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Resistance, drift and the evidence: selection where you can watch it happen questions

How antibiotic resistance arises and spreads, vertical and horizontal gene transfer, why a prescribed course is finished, pesticide and herbicide resistance and the refuge strategy, the fossil, anatomical, biochemical and biogeographical evidence for evolution, and genetic drift, bottlenecks and the founder effect as changes in allele frequency that selection did not cause.

6 original questions · 20 marks · the resistance, drift and the evidence: selection where you can watch it happen notes · Evolution and speciation

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• 4 marks · Compare

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

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  1. 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
  2. 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
  3. 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
  4. B1 a single plasmid may carry resistance to several antibiotics, so horizontal transfer can produce multiple resistance in one step, which vertical transfer cannot

• 4 marks · Describe

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

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  1. 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
  2. B1 comparative anatomy: the forelimb of a human, a bat, a whale and a horse contains the same bones in the same arrangement doing entirely different jobs, which modification of one ancestral limb explains and independent design does not
  3. 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
  4. B1 these lines come from fields that could have contradicted one another and do not, so their agreement is the argument; any single line taken alone is far weaker than several that converge on the same branching order

• 4 marks · Explain

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

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  1. 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
  2. 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
  3. 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
  4. 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

• 3 marks · Explain

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

• 3 marks · Suggest

  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.

• 2 marks · State

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

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