Biology Notes of Chapter 6 Evolution CBSE Class 12

Evolutionary Biology explores the history and changes in life forms on Earth, framed within the context of the evolution of the universe and Earth itself. While various ideas existed, including special creation and spontaneous generation, the widely accepted view is that life originated from pre-existing non-living organic molecules through chemical evolution, followed by the emergence of the first cellular forms. Charles Darwin's theory of evolution by natural selection, supported by Alfred Wallace, explains the diversity and adaptation of life forms. Evidence for evolution comes from fossils, comparative anatomy, embryology, and biochemistry. Key mechanisms driving evolution involve variation, inheritance, and differential survival and reproduction, influenced by factors like mutation, genetic drift, gene flow, and natural selection. The history of life shows the gradual appearance and diversification of organisms over billions of years, culminating in the evolution of modern man.

6.1 Origin of Life

• Study of Evolution: Evolutionary Biology studies the history of life forms on earth. Evolution of life forms must be understood in the context of the evolution of Earth, stars, and the universe.

Universe & Earth Origin:

• The universe is vast and old, approximately 13.8 billion years old .
• Earth is a speck compared to the universe.
• The Big Bang theory attempts to explain the origin of the universe. It describes a singular, huge explosion followed by expansion and cooling, leading to the formation of Hydrogen and Helium, which condensed under gravity to form galaxies.
• Earth is thought to have formed about 4.5 billion years back in the Milky Way galaxy's solar system.
• Early Earth had no atmosphere. Water vapour, methane (CH₄), carbon dioxide (CO₂), and ammonia (NH₃) were released from molten mass, covering the surface.
• UV rays broke down water (H₂O) into Hydrogen (H₂) and Oxygen (O₂). Lighter H₂ escaped. Oxygen combined with NH₃ and CH₄ to form water, CO₂, and others.
• The ozone layer was formed.
• As Earth cooled, water vapour fell as rain, forming oceans.

Origin of Life Theories:

• Considered a unique event in the history of the universe.
• Panspermia: Idea by early Greek thinkers that life units (spores) were transferred to different planets, including Earth. Still favored by some astronomers.
• Theory of Spontaneous Generation: Believed life came out of decaying and rotting matter (straw, mud, etc.).
• Louis Pasteur experimentally disproved spontaneous generation. He showed life only comes from pre-existing life; life did not appear in pre-sterilised flasks with killed yeast, but new organisms arose in flasks open to air with killed yeast. This dismissed the theory.
• Chemical Evolution (Abiogenesis): Proposed by Oparin (Russia) and Haldane (England).
• First life form could have come from pre-existing non-living organic molecules (RNA, protein, etc.).
• Formation of life was preceded by chemical evolution: formation of diverse organic molecules from inorganic constituents.
• Early Earth conditions: high temperature, volcanic storms, reducing atmosphere (containing CH₄, NH₃, etc.).
• S.L. Miller Experiment (1953): Created similar conditions in a lab. Electric discharge in a flask with CH₄, H₂, NH₃, and water vapour at 800°C. Observed formation of amino acids.
• Other similar experiments produced sugars, nitrogen bases, pigments, and fats.
• Analysis of meteorite content also revealed similar compounds, suggesting such processes occur elsewhere in space.
• Chemical evolution is accepted by the majority.

First Life Forms:

• The first non-cellular forms of life could have originated 3 billion years back . These were likely giant molecules (RNA, Protein, Polysaccharides, etc.). They might have reproduced their molecules.
• The first cellular form of life probably originated around 2000 million years ago . These were likely single-cells in a water environment.
• This version of abiogenesis (first life arising from non-living molecules) is accepted by the majority.

6.2 Evolution of Life Forms – A Theory

• Theory of Special Creation: From conventional religious literature.
• Connotations:
1. All living organisms seen today were created as such .
2. Diversity was always the same since creation and will remain the same.
3. Earth is about 4000 years old .
• These ideas were strongly challenged in the nineteenth century.
• Charles Darwin & Natural Selection:
• Based on observations during a sea voyage on the H.M.S. Beagle.
• Concluded that existing life forms share similarities among themselves and with extinct life forms.
• Extinctions have occurred, and new life forms arose over time.
• Life forms have undergone gradual evolution .
• Any population has built-in variation in characteristics.
• Those with characteristics enabling better survival in natural conditions (climate, food, physical factors) would "outbreed" others less equipped.
• Fitness of an individual or population refers ultimately and only to reproductive fitness . Better fit individuals leave more progeny.
• Those that survive more are selected by nature . This mechanism is called natural selection .
• Alfred Wallace , working in Malay Archipelago, reached similar conclusions around the same time.
• Over time, new types of organisms become recognisable.
• Existing life forms share similarities and common ancestors , present at different historical periods.
• Geological history correlates closely with biological history.
• Conclusion: Earth is very old, billions of years, not thousands.
• Comparison: Darwin vs. Lamarck:
• Lamarck believed evolution was driven by the use and disuse of organs.
• Example: Giraffes elongated necks by foraging on tall trees, passing this acquired trait to offspring.
• Lamarck's conjecture is generally not believed anymore.
• Darwin proposed natural selection acting on heritable variations.

6.3 What are the Evidences for Evolution?

• Evidence comes from many sources.

Paleontological Evidence (Fossils):

• Fossils are remains of hard parts of life forms found in rocks.
• Rocks form sediments layered over Earth's history.
• Different-aged sediments contain fossils of different life forms.
• Some fossils are similar to modern organisms, others represent extinct ones (e.g., Dinosaurs).
• Study of fossils in layers shows life forms varied over time and were restricted to certain geological time-spans. New life forms arose at different times.
• Fossil ages can be calculated using radioactive-dating.

Embryological Support:

• Proposed by Ernst Heckel based on observations during embryonic stages common to all vertebrates, absent in adults.
• Example: Embryos of all vertebrates (including humans) develop vestigial gill slits behind the head, functional only in adult fish.
• Disapproved by Karl Ernst von Baer, who noted embryos never pass through adult stages of other animals.

Comparative Anatomy and Morphology:

• Shows similarities and differences among present and past organisms.
• Helps understand shared common ancestors.
• Homologous Structures: Same anatomical structure but perform different functions due to adaptation to different needs (divergent evolution). Indicates common ancestry.
• Example (Animals): Forelimbs of whales, bats, cheetah, humans (share humerus, radius, ulna, carpals, metacarpals, phalanges).
• Example (Animals): Vertebrate hearts or brains.
• Example (Plants): Thorn of Bougainvillea and tendril of Cucurbita .
• Homology is based on divergent evolution .
• Analogous Structures: Different structures evolving for the same function (convergent evolution). Look alike but are not anatomically similar. Result from similar habitats leading to selection of similar adaptive features.
• Example: Wings of butterfly and birds.
• Example: Eye of octopus and mammals.
• Example: Flippers of Penguins and Dolphins.
• Example: Sweet potato (root modification) and potato (stem modification).
• Analogy refers to convergent evolution .

Biochemical Similarities:

• Similarities in proteins and genes for given functions among diverse organisms.
• Provide clues to common ancestry, same as structural similarities.

Artificial Selection (Anthropogenic Action):

• Man has bred selected plants and animals (agriculture, horticulture, sport, security).
• Domesticated wild animals/crops, creating different breeds (e.g., dogs).
• Argument: If man created new breeds in hundreds of years, nature could do the same over millions of years.
• Excess use of herbicides, pesticides results in resistant varieties quickly (months/years).
• Resistance in microbes to antibiotics/drugs appears quickly.
• These are examples of evolution by anthropogenic action .

Industrial Melanism (Moths Example):

• Observation in England: Before industrialization (1850s), more white-winged moths than dark-winged moths on trees. After industrialization (1920), more dark-winged moths. The proportion reversed.
• Explanation: Predators spot moths against contrasting backgrounds.
• Post-industrialization: Tree trunks darkened by smoke/soots. White moths easily spotted, dark moths survived.
• Pre-industrialization: Trees covered in white lichen. White moths camouflaged, dark moths picked out by predators. (Lichens indicate no industrial pollution).
• Moths able to camouflage survived.
• In unpolluted (rural) areas, melanic moth count remained low.
• Conclusion: In a mixed population, those better adapted survive and increase in size. No variant is completely wiped out.

6.4 What is Adaptive Radiation?

• Observed by Darwin in Galapagos Islands (diversity of creatures).
• Darwin's Finches: Small black birds with many varieties on the same island.
• Conjectured to have evolved on the island from an original seed-eating form.
• Altered beaks arose, enabling them to become insectivorous and vegetarian.
• Adaptive Radiation: The process of evolution of different species in a given geographical area starting from a point and radiating to other areas/habitats.
• Darwin's finches are a prime example.
• Another example: Australian marsupials. Many different marsupials evolved from an ancestral stock within the Australian island continent.
• Convergent Evolution: When more than one adaptive radiation occurs in an isolated geographical area, resulting in different species that appear "similar" to species from other areas.
• Example: Placental mammals in Australia exhibiting adaptive radiation into varieties, some of which are similar to corresponding marsupials (e.g., Placental wolf and Tasmanian wolf-marsupial).

6.5 Biological Evolution

• Evolution by natural selection truly began when cellular life forms with metabolic differences originated.
• Essence of Darwinian theory: Natural selection .
• Rate of new form appearance is linked to life cycle/span. Microbes evolving faster than fish or fowl due to rapid reproduction.
• Fitness is better under new conditions, and nature selects for fitness.
• Fitness is based on inherited characteristics ; there must be a genetic basis for selection and evolution.
• Organisms better adapted survive in hostile environments. Adaptive ability is inherited and has a genetic basis .
• Fitness is the end result of adaptation and selection by nature.
• Branching descent and natural selection are the two key Darwinian concepts.
• Is evolution a process or result? Can be seen as a process when describing the world's story, or a consequence of natural selection when describing life's story. Unclear whether evolution and natural selection are processes or end results.
• Influence of Thomas Malthus: Possibly influenced Darwin's ideas on populations.
• Basis of Natural Selection (Factual Observations): Limited natural resources, stable population size (except seasonal), variation among population members (no two alike), most variations are inherited.
• Darwin's Insight: Populations can grow exponentially (like bacteria), but size is limited in reality. This implies competition for resources. Individuals with heritable variations that improve resource utilisation (better adapted) reproduce more, leaving more progeny. Over generations, this changes population characteristics, leading to new forms.

6.6 Mechanism of Evolution

• Origin of variation and speciation mechanism.
• Mendel: Talked about inheritable 'factors' (genes). Darwin did not explicitly incorporate this.
• Hugo deVries: Based on work on evening primrose.
• Idea of mutations – large differences arising suddenly.
• Believed mutation causes evolution, not Darwin's minor variations.
• Mutations are random and directionless .
• Darwinian variations are small and directional.
• Evolution for Darwin was gradual .
• deVries believed mutation caused speciation in a single step ( saltation ).
• Population genetics provided clarity later.

6.7 Hardy - Weinberg Principle

• Applicable to a given population to find allele frequencies.
• States that allele frequencies in a population are stable and constant from generation to generation.
• The gene pool (total genes and alleles) remains constant. This is genetic equilibrium .
• Sum of allele frequencies is 1. For diploid organisms with alleles A and a, frequencies p and q: p + q = 1.
• Genotype frequencies: AA = p², aa = q², Aa = 2pq.
• Equation: p² + 2pq + q² = 1 . This is the binomial expansion of (p+q)².
• Difference between measured and expected frequencies indicates the extent of evolutionary change .
• Evolution is interpreted as a disturbance in genetic equilibrium (Hardy-Weinberg equilibrium), i.e., change of allele frequency.

Factors Affecting Hardy-Weinberg Equilibrium (Leading to Evolution):

1. Gene migration or Gene flow: Migration of a population section changes gene frequencies in both old and new populations. New genes added to new population, lost from old. Gene flow occurs with multiple migrations.
2. Genetic drift: Chance change in allele frequency. If the change is significant in a new sample, it can lead to a different species. Original drifted population becomes founders , effect is founder effect .
3. Mutation: Creates new variations. Pre-existing advantageous mutations selected quickly in microbes.
4. Genetic recombination: During gametogenesis, causes variation.
5. Natural selection: Leads to changed gene/allele frequency. Heritable variations enabling better survival lead to enhanced reproductive success.

Outcomes of Natural Selection (based on Figure 6.8 description):

• Stabilisation: More individuals acquire the mean character value.
• Directional change: More individuals acquire values other than the mean.
• Disruption: More individuals acquire peripheral character values at both ends of the distribution curve.

6.8 A Brief Account of Evolution

• 2000 mya: First cellular forms of life appeared. Mechanism of non-cellular to cellular evolution unknown. Some could release O₂ (similar to photosynthesis light reaction).
• Single-celled became multi-cellular.
• 500 mya: Invertebrates formed and active.
• 350 mya: Jawless fish probably evolved. Fish with stout fins moved on land and back to water. Example: Coelacanth (caught in 1938, thought extinct).
• 320 mya: Sea weeds and few plants existed. Plants invaded land first, widespread when animals invaded.
• Lobefins: Evolved into first amphibians (lived on land/water) around 350 mya. Ancestors of modern frogs/salamanders.
• Amphibians evolved into reptiles . Reptiles laid thick-shelled eggs not drying in sun. Modern descendents: turtles, tortoises, crocodiles.
• Next 200 million years: Reptiles of different shapes/sizes dominated. Giant ferns present, formed coal deposits.
• Some land reptiles returned to water, evolving into fish-like reptiles (e.g., Ichthyosaurs) ~200 mya.
• Land reptiles included dinosaurs . Largest: Tyrannosaurus rex (~20 feet tall, huge teeth).
• 65 mya: Dinosaurs suddenly disappeared. Reason unknown (climatic change? evolution into birds?). Small reptiles of that era still exist.
• First mammals were like shrews. Fossils are small.
• Mammals were viviparous, protecting young internally. More intelligent in sensing/avoiding danger.
• Mammals took over after reptiles declined.
• Continental Drift: South America joining North America led to South American mammals being overridden by North American fauna. Australian pouched mammals survived due to isolation (lack of competition).
• Some mammals live wholly in water (whales, dolphins, seals, sea cows).
• Evolution of horse, elephant, dog are specific stories.
• Most successful story: evolution of man with language/self-consciousness.

6.9 Origin and Evolution of Man

• 15 mya: Primates Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed. Hairy, walked like gorillas/chimpanzees.
• 3-4 mya: Man-like primates walked in eastern Africa (from fossil discoveries in Ethiopia/Tanzania). Probably <4 feet tall, walked upright.
• 2 mya: Australopithecines likely lived in East African grasslands. Hunted with stone weapons, ate fruit.
• First human-like being ( hominid ): Homo habilis . Brain capacity 650-800 cc. Probably did not eat meat.
• 1.5 mya: Homo erectus (from fossils in Java, 1891). Larger brain (~900 cc). Probably ate meat.
• 1,00,000 - 40,000 years back: Neanderthal man lived in near east/central Asia. Brain size 1400 cc. Used hides for body protection, buried their dead.
• Homo sapiens arose in Africa, moved across continents, developed distinct races.
• 75,000 - 10,000 years ago (Ice Age): Modern Homo sapiens arose.
• Pre-historic cave art developed ~18,000 years ago (e.g., Bhimbetka rock shelter).
• Agriculture came ~10,000 years back; human settlements started.

Comparison of Concepts

Frequently Asked Questions

1. What is natural selection according to Darwinian theory?
   
   Natural selection is the mechanism of evolution where individuals with heritable variations better suited (more fit) to their environment survive and reproduce more successfully. This differential reproduction leads to an increase in the frequency of advantageous traits in the population over generations. Fitness is defined as reproductive fitness.
   
   
2. What is the Hardy-Weinberg principle and what disturbs it?
   
   The Hardy-Weinberg principle states that allele frequencies in a population remain stable and constant across generations, maintaining genetic equilibrium. This equilibrium is disturbed, leading to evolution, by factors such as gene migration (gene flow), genetic drift, mutation, genetic recombination, and natural selection.
   
   
3. Explain adaptive radiation with an example from the sources.
   
   Adaptive radiation is the evolutionary process where different species evolve from a common ancestor in a given geographical area, radiating into different habitats. A classic example from the sources is Darwin's Finches on the Galapagos Islands, which evolved different beak types from an ancestral seed-eating form to adapt to diverse diets like insects and vegetation.