A-LevelBiologySelection and evolution9700/43 · May/June 2025 · Q211 Marks

Natural selection and selective breeding (artificial selection) are processes that result in changes in the gene pool of a population. Natural selection and selective breeding have implications for humans.

(a)(i)If a person with a bacterial infection does not finish the course of an antibiotic given, it provides the conditions for a population of bacteria to become resistant to this antibiotic. A mutation in a bacterial gene can give resistance to an antibiotic. Directional selection can occur when the antibiotic is present in the environment. A bacterium can also gain resistance when it receives genetic material from another bacterium in a process known as horizontal gene transfer. Outline how directional selection and horizontal gene transfer result in a new population of bacteria that is resistant to an antibiotic.[4]
(a)(ii)Some bacterial diseases can be treated only with one antibiotic, because the bacterial pathogens are resistant to all other antibiotics. A drug is being developed to help treatment. • The drug is a small polynucleotide. • The drug inhibits translation of the messenger RNA (mRNA) produced by transcription of the gene associated with antibiotic resistance. • The bacteria are then susceptible to more antibiotics. Suggest and explain how the drug could cause bacteria to become susceptible to more antibiotics.[2]
(b)(i)Outline how natural selection differs from selective breeding.[3]
(b)(ii)Selective breeding is used to produce uniform varieties of maize. The maize plants in a crop ripen at the same time and are the same height. The advantage of this is that harvesting is easy and quick. The disadvantage is that farmers must buy new seeds each year. Explain why farmers must buy new seeds each year.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[4] 2(a)(i) any four from: 1 antibiotic acts as selection pressure ; 2 bacteria with antibiotic resistance (gene / allele) have selective advantage / selected for ; ora 3 bacteria with antibiotic resistance (gene / allele), survive / reproduce ; ora 4 pass on, gene / allele, (for antibiotic resistance) or increase in allele frequency (for antibiotic resistance) ; 5 ref. to plasmids ; 6 (horizontal gene transfer) by, transduction / transformation / conjugation; 7 (horizontal gene transfer) gives rapid increase of resistant bacteria ;
[2] 2(a)(ii) any two from: 1 drug prevents attachment of mRNA to ribosome or drug prevents attachment of tRNA to, amino acid / ribosome / mRNA or drug binds to ribosome ; 2 protein (coded for by antibiotic resistance gene) not made ; 3 so antibiotic able to work ;
[3] 2(b)(i) any three from: accept ora throughout natural selection 1 environment as selection pressure; ora human acts as selection pressure 2 random mating; ora humans select organisms to breed 3 takes more generations for effects to be seen / slower process / AW; 4 organisms not selected for desirable phenotypes ; ora humans select features 5 can result in speciation; 6 does not decrease, genetic diversity / hybrid vigor ; 7 inbreeding less likely to occur; 8 greater heterozygosity;
[2] 2(b)(ii) 1 offspring (from F1) will not be genetically similar / AW ; 2 offspring (from F1) will produce variation in next crop / will not be true-breeding plants;