeth).

Origin and Evolution of Man

  • 15 mya: Dryopithecus (ape-like) and Ramapithecus (man-like) existed.
  • 2 mya: Australopithecines lived in East African grasslands (hunted with stones, ate fruit).
  • Homo habilis: First human-like hominid. Brain capacity: 650–800 cc. Did not eat meat.
  • Homo erectus: Existed 1.5 mya. Brain capacity: 900 cc. Ate meat.
  • Neanderthal Man: Lived in Central Asia (100,000–40,000 years ago). Brain capacity: 1400 cc. Used hides to protect bodies and buried their dead.
  • Homo sapiens: Arose in Africa during the Ice Age (75,000–10,000 years ago).
  • Pre-historic cave art: Developed about 18,000 years ago. Agriculture began around 10,000 years ago.
  • Questions count: 10
  • Passing grade: 4
  • Time limit: 30 minutes
  • Number of allowed attempts: 5
Start Quiz
1.

Select all the correct statements. Explain the "Theory of Natural Selection" as proposed by Charles Darwin.

Darwin’s theory is based on the observations of overproduction and limited natural resources.
He noted that individuals within a population show variations in their physical and behavioral traits.
Since resources are limited, there is a "struggle for existence" among members of the population.
Individuals with "favorable" variations are better adapted to survive and reproduce in their environment.
Natural selection leads to the immediate transformation of an individual into a new species within its own lifetime.
Over many generations, the accumulation of these inherited favorable traits leads to speciation.
2.

Discuss the evidence for evolution from Comparative Anatomy (Homology).

Homology refers to the similarity in structures based on a common ancestry or origin.
For example, the forelimbs of whales, bats, cheetahs, and humans share a similar anatomical structure.
While these limbs perform different functions, they all possess humerus, radius, ulna, and phalanges.
This indicates that these diverse animals have evolved from a common ancestral form.
Homologous organs are a result of convergent evolution where different structures evolve for the same function.
This pattern of "divergent evolution" shows how organisms adapt to different ecological needs.
3.

Explain "Adaptive Radiation" with the example of Darwin’s Finches.

Adaptive radiation is the process of evolution of different species in a given geographical area starting from a point.
Darwin observed an amazing diversity of small black birds on the Galapagos Islands.
All these finches were originally meat-eaters that evolved into seed-eaters to avoid competition. (incorrect)
He realized that from the original seed-eating features, many other forms with altered beaks arose.
These variations allowed them to become insectivorous, vegetarian, or cactus-eating finches.
This radiation into new niches is a classic example of how geography influences evolution.
4.

Discuss the "Hardy-Weinberg Principle" and its significance.

This principle provides a mathematical model to study evolutionary changes in a gene pool.
It states that allele frequencies in a stable population will remain constant over generations.
The principle assumes that frequent mutations are necessary to maintain genetic equilibrium.
Factors such as gene flow, genetic drift, and natural selection can disturb this balance.
When the frequency of alleles changes significantly, it indicates that evolution is occurring.
This equilibrium serves as a baseline to measure the rate of evolutionary change in nature.
5.

Describe the "Miller-Urey Experiment" on the origin of life. 

The experiment was designed to test the chemical evolution theory of Oparin and Haldane.
They used a closed flask system to mimic the conditions of the primitive Earth.
The atmosphere inside the flask consisted of methane, ammonia, hydrogen, and water vapor.
Oxygen was pumped into the chamber to simulate the breathable air of the early Earth.
Electric sparks were used to provide the energy that would have come from lightning.
After one week, they found amino acids, which are the building blocks of proteins.
6.

Explain the concept of "Biological Evolution" and the role of Fitness. 

Evolution is a stochastic process based on chance events in nature and mutation.
According to Darwin, the ability of an organism to survive and reproduce is called fitness.
Organisms that are better adapted to their environment will leave more offspring.
Fitness is determined solely by the physical strength and speed of an individual.
Those who are more fit are "selected" by nature to continue the species.
Over long periods of time, this selection leads to the gradual change of the population.
7.

Discuss the "Modern Synthetic Theory" of Evolution. 

The theory suggests that acquired characters are passed on to the next generation. (incorrect)
This theory integrates the findings of genetics with Darwin’s ideas on natural selection.
It emphasizes that variations in a population arise due to gene recombination and mutation.
Natural selection acts on these variations to favor those that provide a survival advantage.
Reproductive isolation is considered a key factor in the formation of new species.
This synthesis provides a more comprehensive explanation of how life evolves over time.
8.

Describe the "Evolution of Man" highlighting the key stages. 

Early primates like Ramapithecus were more man-like and walked more upright.
Australopithecines lived in the East African grasslands and used stone tools for hunting.
Homo habilis is known as the "handy man" and had a brain capacity of about 700cc.
Homo erectus was a small-brained herbivore that lived exclusively in the trees.
Neanderthals lived in Central Asia and used hides to protect their bodies.
Modern Homo sapiens arose in Africa and spread across the globe during the last ice age.
9.

Explain "Industrial Melanism" as an example of Natural Selection. 

This phenomenon was observed in the peppered moth population in England.
Before the industrial revolution, white moths were better camouflaged against light-colored lichens.
After industrialization, tree trunks became dark due to soot and the death of lichens.
In the darkened environment, dark-colored moths had a higher survival rate against birds.
This shift in the moth population clearly demonstrates natural selection in action.
The white moths realized the danger and intentionally changed their wing color to black. (incorrect)
10.

Discuss the evidence for evolution from Embryology. 

Embryology studies the development of an organism from a fertilized egg to adulthood.
Certain features are shared by the embryos of all vertebrates during early development.
For example, all vertebrate embryos possess vestigial gill slits at a certain stage.
These gill slits eventually develop into functional lungs in all adult land animals.
Karl Ernst von Baer noted that embryos do not repeat the adult stages of other animals.
Despite this, the shared embryonic stages suggest a common ancestry for all vertebrates.
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