Evolution | ISC Class 12 Biology Notes

This note covers the origin of life, evidence for evolution, geological history, Darwin's explanation of evolutionary change, variation and selection, population genetics, adaptive radiation and human evolution.

How could life have originated from non-living matter?

Biological evolution is change in inherited characteristics, passed from parents to offspring, within populations over generations. A population is a group of individuals of the same species in an area; a species is a group whose members can interbreed and produce fertile offspring under natural conditions.

Abiogenesis, biogenesis and earlier views

Abiogenesis means the origin of life from non-living matter. Biogenesis means that living organisms arise from pre-existing living organisms. These ideas address different situations when abiogenesis refers to the first origin of life under early terrestrial conditions.

Panspermia proposes that units of life reached Earth from elsewhere in space. Spontaneous generation was the belief that living organisms arose directly from decaying material, such as straw or mud, under ordinary conditions.

Louis Pasteur's experiments opposed spontaneous generation. Pre-sterilised flasks containing killed yeast did not develop life when contamination was prevented; exposure to air allowed new organisms to appear. Sterilisation means destroying existing microorganisms. This result did not explain how the first life originated.

Oparin and Haldane's chemical evolution

Chemical evolution is the formation of increasingly complex organic molecules from simpler, non-living constituents. Oparin and Haldane proposed that such chemical changes preceded life. Their explanation involved high temperatures, volcanic activity and a reducing atmosphere, favouring reduction, or electron gain, and lacking free molecular oxygen.

Free oxygen would oxidise, or remove electrons from, many newly formed organic compounds, hindering their accumulation. Thus, oxygen-poor conditions are important in this proposed route of abiotic synthesis, meaning chemical production without living organisms.

Protobionts are proposed pre-cellular aggregates of organic molecules with a boundary separating their internal contents from the surroundings. Coacervates are droplets formed by aggregation of organic molecules in water. They model compartment formation but are not complete living cells.

A self-replicating system can make copies of its components; metabolism is the set of chemical reactions associated with life. The transition from organic molecules to a system combining these properties is distinct from merely producing organic compounds.

Note: The first non-cellular forms could have originated about three billion years ago. The first cellular forms did not possibly originate until about 2,000 million years ago and were probably single-celled. How the first self-replicating metabolic system arose remains unknown.

What did the Miller and Urey experiment demonstrate?

The Miller and Urey experiment tested whether organic compounds could form under simulated early-Earth conditions. In 1953, S. L. Miller produced an electric discharge in a closed apparatus containing methane, hydrogen, ammonia and water vapour.

Here CH₄ denotes methane, H₂ molecular hydrogen, NH₃ ammonia and H₂O water. Electric sparks supplied energy to the gas mixture. The experiment produced amino acids, the building units of proteins, which are chains of amino acids.

How did the apparatus operate?

1. Water was boiled in a flask, supplying water vapour to the connected apparatus.
2. The vapour circulated with methane, ammonia and hydrogen through the chamber containing electrodes.
3. Electric discharge between the electrodes provided energy for reactions within the gas mixture.
4. A condenser, a device that cools vapour into liquid, cooled the circulating mixture.
5. Liquid collected in a trap, allowing the organic products in the water to be examined.

A boiling-water flask connects to a gas chamber with electrodes and a spark discharge. The return path contains a condenser labelled water in and water out, and a liquid trap containing water with organic compounds.

See Fig. 6.1 in your NCERT textbook

What conclusion is justified?

The result supports the possibility of abiotic formation of organic molecules. It does not show that a complete living cell was created. Organic building materials and a reproducing cellular system are different levels of organisation.

Similar experiments produced sugars, nitrogen bases, pigments and fats. Nitrogen bases are nitrogen-containing components of genetic material; pigments are coloured substances. Related compounds in meteorites provide additional evidence that organic synthesis can occur beyond living systems. These observations support chemical evolution while leaving the origin of the first living system unresolved.

How do fossils and radioactive dating provide evidence for evolution?

Fossils are preserved remains or traces of past organisms; hard parts preserved in rocks are common examples. Palaeontology is the study of past life through fossils. Fossils show that organisms living in earlier times differed from those living today.

Reading the fossil record

Sediments are materials deposited in layers that can form sedimentary rocks. Different-aged layers contain different fossil assemblages, meaning groups of fossils found together. Some forms resemble living organisms, whereas others represent extinct groups. Extinction is the disappearance of a species or group.

Comparing fossil-bearing layers reveals the succession of life through geological time. Certain organisms are restricted to particular time intervals. This supports the conclusion that new forms appeared at different times and that the diversity of life has changed.

What does carbon dating measure?

Radioactive dating estimates age using the predictable decay of unstable atoms. An isotope is a form of an element with the same number of positively charged protons but a different number of uncharged neutrons in its atomic nucleus. Carbon-14 is a radioactive isotope of carbon.

Living organisms exchange carbon with their surroundings. After death, this exchange stops and the carbon-14 already present decays. Measuring the remaining carbon-14 helps estimate the age of suitable organic remains. A half-life is the time taken for half the radioactive atoms present to decay.

Carbon dating applies to relatively recent organic material. It cannot date all fossils, particularly very ancient remains or mineralised material without suitable original carbon. Other radioactive dating methods are needed for much older geological material.

Note: A fossil's position in a sequence of layers and a radioactive age estimate are different kinds of information. The first helps establish relative succession; the second can provide an estimate in years when the material and method are suitable.

What does the geological time scale show about changing life?

The geological time scale arranges Earth's history into named intervals, including eras and their subdivisions, periods. Flora means the plant life of a region or interval; fauna means its animal life. Their distribution through time records major changes in biodiversity, the variety of life.

Major intervals and characteristic organisms

The sequence runs from older to younger intervals and lists characteristic groups. Algae are simple photosynthetic organisms; photosynthesis uses light energy to make organic food. Gymnosperms have seeds not enclosed in fruits; angiosperms are flowering plants with seeds enclosed in fruits.

Invertebrates lack a backbone; vertebrates possess a backbone, or vertebral column; trilobites were extinct marine arthropods. Arthropods have jointed appendages. Mammals are vertebrates that feed their young with milk. Vascular plants possess conducting tissues. Amphibians include frogs and salamanders; reptiles include turtles and crocodiles.

How should the sequence be interpreted?

Plants spread on land before animals invaded it. Lobefinned fishes are linked with the ancestry of amphibians, and amphibians with reptiles. Reptilian eggs had protective shells that prevented drying. These changes connect structure with the conditions under which organisms survived.

Reptiles dominated for a long interval, while later mammals became prominent. This is a branching history involving surviving and extinct groups. It should not be read as a claim that every earlier organism changed directly into a later one.

Monocotyledons have one seed leaf, or cotyledon, whereas dicotyledons have two.

The drawing places plant groups beside labelled periods from Silurian upwards. Branching bands connect earlier forms with later groups, including ferns, conifers and flowering plants; the flowering-plant band divides into monocotyledons and dicotyledons.

See Fig. 6.9 in your NCERT textbook

How do homologous and analogous organs differ?

Comparative anatomy compares internal structures, while morphology concerns form and structural features. Such comparisons help distinguish resemblance caused by shared ancestry from resemblance associated with similar functions.

Divergent evolution involves related forms developing different adaptations from a common ancestral plan. Convergent evolution involves different groups developing similar functional adaptations. An adaptation is an inherited feature that helps survival or reproduction in particular conditions.

Homology and analogy compared

Which examples illustrate the distinction?

A tendril is a slender, coiling structure providing climbing support. A thorn is a modified stem, while the cactus spine in this comparison is a modified leaf.

The plant panel labels the Bougainvillea thorn and Cucurbita tendril. The animal panel places human, cheetah, whale and bat forelimb drawings beneath the corresponding animals, showing their shared bone arrangement despite different shapes.

See Fig. 6.3 in your NCERT textbook

How do embryos, vestigial organs and molecules support evolution?

Embryology and recapitulation

Embryology studies early development. Ontogeny is the development of an individual organism; phylogeny is the evolutionary history of a species or group. Ontogeny concerns one life history, whereas phylogeny concerns descent across generations.

Ernst Haeckel's theory of recapitulation proposed that ontogeny repeats phylogeny. However, Karl Ernst von Baer's observations showed that embryos do not pass through the adult stages of other animals. Early developmental similarities must not be interpreted as literal repetition of ancestral adult forms.

Vertebrate embryos share features around the region behind the head associated with the gill apparatus of fishes. Gills are organs that exchange gases with water. Such similarities can support relationship, but a human embryo is not an adult fish at any developmental stage.

Vestigial organs

Vestigial organs are reduced structures whose ancestral function has been lost or greatly reduced. Examples include the human coccyx, the remnant of an ancestral tail, and the reduced hindlimb bones of whales. A vestigial structure need not lack every present function.

Molecular evidence

Molecular evidence includes similarities in genes and proteins among organisms. A gene is a unit of heredity represented by a sequence of genetic material; heredity is the transmission of characteristics between generations. A genome is an organism's complete genetic material.

Similarities between corresponding genes and proteins give clues to common ancestry. DNA, deoxyribonucleic acid, stores genetic information; RNA, ribonucleic acid, participates in its expression. The genetic code links information in RNA with the amino-acid sequence of a protein.

A codon is a three-base sequence specifying an amino acid or a stop signal during protein synthesis. Bases are the information-bearing chemical components of nucleic acids. The genetic code is nearly universal, with exceptions including mitochondrial codons and some protozoans.

Mitochondria are cellular structures involved in energy release; protozoans are single-celled, animal-like organisms.

How did Darwin explain evolution by natural selection?

Natural selection is differential survival and reproduction associated with inherited differences under particular environmental conditions. Darwin's two central concepts were natural selection and branching descent, the diversification of lineages, or lines of descent, from shared ancestors. Alfred Wallace independently reached similar conclusions.

From variation to reproductive success

Variation means differences among individuals. Heritable variation can be passed to offspring. Darwin connected population differences with limited resources and unequal reproductive success. It is possible that Thomas Malthus's work on populations influenced this reasoning.

1. Organisms have the capacity to produce more offspring than limited resources can support.
2. Competition for resources creates a struggle for existence among individuals.
3. Individuals differ in characteristics, and most of these variations are inherited.
4. Some inherited differences improve survival and resource use in the prevailing environment.
5. Individuals with advantageous differences leave more offspring, changing population characteristics over many generations.

Reproductive fitness means success in leaving offspring. A useful trait must be considered in relation to the environment in which its bearer survives and reproduces.

Giraffes, Lamarck and the limits of Darwinism

Lamarck proposed that use and disuse altered organs and that acquired changes were inherited. His giraffe explanation involved repeated stretching to reach high leaves, followed by inheritance of an elongated neck. This acquired-character explanation is not accepted as the explanation of giraffe evolution.

The natural-selection explanation instead begins with inherited variation in neck length. Where higher foliage provides a feeding advantage, longer-necked variants may survive and reproduce more successfully. Across generations, selection can increase the representation of this inherited characteristic.

Darwin's account explained the selection of useful variations but did not adequately explain their genetic origin or mechanism of inheritance. Later genetics supplied these missing mechanisms. Selection acts on existing heritable differences; need alone does not produce the required inherited change.

How do industrial melanism and resistance demonstrate selection?

Industrial melanism

Industrial melanism is the increased representation of dark forms in populations associated with industrial environmental change. In English moth collections from the 1850s, white-winged forms were more numerous. In a collection from the same area in 1920, dark forms were more numerous.

Before industrialisation, pale lichens helped conceal pale moths. A lichen is an association between a fungus and an alga or cyanobacterium, a photosynthetic bacterium. Soot darkened tree trunks, changing which moths predators could detect more easily.

Camouflage means concealment against the background. Dark moths were better concealed on darkened trunks, whereas pale moths stood out. Differential predation changed the relative abundance of the forms. No variant was completely wiped out.

Antibiotics and insecticides

Resistance is the ability to survive a treatment that harms susceptible organisms. An antibiotic acts against bacteria; an insecticide acts against insects. DDT, dichlorodiphenyltrichloroethane, is an insecticide used in the mosquito-resistance example.

If resistant variants already exist, exposure removes more susceptible individuals and favours resistant survivors. Their descendants increase the resistant fraction. Mosquito resistance to DDT and bacterial antibiotic resistance illustrate this process. Exposure does not imply that every individual deliberately develops a needed adaptation.

What did Lederberg's replica plating show?

Replica plating transfers bacteria from colonies in a preserved spatial arrangement to fresh growth surfaces. A colony is a visible population descended from multiplying bacterial cells. A master plate is the original culture plate from which replicas are made.

1. Grow separate bacterial colonies on a master plate without the selecting antibiotic.
2. Use sterile velvet to pick up cells while preserving the positions of colonies.
3. Transfer the pattern to plates containing antibiotic and compare growth with an antibiotic-free control plate.
4. Locate resistant colonies growing at corresponding positions on the selective replicas.
5. Recover cells from the matching positions on the original master plate to show that resistant bacteria existed before exposure.

The result supports selection of pre-existing variants. The antibiotic reveals resistant variants already represented on the master plate; it does not instruct bacteria to make a specific useful mutation. A mutation is a change in genetic material.

How do stabilising, directional and disruptive selection differ?

A phenotype is an observable characteristic of an organism. A population may contain a range of values for a characteristic. Selection changes the representation of those values when phenotypes differ in survival and reproduction.

Three patterns of selection

Stabilising selection favours the central form rather than causing a progressive shift towards an extreme. Directional selection shifts the distribution in one direction. Disruptive selection favours contrasting forms at both ends rather than one central form.

The vertical axis represents numbers of individuals with a phenotype. Panel (a) shows a higher, narrower central peak; panel (b) shows a peak shifted to one side; panel (c) shows two separated peaks.

See Fig. 6.8 in your NCERT textbook

The horizontal direction represents variation in the characteristic. Before interpreting any selection graph, identify both axes and compare the earlier distribution with the later one.

These patterns concern which variants reproduce successfully. They do not mean that mutation itself anticipates environmental requirements. Mutations are random and directionless with respect to the useful adaptation needed.

How does the modern synthetic theory explain evolutionary change?

Neo-Darwinism, or the modern synthetic theory, integrates natural selection with genetics and changes in populations. An allele is an alternative form of a gene. Allele frequency is its proportion among all copies of that gene in a population.

Mutation and recombination

Genetic recombination produces new combinations of existing genetic material during sexual reproduction. Mutation can create new alleles, whereas recombination reshuffles inherited variants. These processes provide heritable differences on which selection can act.

Hugo de Vries, working with evening primrose, proposed that large, sudden mutations could produce new species. He called such a single-step evolutionary jump saltation. His mutation theory contrasted with Darwin's emphasis on gradual change through small heritable variations.

Speciation is the formation of new species. Mutation contributes to variation, but evolutionary change is not explained by mutation alone. Selection, migration and chance changes also affect the genetic composition of populations.

Gene flow, drift and small populations

Gene flow is the transfer of alleles between populations through migration and reproduction. Such movement can alter allele frequencies in both the population receiving migrants and the population they leave. Genetic drift is change in allele frequencies through chance sampling.

The founder effect occurs when a small founding group carries an unrepresentative sample of the original population's alleles. The bottleneck effect follows a sharp reduction in population size, leaving survivors whose allele frequencies may differ by chance from the earlier population.

Both effects involve sampling. Neither guarantees improvement or the formation of a new species. Natural selection differs because the change is associated with differences in inherited reproductive success under environmental conditions.

How is Hardy-Weinberg equilibrium used to study populations?

The Hardy-Weinberg principle describes constancy of allele frequencies across generations under specified conditions. The gene pool comprises all genes and their alleles in a population. Constancy of its allele frequencies is called genetic equilibrium.

Symbols, assumptions and equations

Consider a gene with two alleles, A and a, merely labels for the alternative forms. Let p be the frequency of A and q the frequency of a. Together they account for all copies of that gene.

A diploid individual has two sets of chromosomes, the structures carrying genes. A genotype is an individual's genetic constitution. At this gene, AA and aa are homozygous, having identical alleles; Aa is heterozygous, having different alleles.

p + q = 1. Under equilibrium conditions, p² + 2pq + q² = 1. Here p² is the frequency of AA, 2pq the frequency of Aa, and q² the frequency of aa. The superscript ² means squared; pq means p multiplied by q.

The model assumes a sufficiently large population, random mating, and no mutation, migration or natural selection affecting the gene. Random mating means pairing is independent of the genotype being considered. A large population minimises the effect of chance sampling.

How are the genotype frequencies obtained?

1. An offspring receives one allele from each parent through their reproductive cells, or gametes.
2. The probability of receiving A from each side is p multiplied by p, giving p² for AA.
3. The probability of receiving a from each side is q multiplied by q, giving q² for aa.
4. Aa can arise through either order of contributions, giving pq + qp = 2pq.
5. Adding the three genotype frequencies gives the total population frequency of one.

Worked calculation: If the two alleles are equally frequent, p = q. Since p + q = 1, each equals 0.5. Therefore AA = 0.25, Aa = 0.50 and aa = 0.25. These are calculated proportions for the stated condition, not observations from a sampled population.

Gene migration, genetic drift, mutation, genetic recombination and natural selection are factors involved in evolutionary change. A departure from expected proportions calls for examining the model's assumptions; a measured change in allele frequencies across generations indicates genetic change in the population.

Do not equate q with q²: the former counts the proportion of one allele, while the latter is an expected genotype proportion. Recombination reshuffles combinations and does not, by itself, necessarily change the frequency of an allele at a single gene.

How does adaptive radiation relate to biogeography?

Biogeography studies the geographical distribution of organisms. Geographical separation and differing habitats help explain why related organisms may become different and why distinct groups may develop similar adaptations.

Adaptive radiation is diversification from an ancestral stock into different forms adapted to different habitats or ways of life within a geographical region. It is a pattern of branching evolution, involving related forms with contrasting adaptations.

Darwin's finches

On the Galapagos Islands, Darwin observed finches with different beaks. He conjectured that the varieties evolved on the islands. From original seed-eating features, forms with altered beaks arose, enabling different feeding habits, including insect-eating and other plant-based feeding.

The connection between related birds, a shared geographical setting and modified feeding structures makes the finches an example of adaptive radiation. The explanation depends on inherited differences accumulating through generations, not beaks changing because individual birds decide to eat different food.

Australian marsupials and convergence

Marsupials are mammals whose young are born relatively undeveloped and usually continue development attached to maternal teats, often in a pouch. Australian marsupials diversified from ancestral stock within the Australian continent, producing forms adapted to different ways of life.

Placental mammals nourish developing young through a well-developed placenta, an organ linking maternal and embryonic tissues. Similarities between some placental mammals and marsupials illustrate convergence between separate lineages.

Arrows extend from a central circle labelled marsupial radiation to drawings including kangaroo, wombat, koala, marsupial mole and Tasmanian wolf. The branching arrangement represents diversification within the group.

See Fig. 6.6 in your NCERT textbook

The placental wolf and Tasmanian wolf illustrate similar outward adaptations in different mammalian groups. Radiation compares diversification within an ancestral group; convergence compares similarities that develop in distinct groups.

What features distinguish the major forms in human evolution?

Human evolution is reconstructed from fossils and other evidence of past life. Primates are the mammalian group including apes and humans. Cranial capacity is the internal volume of the braincase; cm³ means cubic centimetres. It provides evidence about brain size, not a complete measure of behaviour.

Comparing physical and behavioural features

Bipedal means walking on two legs. Posture describes how the body is held. Cro-Magnon refers to early modern humans in Europe.

The named forms should not be interpreted as a proven single-file sequence of direct ancestors.

What broad chronology accompanies these forms?

Dryopithecus and Ramapithecus existed about 15 million years ago. Australopithecines probably lived in East African grasslands about two million years ago. Fossils from Java represent Homo erectus about 1.5 million years ago.

Neanderthals lived in the Near East and central Asia between 1,00,000 and 40,000 years ago. Homo sapiens arose in Africa and spread across continents. Modern Homo sapiens arose during the ice-age interval between 75,000 and 10,000 years ago.

Prehistoric cave art developed about 18,000 years ago. Agriculture began around 10,000 years ago, followed by human settlements. The record combines changes in body structure with changes in food use, protection, communication and social behaviour.

Glossary

• Abiogenesis — Origin of life from non-living material through processes proposed for early terrestrial conditions.
• Biogenesis — The production of living organisms from other living organisms already present.
• Coacervate — A droplet formed by aggregation of organic molecules in an aqueous environment.
• Homology — Correspondence in basic structure and origin indicating shared ancestry despite possible functional differences.
• Analogy — Functional similarity between structures with different evolutionary origins and underlying construction.
• Ontogeny — The development of an individual organism through its own life history.
• Phylogeny — The evolutionary history and relationships of a species or larger group.
• Reproductive fitness — Relative success in producing offspring that contribute to subsequent generations.
• Natural selection — Differential survival and reproduction associated with inherited differences under particular environmental conditions.
• Genetic drift — A change in population allele frequencies resulting from chance sampling effects.
• Gene flow — Transfer of alleles between populations through migration followed by reproductive contribution.
• Founder effect — Genetic drift associated with an unrepresentative sample establishing a new population.
• Bottleneck effect — Chance alteration of genetic composition following a sharp reduction in population size.
• Genetic equilibrium — Constancy of allele frequencies across generations under the conditions of the population model.
• Adaptive radiation — Diversification from ancestral stock into forms adapted to different habitats or ways of life.

Common errors and misconceptions

• Misconception: Miller produced living cells. Correct: His experiment produced organic compounds, including amino acids, supporting chemical evolution without creating a complete living organism.
• Misconception: Organs with the same function must be homologous. Correct: Common origin and basic structure establish homology; similar functions can evolve independently.
• Misconception: A human embryo passes through an adult fish stage. Correct: Developmental similarities do not mean that embryos repeat the adult stages of other animals.
• Misconception: Fitness means strength alone. Correct: Darwinian fitness concerns reproductive success under particular environmental conditions.
• Misconception: Antibiotics direct bacteria to produce the needed resistance. Correct: Replica plating demonstrates the selection of resistant variants that already existed before exposure.
• Misconception: Genetic drift always improves adaptation. Correct: Drift is a chance process and need not favour an advantageous allele.
• Misconception: q and q² describe the same frequency. Correct: q is an allele frequency; q² is the expected frequency of the aa genotype under Hardy-Weinberg conditions.
• Misconception: All human fossil forms form a confirmed direct ancestral chain. Correct: Fossils support comparisons and inferred relationships, not an automatically proven single sequence.

Exam-style questions with model answers

Q1. Define abiogenesis and biogenesis, distinguishing their proposed sources of living organisms. [2 marks]

1. Abiogenesis is the origin of life from non-living material, as proposed for the first emergence of life.
2. Biogenesis is the production of living organisms from pre-existing living organisms, as demonstrated under ordinary conditions by Pasteur's experiments.

Q2. A closed apparatus contains methane, ammonia, hydrogen and water vapour. It has electrodes, a condenser and a liquid trap; amino acids are recovered after sparking. Explain the energy source, cooling step, result and justified conclusion. [4 marks]

1. The electric sparks provide energy that allows reactions among the gases and water vapour in the apparatus.
2. The condenser cools the circulating vapour into liquid, which can collect in the trap for examination.
3. The recovered amino acids show that organic building materials of proteins have formed from the supplied mixture.
4. The experiment supports abiotic organic synthesis under the stated conditions, but it does not demonstrate creation of a living cell.

Q3. Bougainvillea thorns are modified stems used for protection, and Cucurbita tendrils are stem modifications for climbing. Sweet potato stores food in a modified root, whereas potato stores food in a modified stem. Classify each pair and explain its evolutionary significance. [4 marks]

1. Bougainvillea thorns and Cucurbita tendrils are homologous because both are modifications of the same original organ, the stem.
2. Their different protective and climbing functions illustrate divergence from a common structural plan into different adaptations.
3. Sweet potato and potato are analogous storage organs because the storage structures have different origins, as root and stem respectively.
4. The shared storage function despite different origin illustrates functional convergence rather than homology of the compared organs.

Q4. A mixed moth population contains inherited pale and dark forms. Pale lichens initially cover the trunks; industrial soot then darkens them. Birds more easily capture contrasting moths, and better-concealed moths leave more offspring. Explain the resulting natural selection in five stages. [5 marks]

1. The starting population already contains inherited pale and dark variants. The environmental change therefore acts on existing variation rather than requiring a new colour to appear deliberately.
2. On the initially pale, lichen-covered trunks, pale moths are less conspicuous and have an advantage in avoiding capture by the birds.
3. Soot darkens the trunks and changes the background. Dark moths now blend into the surroundings more effectively than pale moths.
4. Birds capture more conspicuous moths more easily. Better-concealed dark individuals consequently survive and leave more offspring under the stated conditions.
5. Inheritance and unequal reproduction increase the dark form's representation over generations. This population change illustrates natural selection associated with industrial melanism.

Q5. Colonies from an antibiotic-free master plate are copied with sterile velvet onto antibiotic plates, preserving their positions. Resistant colonies grow at matching positions, and resistant cells can be recovered from those positions on the original plate. Explain what the procedure establishes in three points. [3 marks]

1. Preserving colony positions makes it possible to identify which original colonies correspond to the survivors on the antibiotic-containing replicas.
2. Recovery of resistant cells from the original antibiotic-free plate shows that resistant variants were present before the selecting treatment.
3. The antibiotic therefore selects existing resistant bacteria. The observations do not support the claim that exposure directs the production of the particular resistance needed.

Q6. A diploid population is in Hardy-Weinberg equilibrium for a gene with alleles A and a, present equally frequently. Let p and q be their respective frequencies. Using p + q = 1 and genotype frequencies AA = p², Aa = 2pq and aa = q², calculate p, q and all three genotype frequencies in four steps. [4 marks]

1. Equal allele frequencies mean p = q. Substitution into p + q = 1 gives 2p = 1, so p = 0.5 and q = 0.5.
2. The expected frequency of AA is p² = 0.5 × 0.5 = 0.25, the proportion receiving A from both parental contributions.
3. The expected frequency of Aa is 2pq = 2 × 0.5 × 0.5 = 0.50, including both possible orders of the two different alleles.
4. The expected frequency of aa is q² = 0.5 × 0.5 = 0.25. The genotype frequencies sum to 0.25 + 0.50 + 0.25 = 1.

Q7. Define adaptive radiation and explain it using Darwin's finches and Australian marsupials. Distinguish it from convergence between placental and marsupial mammals. [5 marks]

1. Adaptive radiation is diversification from ancestral stock into different forms adapted to different habitats or ways of life within a geographical region.
2. Darwin's finches illustrate this pattern through related island birds with different beaks associated with different kinds of feeding, including seed-eating and insect-eating.
3. Darwin conjectured that the finch varieties evolved on the islands, with modified beaks arising from the original seed-eating features of ancestral forms.
4. Australian marsupials provide another example because different forms developed from ancestral stock within Australia, occupying different habitats and ways of life.
5. Convergence instead concerns similar adaptations in distinct lineages, illustrated by the placental wolf and Tasmanian wolf. Similarity between those groups differs from radiation within one ancestral group.

Q8. Compare Homo habilis, Homo erectus and Homo neanderthalensis, giving one brain-size feature and one dietary or behavioural feature for each. [3 marks]

1. Homo habilis had a brain capacity between 650 and 800 cm³, where cm³ means cubic centimetres. It probably did not eat meat.
2. Homo erectus had a brain around 900 cm³. Dietary reconstruction indicates that this human form probably ate meat.
3. Homo neanderthalensis had a brain size of 1,400 cm³. A behavioural feature was burial of the dead, indicating a social practice in this group.

Key takeaways

• Chemical evolution explains a proposed route towards life's building materials, while the origin of the first self-replicating living system remains unresolved.
• Fossils, structural comparisons, developmental similarities and molecular evidence provide different ways of investigating evolutionary relationships.
• Homologous organs share an underlying origin; analogous organs perform similar functions despite different evolutionary origins.
• Natural selection depends on inherited variation and unequal reproductive success within a particular environment.
• Replica plating supports the existence of resistant bacterial variants before exposure to the selecting antibiotic.
• Mutation, recombination, gene flow, drift and selection contribute differently to evolutionary changes in populations.
• Hardy-Weinberg calculations distinguish allele frequencies from genotype frequencies and depend on the assumptions of the population model.
• Adaptive radiation describes diversification within ancestral groups; human evolution combines evidence of anatomical change with behaviour and culture.

Test yourself

Why did Pasteur's experiments not settle the first origin of life?

They tested whether life appeared from non-living material under the experimental conditions, not how the first life arose on early Earth.

Why is “nearly universal” more accurate than “universal” for the genetic code?

The code is widely shared, but exceptions occur in mitochondrial codons and some protozoans.

What distinguishes ontogeny from phylogeny?

Ontogeny concerns an individual's development; phylogeny concerns the evolutionary history of a species or group.

What feature distinguishes disruptive from directional selection?

Disruptive selection favours variants at both extremes, whereas directional selection favours values towards one extreme.

How does a bottleneck differ from a founder effect?

A bottleneck follows a sharp reduction in an existing population; a founder effect involves a small group establishing another population.

In the Hardy-Weinberg equation, what does 2pq represent?

It represents the expected frequency of heterozygous Aa individuals when the model's conditions apply.

Why should resistance not be described as a deliberate response?

Selection favours heritable resistant variants; organisms do not direct mutations towards whatever adaptation a treatment requires.

Why is brain size alone insufficient for comparing human fossil forms?

Comparison also requires other evidence, including posture, dietary reconstruction, tool use and social behaviour.