Samacheer Kalvi 10th Science Chapter 18 Long Answer Questions — Evolution with Answers

Answer:

Origin of Life — Chemical Evolution:

The prevailing scientific theory for the origin of life on Earth is chemical evolution — the idea that life arose from simple inorganic chemicals through a series of chemical reactions over millions of years, approximately 3.5–4 billion years ago.

Oparin-Haldane Hypothesis (Primordial Soup Theory):

The hypothesis was proposed independently by A.I. Oparin (Russian chemist, 1924) and J.B.S. Haldane (British biochemist, 1929).

Conditions on Early Earth:

• The early Earth (about 4 billion years ago) had very different conditions than today
• No free oxygen (O2) in the atmosphere — the atmosphere was reducing (contained CH4, NH3, H2, H2O vapour)
• High temperature (volcanic activity)
• Intense ultraviolet radiation (no ozone layer to filter it)
• Lightning, heat from volcanoes, and radiation provided energy

Key Steps in Chemical Evolution (Oparin-Haldane):

Step 1: Formation of Simple Organic Molecules

• Inorganic gases (methane, ammonia, water vapour, hydrogen) in the early atmosphere reacted under the influence of lightning, UV radiation, and heat
• This produced simple organic molecules: amino acids, sugars, fatty acids, and nitrogenous bases

Step 2: Accumulation in the Ocean

• These organic molecules accumulated in the ancient oceans, forming a warm, dilute solution called the "primordial soup" or "prebiotic soup"
• The oceans became a huge reservoir of organic molecules

Step 3: Formation of Complex Biomolecules

• Simple organic molecules joined together to form complex macromolecules:
• Amino acids → Polypeptides (proteins)
• Nucleotides → Nucleic acids (RNA and DNA)
• Fatty acids + glycerol → Lipids

Step 4: Formation of Protobionts (Coacervates)

• Oparin proposed that complex organic molecules in water spontaneously organised into colloidal aggregates called coacervates
• Coacervates had a surrounding membrane-like layer, could absorb molecules from the surrounding solution, and could grow and divide
• These are considered the precursors to the first cells

Step 5: Origin of the First Cells

• Eventually, some coacervates developed the ability to replicate (RNA was probably the first self-replicating molecule — "RNA World Hypothesis")
• The first true cells were prokaryote-like: no membrane-bound nucleus
• They were anaerobic (obtained energy without oxygen) — fermenters
• Over time, photosynthetic bacteria evolved → released O2 → changed the atmosphere → allowed aerobic organisms to evolve

Miller-Urey Experiment (1953):

Stanley Miller (a graduate student) and Harold Urey (his professor) designed an experiment to test the Oparin-Haldane hypothesis.

Experimental Setup:

• A flask simulating the ocean: contained water (H2O)
• A larger flask with a mixture of gases simulating the early atmosphere: methane (CH4), ammonia (NH3), and hydrogen (H2)
• Electrodes to produce electric sparks (simulating lightning)
• A condenser (cold water) to cool the gases back to liquid (simulating rain falling into the ocean)
• The apparatus was sealed and run continuously for one week

Results:

• After one week, the water turned reddish-brown
• Analysis showed the presence of various organic compounds, including:
• Several amino acids (notably glycine, alanine, aspartic acid, glutamic acid)
• Organic acids (lactic acid, formic acid)
• Urea
• Hydrogen cyanide (HCN) and other small molecules

Significance of Miller-Urey Experiment: 1. Experimentally proved that organic molecules (including amino acids — the building blocks of proteins) can form from inorganic molecules under conditions similar to the early Earth 2. Provided strong support for the Oparin-Haldane hypothesis 3. However, it did not create life itself — only demonstrated the first step of chemical evolution

Limitations:

• The exact composition of the early Earth's atmosphere is debated
• The experiment did not show how life itself (with replication and metabolism) originated from these molecules

Answer:

Lamarck's Theory (Theory of Inheritance of Acquired Characteristics):

Jean-Baptiste de Lamarck (1744–1829) was a French naturalist who proposed the first comprehensive theory of evolution in 1809 in his work "Philosophie Zoologique."

Lamarck's Proposals:

Darwin's Theory of Natural Selection:

Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently proposed the theory of natural selection, based on observations made during the voyage of HMS Beagle (1831–1836).

Darwin's Observations and Deductions:

Observation 1: Organisms produce far more offspring than can survive (overproduction / superfecundity).

• e.g., A single pair of elephants could have 19 million descendants in 750 years
• But elephant populations remain relatively stable → most offspring die

Observation 2: The number of individuals in a population remains relatively constant over time.

Deduction 1: There must be a struggle for existence — competition for food, space, mates, and resources. Only some survive.

Observation 3: Individuals in a population show natural variation — they differ in their traits.

Observation 4: Some variations are heritable (passed from parents to offspring).

Deduction 2: Those individuals with heritable variations that give them an advantage in the struggle for existence are more likely to survive, reproduce, and pass on those advantageous traits to their offspring. This is Natural Selection.

Key Concepts of Darwin's Theory:

1. Variation: Natural heritable variation exists in all populations 2. Overproduction: More offspring are produced than can survive 3. Struggle for Existence: Competition for limited resources 4. Survival of the Fittest (Natural Selection): Individuals with favourable variations survive and reproduce; those with unfavourable variations are eliminated 5. Inheritance: Favourable traits are passed to offspring 6. Speciation: Over many generations, accumulated changes lead to new species

Darwinian explanation of giraffe's long neck:

• There was natural variation in neck length in the ancestral giraffe population
• Longer-necked giraffes could reach more food (leaves on tall trees)
• Longer-necked giraffes had better survival and reproduction (natural selection)
• They passed the long-neck trait to offspring
• Over many generations, long-necked individuals dominated the population → evolution of long neck

Comparison Table:

Answer:

Multiple lines of evidence from different fields of biology support the theory of evolution by natural selection.

1. Fossil Record:

A fossil is any preserved trace or remains of a past organism (bones, shells, imprints, footprints, teeth in rock strata).

• Fossils are found in sedimentary rock layers (strata) — older strata are deeper, newer strata are at the surface
• The fossil record shows a progression from simpler to more complex organisms through time
• Transitional fossils connect major groups of organisms:
• Archaeopteryx — transitional between reptiles and birds (had reptile features like teeth and clawed wings, but also bird features like feathers)
• Tiktaalik — transitional between fish and amphibians
• Pakicetus — transitional between land mammals and whales
• Limitations: Fossil record is incomplete (soft-bodied organisms rarely fossilise)

2. Comparative Anatomy:

Homologous Organs:

• Organs that have the same basic structural plan (same bones, same embryonic origin) but may differ in function
• Show divergent evolution from a common ancestor
• e.g., Forelimb bones of humans (grasping), whales (paddling), bats (flying), horses (running) — all have humerus, radius, ulna, carpals, metacarpals, phalanges
• Presence of homologous organs suggests common ancestry

Analogous Organs:

• Organs that look similar and have the same function but have different structural origins and evolutionary history
• Show convergent evolution — different organisms evolved similar solutions to similar environmental challenges
• e.g., Wings of insects (exoskeletal, chitin) and wings of birds (endoskeletal, bone + feather) — look similar (both used for flying) but are structurally completely different

Vestigial Organs:

• Organs that have reduced in size and lost their original function but are still present as remnants — evidence of the organism's evolutionary past
• e.g., In humans: coccyx (remnant of a tail), vestigial muscles around ear (no longer used to move ears), appendix, wisdom teeth, body hair, plica semilunaris (remnant of nictitating membrane/third eyelid)
• e.g., Vestiges of hindlimb bones in whales and snakes

3. Embryology (Comparative Embryology):

• Ernst Haeckel observed that embryos of different vertebrates (fish, frog, lizard, chick, human) look remarkably similar in early developmental stages
• All vertebrate embryos have pharyngeal (gill) pouches in early stages, a tail, and similar basic body plan
• As development proceeds, embryos diverge in appearance
• This suggests all vertebrates share a common ancestry
• Haeckel's "Biogenetic Law" ("ontogeny recapitulates phylogeny") — is an oversimplification but embryological similarities do reflect shared ancestry

4. Molecular Evidence:

DNA and Protein Comparisons:

• The degree of similarity in DNA sequences and protein sequences between species reflects how closely related they are
• Closely related species have more similar DNA sequences; distantly related species have more different sequences
• e.g., Human and chimpanzee DNA is approximately 98–99% identical — reflecting very recent common ancestry
• Cytochrome c (a protein in cellular respiration) is nearly identical across all eukaryotes, reflecting common ancestry

Molecular Clocks:

• The rate of mutation accumulation in DNA can be used to estimate when two species diverged from a common ancestor

5. Industrial Melanism (Direct Evidence of Natural Selection):

A classic example of natural selection observed directly in nature.

Background:

• Peppered moth (Biston betularia) in England existed in two forms: light-coloured (peppered) and dark (melanic)
• Before the Industrial Revolution (before 1850): Light form was common; dark form was rare
• Light moths rested on lichen-covered tree trunks and were camouflaged; dark moths were easily visible to predatory birds and were eaten

During Industrial Revolution:

• Pollution killed the lichens; tree bark turned dark from soot
• Now dark moths were better camouflaged; light moths became visible and were eaten by birds

• In non-industrial areas, light form remained common

After Clean Air Acts (post-1950s):

• As pollution decreased, lichens returned and tree bark became lighter
• Light moths regained their advantage; dark moths became less common again

Significance: This is a directly observed, documented case of natural selection changing allele frequencies in a population within a short time frame — strong evidence for Darwin's theory.

6. Biogeography:

• The geographical distribution of organisms provides evidence for evolution
• Island species often closely resemble mainland species of the nearest continent (e.g., Darwin's finches on Galapagos Islands resembled South American finches — they descended from a common ancestor that colonised the islands)
• Marsupials (pouched mammals) are found almost exclusively in Australia — they evolved in isolation after Australia separated from other continents

Answer:

Natural Selection:

Natural selection is the primary mechanism of evolution proposed by Darwin. It is a process by which organisms with heritable traits that improve their survival and reproductive success in a given environment tend to leave more offspring, causing those traits to become more common in subsequent generations.

Requirements for Natural Selection:

1. Variation: Individuals in a population differ from each other in heritable traits 2. Heritability: Variations must be transmissible from parents to offspring 3. Differential Reproduction: Different variants have different rates of survival and reproduction

Types of Natural Selection:

Artificial Selection (Selective Breeding):

Artificial selection is the deliberate, human-directed process of selecting organisms with desired traits for breeding, to produce offspring with those traits. Humans act as the "selector" instead of the natural environment.

Process: 1. Identify individuals with the desired trait 2. Allow only those individuals to breed 3. Select the best offspring with the desired trait in each generation 4. Repeat over many generations

Examples:

• Dog breeds: All domestic dog breeds (German Shepherd, Chihuahua, Great Dane) were derived from the wolf over thousands of years of artificial selection for different traits (herding, guarding, companionship)
• Crop plants:
• Modern maize (corn) was bred from teosinte (a wild grass) over thousands of years
• All varieties of Brassica (cabbage, broccoli, cauliflower, kale, Brussels sprouts, kohlrabi) were developed from wild mustard by selecting for different plant parts
• Livestock: High-milk-producing dairy cows, fast-growing broiler chickens, disease-resistant cattle
• Dog breeding: Greyhounds selected for speed; Labradors selected for intelligence and temperament

Darwin's Insight: Darwin used artificial selection as an analogy to explain natural selection: "If humans can create such dramatic changes in domestic animals and crops within hundreds of years, imagine what natural selection acting over millions of years can achieve."

Comparison: Natural vs Artificial Selection:

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