Questions on evidence for evolution, resistance and genetic drift

Question 1

Compare vertical and horizontal gene transfer as ways in which antibiotic resistance spreads among bacteria.

Accepted Answer

B1 vertical transfer passes the gene from a parent cell to its daughter cells at binary fission, whereas horizontal transfer passes it between bacteria that are not parent and offspring B1 vertical transfer raises the frequency of the allele within one population as resistant lineages increase, whereas horizontal transfer puts the gene into cells that never carried it and never mutated B1 vertical transfer is confined to descendants of the original resistant cell, whereas a plasmid passed through a conjugation tube can carry the gene to bacteria of a different species, which is how a gene first seen in a harmless gut bacterium turns up later in a pathogen B1 a single plasmid may carry resistance to several antibiotics, so horizontal transfer can produce multiple resistance in one step, which vertical transfer cannot

Question 2

Describe three independent lines of evidence for common ancestry, and describe why the agreement between such lines matters.

Accepted Answer

B1 the fossil record places organisms in an order, with simple prokaryote-like forms in the oldest rocks and major groups appearing in a sequence matching that predicted by their anatomy, including transitional forms such as Tiktaalik between lobe-finned fish and tetrapods B1 comparative anatomy: the forelimb of a human, a bat, a whale and a horse is built on the same plan of bones, used for entirely different jobs, which modification of one ancestral limb explains and independent design does not B1 comparative biochemistry: all life uses the same genetic code, the same twenty amino acids and ATP, and conserved molecules such as cytochrome c differ at more positions between more distantly related species B1 these lines come from fields that could have contradicted one another and do not, so their agreement is the argument

Question 3

Explain how genetic drift changes the allele frequencies of a population, and explain why its effect is largest in a small population.

Accepted Answer

B1 the alleles reaching the next generation are a sample of those present in this one, decided by which individuals happen to breed and which gametes happen to meet B1 events such as which juvenile happens to be standing under a falling branch have nothing to do with fitness, so the frequencies shift with no selection acting at all B1 a small sample deviates further from what it is drawn from than a large one, for the same reason that ten coin tosses stray further from half than a thousand do, so the change each generation is larger in a small population B1 drift can therefore fix one allele and lose the other altogether, including a mildly beneficial one, because in a small population chance overwhelms a small difference in fitness, and a lost allele returns only by mutation or migration

Question 4

Glyphosate-resistant weed populations have become established on farmland in North America. Explain how repeated use of a single herbicide leads to a weed population the herbicide no longer controls, and explain why farmers are advised to rotate between herbicides with different modes of action.

Accepted Answer

B1 a rare mutation giving resistance is already present in the weed population before the herbicide is used B1 the herbicide is a severe selection pressure: susceptible plants are killed while resistant plants survive and reproduce B1 applied year after year, the herbicide removes susceptible weeds every season, so the frequency of the resistance allele in the weed population's gene pool rises B1 rotating between herbicides with different modes of action means no single selection pressure is applied for long, so a mutation giving resistance to one herbicide is not favoured every year, slowing the rise in frequency of any one resistance allele

Question 5

Compare homologous structures with analogous structures, using the forelimb of a bat and the forelimb of a human as one example and the wing of a bird and the wing of an insect as the other.

Accepted Answer

B1 homologous structures, such as the forelimb of a bat and of a human, share the same underlying bone arrangement but perform different functions, which is evidence of divergent evolution from a common ancestor B1 analogous structures, such as the wing of a bird and the wing of an insect, perform the same function but have different underlying structures, which is evidence of convergent evolution rather than of common ancestry B1 independent origins would not be expected to produce the shared bone arrangement seen in the bat and human forelimb, whereas descent with modification from one ancestral limb explains it directly B1 analogous structures arise because unrelated lineages face the same functional problem, such as flight, and evolve similar solutions independently rather than because they share a recent common ancestor

Question 6

The Galápagos Islands are volcanic islands about 1000 km off the coast of South America and have never been joined to a continent. More than a dozen species of finch live there and nowhere else. DNA comparisons show that these species are more closely related to one another than to any bird on the mainland. The species differ mainly in the size and shape of their beaks, each suited to the food it eats. Explain how this pattern provides evidence that these species evolved from a common ancestor.

Accepted Answer

B1 the islands were never joined to the mainland, so the first finches must have arrived across the sea, and the close relationship of all the species to one another points to a single founding population B1 populations on different islands were separated by sea, so there was little or no gene flow between them B1 different food on different islands meant different selection pressures: birds whose beaks suited the available food survived and reproduced more and passed on their alleles, so beak form changed in each population B1 over many generations the populations became so different that they could no longer interbreed, giving several species descended from one ancestor, which is why they are most closely related to one another

Question 7

Cytochrome c differs from the human sequence by about a dozen amino acids in the horse, about twenty in the tuna, and by more than forty in yeast. Explain what this pattern shows about the evolutionary relationships between humans and these species, and explain the principle of a molecular clock.

Accepted Answer

B1 the number of differences increases with evolutionary distance from humans in the order predicted by anatomy and the fossil record, so the horse is more closely related to humans than the tuna, which is more closely related than yeast B1 this agreement between an independent source of evidence, molecular sequence comparison, and the branching order already given by anatomy and fossils supports common ancestry, since the two kinds of evidence could have disagreed and do not B1 where mutations accumulate at a roughly steady rate over time, the number of sequence differences between two species estimates how long ago they diverged from a common ancestor, which is the principle of a molecular clock B1 more amino acid differences therefore indicate an older common ancestor and a longer time since the two lineages diverged

Question 8

Penicillin is bactericidal and tetracycline is bacteriostatic. Describe how each antibiotic acts on a bacterial cell to produce this effect.

Accepted Answer

B1 penicillin inhibits the enzymes (transpeptidases) that form the peptide cross-links between the polysaccharide chains of the peptidoglycan (murein) cell wall B1 a growing cell therefore makes a weak wall; water enters by osmosis and the cell bursts (osmotic lysis), so the cell is killed B1 tetracycline binds to the 30S subunit of the bacterial 70S ribosome and prevents tRNA carrying amino acids from binding, so translation stops B1 without new proteins the cell cannot grow or divide, but it is not destroyed, so the effect is to hold the population still rather than to kill it

Question 9

Three flasks of broth were inoculated with the same bacterium. Antibiotic X was added to flask 1, antibiotic Y to flask 2 and nothing to flask 3. The number of living bacteria was estimated at intervals by dilution plating. The results are in the table, which is constructed for this question. Use the results to explain which antibiotic is bacteriostatic and which is bactericidal, and describe how a sample from flask 2 could be used to confirm your answer.

Accepted Answer

B1 Y is bacteriostatic: the number of living cells in flask 2 stays at about 1 × 10⁶ throughout, so the cells are alive but not dividing B1 X is bactericidal: the number of living cells in flask 1 falls about 250-fold, from 1.0 × 10⁶ to 4.0 × 10³, in six hours, so cells are being killed B1 flask 3 is the control and shows the bacteria grow in this broth without an antibiotic, so the level count in flask 2 is caused by Y B1 transfer a sample from flask 2 by aseptic technique into fresh broth or agar with no antibiotic and incubate it; growth shows the cells were inhibited, not killed

Question 10

Explain why infections with antibiotic-resistant bacteria such as MRSA are more common in hospitals than in the wider community.

Accepted Answer

B1 antibiotics are used far more heavily in hospitals, so there is a strong, continuous selection pressure favouring resistant bacteria B1 many patients are elderly, recovering from surgery or immunocompromised, so they are more easily infected B1 wounds, catheters, cannulas and ventilator tubes give bacteria a route past the skin and mucous membranes B1 patients are close together and staff and equipment move between them, so bacteria are transmitted on hands and surfaces, including from symptomless carriers

Question 11

Describe codes of practice for antibiotic prescription in hospitals, and explain how they reduce hospital-acquired infections caused by resistant bacteria.

Accepted Answer

B1 antibiotics are prescribed only when a bacterial infection is likely, and not for viral infections B1 a sample is cultured and tested for sensitivity, so a narrow-spectrum antibiotic that targets the identified bacterium can replace a broad-spectrum one B1 the prescription is reviewed after two or three days and stopped if not needed, and some antibiotics are kept in reserve for use only on specialist advice B1 each of these reduces how often and how widely antibiotics are used, so the selection pressure for resistant strains is lower and harmless competing bacteria are not killed

Question 12

A student tested four antibiotics on a lawn of bacteria and concluded that the antibiotic giving the largest zone of inhibition would be the best one to treat an infection caused by this bacterium. Explain why this conclusion may not be valid.

Accepted Answer

B1 the discs may carry different amounts of each antibiotic, so the zones do not compare equal doses B1 antibiotics diffuse through agar at different rates, so zone size reflects diffusion as well as how strongly the antibiotic acts on the bacterium B1 a clear zone does not show whether the antibiotic is bactericidal or bacteriostatic, which affects whether it suits a patient whose immune system is weakened B1 conditions on an agar plate differ from those in the body: the antibiotic must reach the site of infection and must not harm the patient, which the plate does not test

Question 13

Explain the biological reason a patient is told to finish a prescribed course of antibiotics.

Accepted Answer

B1 the bacteria in an infection are not all equally susceptible, and the most susceptible cells are the ones killed first B1 if treatment stops when the symptoms go, the cells left alive are a biased sample: the least susceptible ones, including any carrying partial resistance B1 those cells reproduce, so the population that regrows is less susceptible than the one treatment started with, whereas finishing the course removes the tail of the distribution as well as its head

Question 14

A crop is engineered to make a toxin that kills a pest insect, and resistance to that toxin is recessive. Farmers growing it are required to plant a refuge of untreated plants alongside. Suggest how the refuge slows the spread of resistance.

Accepted Answer

B1 the refuge produces no toxin, so susceptible insects go on breeding there in large numbers instead of being killed B1 a rare resistant survivor emerging from the treated crop is therefore far more likely to mate with a susceptible insect than with another resistant one B1 the offspring of such a pairing are heterozygous, and because resistance is recessive the toxin still kills them, so the allele is not passed on and its frequency rises far more slowly

Question 15

Streptomycin kills bacteria by binding to their ribosomes. In resistant strain A, a mutation has changed the amino acid sequence of one ribosomal protein. Strain B makes an enzyme that chemically modifies the streptomycin molecule. Strain C has more copies of a membrane protein that pumps streptomycin out of the cell. Explain how each change allows the bacteria to survive treatment with streptomycin.

Accepted Answer

B1 strain A: the changed sequence changes the tertiary structure of the ribosome where the drug binds, so streptomycin no longer fits and translation continues B1 strain B: the modified streptomycin has a different shape, so it no longer binds to the ribosome B1 strain C: the drug is removed as fast as it enters, so its concentration inside the cell stays too low to stop enough ribosomes

Question 16

A warfarin-resistance allele in British rats spread rapidly after warfarin was introduced but settled at an intermediate frequency rather than reaching fixation. Homozygous resistant rats need far more vitamin K than they can normally obtain. Suggest why the resistance allele did not spread to fixation where warfarin is used.

Accepted Answer

B1 homozygous resistant rats suffer from a shortage of vitamin K and are at a disadvantage even where warfarin is used, because they cannot obtain enough of it B1 heterozygotes are resistant to warfarin without needing unusually large amounts of vitamin K, so heterozygotes have the highest fitness of the three genotypes: a heterozygote advantage B1 selection therefore favours an intermediate allele frequency at which both homozygous genotypes are at a disadvantage relative to heterozygotes, rather than driving the allele to fixation or eliminating it

Question 17

Outline the roles played by Charles Darwin and Alfred Russel Wallace in the formulation of the theory of natural selection.

Accepted Answer

B1 both men independently arrived at the same mechanism of natural selection from their own observations, Darwin from the Beagle voyage and from reading Malthus, Wallace from his own collecting in the Amazon and the Malay Archipelago, without knowing of each other's theory until Wallace wrote to Darwin in 1858 B1 their work was presented jointly to the Linnean Society in July 1858, and it was Darwin who then published On the Origin of Species in 1859, which is why the theory is usually associated with his name alone despite its joint origin B1 neither man knew how inheritance worked or where new variation came from, since Mendel's work was not recognised until 1900 and mutation was not described until the twentieth century, so the modern synthesis of the 1930s and 1940s later supplied the missing mechanisms

Question 18

The Old Order Amish population is descended from around two hundred eighteenth-century founders, and Ellis-van Creveld syndrome, caused by a recessive allele, is far more common among them than in the world population as a whole, with no evidence that the allele affects survival or reproduction. Suggest why this recessive allele is unusually common in this population.

Accepted Answer

B1 the founding population was small, so by chance the frequency of the allele among the individuals who founded the population was higher than its frequency in the larger population they came from: the founder effect B1 that unrepresentative frequency has been carried through many generations of a population that has largely bred within itself, keeping the allele common B1 no selection is shown, since the question gives no evidence that carriers or affected individuals differ in fitness, so the high frequency is better explained by chance sampling at founding than by any advantage the allele confers

Question 19

Compare a genetic bottleneck with a founder effect as causes of reduced genetic diversity in a population.

Accepted Answer

B1 a bottleneck happens when a large existing population is sharply reduced in size, so the survivors carry only a sample of the original population's alleles, whereas a founder effect happens when a small number of individuals establish a new population separate from a larger source population that continues to exist B1 both are forms of genetic drift and both leave the resulting population with lower genetic diversity than before, because in each case only a fraction of the original alleles is carried forward B1 in a bottleneck the reduction happens to one continuous population, whereas in a founder effect two populations exist afterwards, the small founding one and the larger source population it came from, so the two differ in the fate of the original population rather than in the mechanism reducing diversity

Question 20

Explain the difference between a bactericidal antibiotic and a bacteriostatic antibiotic, and explain why a patient treated with a bacteriostatic antibiotic still depends on their immune system.

Accepted Answer

B1 a bactericidal antibiotic kills bacteria, so the number of living bacteria falls while the drug is present B1 a bacteriostatic antibiotic stops bacteria growing and dividing but does not kill them, so the number of living bacteria stays about the same B1 the bacteria that remain alive must be removed by phagocytes and antibodies, and they can grow again if the drug is withdrawn before they are cleared

Question 21

On a ward with cases of Clostridioides difficile infection, staff are told to wash their hands with soap and water rather than rely on alcohol gel, and surfaces are cleaned with a chlorine-based disinfectant. Explain these measures, and explain why treatment with broad-spectrum antibiotics increases a patient's risk of this infection.

Accepted Answer

B1 C. difficile forms spores, which are not killed by alcohol gel; washing with soap and water removes them physically from the hands B1 the spores survive for long periods on surfaces, and a chlorine-based disinfectant kills them, breaking transmission between patients B1 a broad-spectrum antibiotic kills many of the harmless gut bacteria that compete with C. difficile, so it can multiply and release toxins

Question 22

A hospital introduces screening of all patients on admission for MRSA. Suggest how this reduces MRSA infection in the hospital, and suggest why a fall in cases after it was introduced might not be caused by screening alone.

Accepted Answer

B1 nose or skin swabs identify patients carrying MRSA without symptoms, who would otherwise not be recognised as a source of infection B1 carriers can be isolated in a side room and treated with antiseptic washes and nasal ointment, so MRSA is not passed to other patients B1 other measures such as hand hygiene, cleaning and prescribing rules may have been introduced or improved at the same time, so the fall cannot be attributed to screening without a comparison in which only screening changed

Question 23

In an investigation of the effect of different antibiotics on bacteria, discs of antibiotics X and Y were placed on a lawn of bacteria. The mean diameter of the zone of inhibition was 21 mm for X and 13 mm for Y; the results are invented for this question. Calculate the area of each zone, and the ratio of the area of the X zone to the area of the Y zone.

Accepted Answer

M1 radii 10.5 mm and 6.5 mm, half of each diameter M1 areas π × 10.5² = 346 mm² and π × 6.5² = 133 mm² A1 ratio 346 ÷ 133 = 2.6; accept 2.6 to 2.61, or 2.6 : 1

Question 24

State what a genetic bottleneck is, and state one species that still shows the effect of one.

Accepted Answer

B1 a sharp reduction in population size that leaves the survivors carrying only a fraction of the original genetic diversity B1 northern elephant seals, hunted down to a few dozen animals in the 1890s, now number well over a hundred thousand and remain strikingly uniform genetically

Question 25

A patient whose neutrophil count has been lowered by chemotherapy develops a bacterial infection. Suggest why a doctor would choose a bactericidal antibiotic rather than a bacteriostatic one.

Accepted Answer

B1 a bacteriostatic antibiotic only stops the bacteria dividing, so the living bacteria must still be destroyed by the patient's phagocytes B1 with few neutrophils the bacteria would not be cleared and could grow again, whereas a bactericidal drug kills them without relying on the immune system

Time / hours	Flask 1 (X): living cells per cm³	Flask 2 (Y): living cells per cm³	Flask 3 (none): living cells per cm³
0	1.0 × 10⁶	1.0 × 10⁶	1.0 × 10⁶
2	1.6 × 10⁵	1.1 × 10⁶	1.5 × 10⁷
4	2.5 × 10⁴	0.9 × 10⁶	1.9 × 10⁸
6	4.0 × 10³	1.0 × 10⁶	6.2 × 10⁸