Chapter 6: Evolution Long Questions | biology Class 12 CBSE

ByteNotes

Chapter 6: Evolution Long Questions | biology Class 12 CBSE | ByteNotes.in

Long Answer

Hard difficulty • Structured explanation

Hard

Question 1

Long Form

Compare the views of Lamarck and Darwin on the mechanism of evolution. Which theory is currently accepted and why?

Lamarck proposed that evolution is driven by use and disuse of organs; acquired characteristics (e.g., giraffe's elongated neck from stretching) are passed on to offspring, driving evolutionary change.
Darwin proposed natural selection: populations have heritable variation; individuals with traits better suited to the environment reproduce more and pass those traits to offspring.
Darwin's theory is supported by genetic evidence — acquired characteristics are not heritable because somatic changes are not passed via gametes, invalidating Lamarck's core premise.
Darwin's two key concepts — branching descent and natural selection — explain both diversification and the pattern of similarities among organisms, which Lamarck's theory cannot account for.
Lamarck's theory is no longer accepted; Darwinian evolution, integrated with Mendelian genetics (Neo-Darwinian synthesis), is the currently accepted explanation for the mechanism of evolution.

ByteNotes

Long Answer

Medium difficulty • Structured explanation

Medium

Question 2

Long Form

Describe Miller's experiment. What conclusions did it support and what questions did it leave unanswered?

Miller created conditions resembling early earth in a closed flask: electric discharge through CH4, H2, NH3, and water vapour at 800°C to simulate lightning in the primitive atmosphere.
He observed the formation of amino acids; similar experiments by others produced sugars, nitrogen bases, pigments, and fats, and meteorite analysis revealed comparable compounds.
The experiment supported Oparin and Haldane's hypothesis of chemical evolution — that diverse organic molecules could form abiotically from inorganic precursors under early-earth conditions.
However, the experiment did not explain how the first self-replicating metabolic capsule of life arose from these organic molecules — the crucial step from chemistry to biology remains unexplained.
The experiment only demonstrated the first part of the conjectured story (chemical evolution); how non-cellular aggregates of macromolecules evolved into membrane-bound cells is still not known.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 3

Long Form

Analyse how the Hardy-Weinberg principle connects genetics to evolution, and explain how its violation leads to evolutionary change.

Hardy-Weinberg principle states that allele frequencies in a population remain constant generation to generation (genetic equilibrium), expressed as p² + 2pq + q² = 1, showing no evolution occurs when equilibrium holds.
Any disturbance in equilibrium — measured as a difference between expected and observed allele frequencies — directly indicates that evolution is occurring in that population.
Five factors disturb equilibrium: gene flow (migration adds/removes alleles), genetic drift (random chance changes allele frequencies), mutation (introduces new alleles), recombination (reshuffles alleles), and natural selection (differentially favours certain alleles).
Genetic drift is particularly powerful in small populations and can lead to founder effect, where the founding group's gene pool diverges so significantly it may lead to a new species.
Thus, Hardy-Weinberg provides a mathematical null model for evolution: it defines the baseline of no evolution, and any deviation from it, caused by the five factors, constitutes evolutionary change.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 4

Long Form

Trace the chronological evolution of life forms on earth from the first cellular organisms to the appearance of mammals.

The first cellular forms of life appeared about 2000 mya; some cells could release O2 through light-driven water-splitting reactions similar to photosynthesis, gradually oxygenating the atmosphere.
By 500 mya, invertebrates were active; jawless fish evolved around 350 mya; plants invaded land first, followed by lobefin fish with strong fins that could move onto land around 350 mya.
Lobefins evolved into amphibians (ancestors of frogs and salamanders); amphibians evolved into reptiles, which lay thick-shelled eggs not prone to desiccation, enabling full colonisation of land.
Reptiles dominated for 200 million years, diversifying into dinosaurs and marine reptiles (e.g., Ichthyosaurs); dinosaurs disappeared about 65 mya due to reasons still debated.
The first mammals (shrew-like, viviparous) were present alongside dinosaurs; after reptile decline, mammals diversified, took over diverse niches, and eventually gave rise to primates and modern humans.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 5

Long Form

Explain the different types of evidence that support the theory of evolution, and evaluate their relative strengths.

Palaeontological evidence (fossils in sedimentary rocks) shows life forms varied over time and certain species are restricted to specific geological periods, providing a direct record of past life.
Comparative anatomy provides homologous structures (e.g., vertebrate forelimbs with humerus, radius, ulna) indicating divergent evolution from a common ancestor, and analogous structures showing convergent evolution.
Embryological evidence — vertebrate embryos sharing features like vestigial gill slits — suggests shared ancestry, though von Baer refined this by showing embryos don't pass through adult stages of other animals.
Biochemical evidence (similarities in proteins and genes performing the same function across diverse organisms) independently supports common ancestry without relying on morphology.
Artificial selection (human breeding of dogs and crops within hundreds of years) and industrial melanism in peppered moths provide direct observational evidence that populations change under selection pressure, supporting the mechanism of natural selection.

ByteNotes

Long Answer

Medium difficulty • Structured explanation

Medium

Question 6

Long Form

Describe the origin of the universe and earth, and explain how conditions on early earth set the stage for the origin of life.

The Big Bang, approximately 13.8 billion years ago, was a singular explosion; the universe expanded, temperatures dropped, and hydrogen and helium formed; gases condensed to form galaxies and stars.
Earth formed about 4.5 billion years ago in the solar system of the Milky Way galaxy; early earth had no atmosphere, and water vapour, methane, CO2, and ammonia were released from the molten mass.
UV rays from the sun broke up water into hydrogen and oxygen; lighter hydrogen escaped; oxygen combined with methane and ammonia to form water, CO2 and other compounds; the ozone layer formed.
As earth cooled, water vapour condensed as rain and filled depressions to form oceans; these oceans, rich in organic molecules formed by abiotic chemical reactions, created the 'primordial soup.'
Life appeared approximately 500 million years after earth formed (about 4 billion years ago); the accumulation of organic molecules — precursors to proteins and nucleic acids — in the oceans constituted chemical evolution, setting the stage for the first cellular forms of life.

ByteNotes

Long Answer

Medium difficulty • Structured explanation

Medium

Question 7

Long Form

Explain adaptive radiation with two examples. How does it relate to the concept of convergent evolution?

Adaptive radiation is the evolution of multiple species from a single ancestral stock in a given geographical area, with descendants radiating into different habitats and adapting to different ecological niches.
Darwin's finches: from a single seed-eating ancestor on the Galapagos Islands, multiple varieties evolved with different beak shapes suited for insectivorous, vegetarian, and other feeding strategies.
Australian marsupials: from a single marsupial ancestor, many different marsupial species (resembling wolves, bears, rabbits, horses) evolved within the isolated Australian continent.
When multiple adaptive radiations occur in isolated geographical areas with similar habitats, it is called convergent evolution — unrelated lineages independently evolve similar traits due to similar selection pressures.
The placental wolf of North America and the Tasmanian wolf (marsupial) of Australia are an example of convergent evolution — their similar body plans evolved independently in response to similar ecological roles, not from a common ancestor.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 8

Long Form

Trace the evolution of man from early primates to modern Homo sapiens, highlighting key changes in brain size and behaviour.

About 15 mya, Dryopithecus (ape-like) and Ramapithecus (man-like) existed; hominid fossils from Ethiopia and Tanzania show that 3–4 mya, primates walked upright in eastern Africa but were not taller than 4 feet.
Two mya, Australopithecines lived in East African grasslands, hunted with stone weapons, and ate fruit; Homo habilis, the first hominid, had a brain capacity of 650–800cc and probably did not eat meat.
Homo erectus (1.5 mya), discovered in Java, had a larger brain of ~900cc, ate meat, and represents a significant advance in cognitive and dietary evolution.
Neanderthal man (100,000–40,000 years ago), with a brain of 1400cc, used hides, buried dead, and showed cultural behaviour; Homo sapiens arose in Africa and spread to develop distinct racial groups.
Modern Homo sapiens emerged during the ice age (75,000–10,000 years ago); cave art appeared ~18,000 years ago (e.g., Bhimbetka, Madhya Pradesh); agriculture and permanent settlements began ~10,000 years ago, marking the beginning of human civilisation.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 9

Long Form

Analyse the mechanisms by which natural selection can alter the distribution of traits in a population. Describe all three possible outcomes.

Natural selection acts on heritable variation in a population, favouring individuals whose traits better suit the environment; over generations this changes the frequency of traits in the population.
Stabilising selection: more individuals acquire the mean character value; extreme variants are selected against, reducing variation and maintaining the population close to the current mean (e.g., birth weight in humans).
Directional selection: more individuals acquire a value other than the current mean, shifting the population in one direction (e.g., industrial melanism where darker moths increased in frequency under soot-covered conditions).
Disruptive selection: more individuals acquire peripheral character values at both ends of the distribution, while intermediate forms are selected against; this can lead to two distinct subpopulations and eventually speciation.
All three outcomes demonstrate that natural selection is not always progressive or directed — it can maintain, shift, or split populations depending on which phenotypes confer reproductive advantage in a given environment.

ByteNotes

Long Answer

Hard difficulty • Structured explanation

Hard

Question 10

Long Form

Explain how Darwinian theory of natural selection accounts for the origin of new species from an existing population.

All populations have built-in heritable variation; no two individuals are exactly alike, and these variations are mostly inherited, providing the raw material for natural selection.
Natural resources are limited and population sizes are relatively stable; this means there is competition for survival, and not all individuals can reproduce maximally.
Individuals whose heritable traits enable better resource utilisation and survival in the current environment leave more progeny than others — these traits increase in frequency over generations.
Over many generations, the survivors (well-adapted individuals) leave more progeny, gradually shifting population characteristics; over long time spans (especially in organisms with long life cycles), populations diverge significantly.
Through branching descent — when populations are geographically or reproductively separated — different selection pressures on the two groups can lead to such divergence that they become distinct species, illustrating how new species arise from existing ones.

ByteNotes

Premium Access

Unlock all Long Questions

You are viewing 10 free long questions from this chapter. Upgrade to Premium to unlock the remaining 10 questions.

Upgrade to Premium10 questions locked

Chapter sections

Chapter 6: Evolution | Long Questions

Compare the views of Lamarck and Darwin on the mechanism of evolution. Which theory is currently accepted and why?

Describe Miller's experiment. What conclusions did it support and what questions did it leave unanswered?

Analyse how the Hardy-Weinberg principle connects genetics to evolution, and explain how its violation leads to evolutionary change.

Trace the chronological evolution of life forms on earth from the first cellular organisms to the appearance of mammals.

Explain the different types of evidence that support the theory of evolution, and evaluate their relative strengths.

Describe the origin of the universe and earth, and explain how conditions on early earth set the stage for the origin of life.

Explain adaptive radiation with two examples. How does it relate to the concept of convergent evolution?

Trace the evolution of man from early primates to modern Homo sapiens, highlighting key changes in brain size and behaviour.

Analyse the mechanisms by which natural selection can alter the distribution of traits in a population. Describe all three possible outcomes.

Explain how Darwinian theory of natural selection accounts for the origin of new species from an existing population.

Unlock all Long Questions