RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Evolution asks the largest question in biology: how did the enormous variety of living forms come to exist from a lifeless early Earth? You will move from cosmic origins and Oparin's chemical evolution to Darwin's natural selection, the evidence from fossils, homologous organs and molecular sequences, and the algebra of Hardy-Weinberg equilibrium. Notice throughout that evolution is a change in the gene pool of a population, never in a single individual.
The wings of a butterfly and the wings of a bat perform the same function but have entirely different structural origins. Such organs are described as:
Answer
Analogous is correct — analogous structures have different embryonic and anatomical origins but serve a similar function because the organisms adapted to a similar way of life, an outcome called convergent evolution. Homologous organs are the opposite case, sharing a common origin while differing in function, and vestigial organs are reduced remnants that no longer serve their original purpose.
In a large randomly mating population at Hardy-Weinberg equilibrium, 16 percent of individuals show a recessive trait. What is the frequency of the dominant allele?
Answer
0.6 is correct — the recessive phenotype frequency is q² = 0.16, so q = 0.4 and therefore p = 1 − q = 0.6. The value 0.4 is the recessive allele frequency, 0.16 is the recessive genotype frequency, and 0.84 is the frequency of all individuals showing the dominant phenotype rather than the allele frequency.
The Miller and Urey experiment provided experimental support for the idea that:
Answer
Simple organic molecules could form abiotically under primitive Earth conditions is correct — by passing electric discharges through a closed flask containing CH₄, NH₃, H₂, and water vapour at high temperature, they obtained amino acids and other organic compounds. This supported the Oparin-Haldane proposal of chemical evolution preceding biological evolution; it says nothing about panspermia, common descent from a fossil species, or Lamarckian inheritance.
A severe cyclone wipes out most of a small island bird population at random, and the few survivors happen to carry allele frequencies quite different from the original population. This is an example of:
Answer
Genetic drift is correct — specifically the bottleneck effect, a chance change in allele frequency caused by the random survival of a small number of individuals. It is not selection of any kind, because the survivors were not better adapted; they simply happened to survive. Gene flow would require migration of individuals between populations.
Assertion (A): The widespread use of antibiotics has led to bacterial populations in which most cells are resistant. Reason (R): Exposure to an antibiotic causes bacteria to develop resistance genes in response to the drug.
Answer
A is true but R is false — resistance does become widespread, but not because the antibiotic instructs bacteria to develop resistance. Resistant variants arise beforehand by random mutation; the antibiotic then acts as a selective agent, killing the susceptible cells and leaving the pre-existing resistant ones to multiply and dominate the population. The reason as stated is a Lamarckian explanation and is not accepted.
Distinguish between divergent and convergent evolution, giving one example of each.
Answer
Divergent evolution occurs when a single ancestral structure is modified in different descendant species to perform different functions, because the descendants adapt to different environments. The forelimbs of a human, a whale, a bat, and a cheetah all share the same basic bone plan but are used for grasping, swimming, flying, and running respectively. Convergent evolution occurs when unrelated species independently develop similar structures because they face similar environmental demands. The streamlined body of a shark and that of a dolphin, or the wings of an insect and of a bird, are similar in function but have entirely different origins.
State the Hardy-Weinberg principle and list any three factors that can disturb the equilibrium it describes.
Answer
The Hardy-Weinberg principle states that in a large, randomly mating population the allele frequencies and genotype frequencies remain constant from generation to generation, provided no evolutionary force acts on it. If p is the frequency of one allele and q of the other, then p + q = 1 and the genotype frequencies follow p² + 2pq + q² = 1. A measured departure from these expected values is taken as evidence that evolution is occurring. Three factors that disturb the equilibrium are: gene flow, in which migration adds or removes alleles from the population; genetic drift, in which allele frequencies change purely by chance in a small population; and natural selection, in which certain phenotypes survive and reproduce more successfully than others. Mutation and genetic recombination are two further factors.
Explain how the peppered moth population of industrial England illustrates natural selection.
Answer
Before industrialisation, tree trunks were covered with pale lichens, so the light-coloured moths were well camouflaged while dark moths were conspicuous and were eaten more often by birds; light moths were therefore the majority. As soot from factories killed the lichens and blackened the trunks, the situation reversed — dark moths blended in and light moths stood out and were picked off. Within a few decades the dark form came to predominate in industrial areas, while unpolluted rural areas still had mostly light moths. No moth changed colour; the environment simply changed which pre-existing variant survived and bred better, which is precisely what natural selection means.
In a village of 2,500 people, 225 individuals are found to have a recessively inherited condition. Assuming the population is at Hardy-Weinberg equilibrium, calculate the allele frequencies and the expected number of carriers, showing your working.
Answer
Step 1 — find q². The recessive condition appears only in homozygous recessive individuals, whose frequency is q². Here q² = 225 ÷ 2500 = 0.09. Step 2 — find q. Taking the square root, q = √0.09 = 0.3. So the recessive allele has a frequency of 0.3, or 30 percent of all alleles at this locus in the population. Step 3 — find p. Since p + q = 1, p = 1 − 0.3 = 0.7. The dominant allele frequency is 0.7. Step 4 — find the carrier frequency. Carriers are heterozygotes, whose frequency is 2pq = 2 × 0.7 × 0.3 = 0.42, that is 42 percent of the population. Step 5 — convert to numbers. Expected carriers = 0.42 × 2500 = 1,050 individuals. For completeness, homozygous dominant individuals number p² × 2500 = 0.49 × 2500 = 1,225, and 1,225 + 1,050 + 225 = 2,500, which confirms the arithmetic. Note that carriers outnumber affected individuals by more than four to one — this is why a recessive allele can persist at appreciable frequency in a population even when the condition itself is uncommon. This calculation is valid only if the assumptions hold: a large population, random mating, and no mutation, migration, drift, or selection acting on this locus.
Summarise the main lines of evidence that support the theory of organic evolution.
Answer
Palaeontological evidence: fossils found in sedimentary rock layers can be dated, and the deeper the layer the older the fossil. The sequence shows simple forms in older strata and progressively more complex forms above, and includes transitional forms linking major groups, which indicates that life forms have changed over geological time rather than appearing all at once. Anatomical evidence — homology: the forelimbs of a human, a whale, a bat, and a horse are built from the same set of bones — humerus, radius and ulna, carpals, metacarpals, and phalanges — although they are used for grasping, swimming, flying, and running. Such a shared plan modified for different uses is best explained by descent from a common ancestor, that is, divergent evolution. Vestigial organs such as the reduced ear muscles and the vermiform appendix in humans point the same way. Anatomical evidence — analogy: the wings of a butterfly and a bird, or the eyes of an octopus and a mammal, are similar in function but structurally quite different. This shows that similar environmental pressures can produce similar solutions in unrelated lineages, which is convergent evolution. Embryological evidence: the early embryos of fishes, amphibians, reptiles, birds, and mammals show striking resemblances, including structures such as pharyngeal pouches that appear even in species that never develop gills, suggesting shared ancestry. Molecular and biochemical evidence: all organisms use DNA as genetic material, share essentially the same genetic code, and use similar metabolic pathways. The degree of similarity in the sequences of DNA and of proteins such as cytochrome c closely parallels the relatedness worked out from anatomy — the more recently two species shared an ancestor, the fewer the differences. Direct observation: industrial melanism in the peppered moth, the rise of antibiotic-resistant bacteria, and the appearance of pesticide-resistant insects all show natural selection changing populations within a human lifetime.
A team studying a chain of volcanic islands finds a group of ground-feeding finches. On the largest island the birds vary widely in beak depth. On three smaller islands, each dominated by a different food source — hard-shelled nuts on one, small soft seeds on another, and flower nectar on the third — the finches have short deep beaks, small fine beaks, and long slender beaks respectively. Genetic work shows all four populations descend from a single ancestral flock that arrived from the mainland. (a) Name the evolutionary process that produced the three island forms from one ancestral stock. (b) Are the beaks of the three island forms homologous or analogous? Justify. (c) Explain how natural selection could have produced the deep-beaked form on the nut island. (d) The founding flock was small and by chance carried allele frequencies unlike the mainland population. Name this effect.
Answer
(a) The process is adaptive radiation — a single ancestral species entering a new area with varied habitats diversifies into several forms, each adapted to a different mode of life. Because these forms all diverge from one common ancestor, it is also an instance of divergent evolution. (b) They are homologous. All three beaks are modifications of the same ancestral beak inherited from the common founding stock, and they now differ because each is adapted to a different food. Same origin with different function is the definition of homology; analogy would require different origins. (c) The ancestral population already contained heritable variation in beak depth. On the nut island, birds that happened to have deeper, stronger beaks could crack the hard shells and feed successfully, while those with fine beaks could not obtain enough food and left fewer offspring. Over many generations the alleles for deeper beaks increased in frequency in that gene pool until the deep-beaked form predominated. The variation arose first, at random; the environment merely selected among it. (d) This is the founder effect, a form of genetic drift in which a small number of individuals starting a new population carry, purely by chance, a non-representative sample of the parent population's alleles.
RowQ generates fresh questions on Evolution, marks your answers, and explains every step.
Start free