Question
Antibiotics such as penicillin are secreted by fungi and other microorganisms in soil. By secreting them, a microorganism can inhibit the growth of a competitor. In research published in 2014, nearly 3000 antibiotic resistance genes were discovered in soil microorganisms, giving resistance to 18 different antibiotics. The types of antibiotic resistance gene varied between soil types.
(a) Explain how natural selection could increase the prevalence of an antibiotic resistance gene in a species of soil bacterium.
(b) (i) There are viruses in soils that are pathogens of animals. Outline a reason for antibiotics in soil not eliminating these viruses.
(ii) Explain the reasons for antibiotics secreted into soil not harming insects or other animals in the soil.
Answer/Explanation
Answer:
(a)
- If antibiotics are present in the soil, they create a selection pressure on bacteria.
- Bacteria without the antibiotic resistance gene will die or have reduced survival.
- Bacteria with the resistance gene survive and reproduce more successfully.
- Resistance genes can also spread between bacteria via horizontal gene transfer (e.g., conjugation with plasmids).
- Over time, the frequency of the antibiotic resistance gene increases in the bacterial population through natural selection.
(b)(i)
- Viruses are not living cells and lack metabolic machinery, plasma membranes, or cell walls.
- Antibiotics target bacterial cell structures and metabolic processes, which viruses do not have.
- Therefore, antibiotics do not affect viruses, including those pathogenic to animals in soil.
(b)(ii)
- Antibiotics specifically target structures/processes found only in bacteria (e.g., bacterial cell walls, 70S ribosomes).
- Insects and other animals are eukaryotes with different cell structures (e.g., no cell walls, 80S ribosomes).
- Because of these differences, antibiotics do not harm insects or other soil animals.
- Microbes secrete antibiotics primarily to inhibit competing microbes, not multicellular organisms.
Question
a. List two causes of variation within a gene pool.[2]
b. Describe how variation contributes to evolution by natural selection.[3]
c. Outline what is required for speciation to occur.[3]
▶️Answer/Explanation
(a) Two causes of variation within a gene pool:
- Sexual reproduction (e.g., random fertilization, meiosis)
- Mutation
(b)
- Variation causes differences in phenotypes between individuals in a population.
- There is competition/struggle for survival among individuals.
- Individuals with advantageous traits have a higher chance of survival and reproduction, so favourable alleles are passed on to offspring.
Note: Do not say “pass on phenotypes” it’s the alleles that are inherited.
(c) Requirements for speciation:
- A population must be divided, creating separate gene pools (e.g., by geographic separation).
- Reproductive isolation occurs (lack of interbreeding between separated groups).
- Different selective pressures act on each group (e.g., due to temporal, behavioral, or geographic isolation), leading to genetic divergence.
——————————————————————————————–Markscheme——————————————————————————————–
a. a. sexual reproduction / random fertilization / meiosis
b. mutation
No mark for crossing over unqualified.
Reject natural selection/evolution as causes of variation.
b. a. (variation is) different phenotypes/differences between individuals in a population/species
b. struggle/competition for survival
c. some individuals have advantageous characteristics/are better adapted/have greater chance of survival/reproduction (than others)
d. favourable alleles/genetic variations passed on/inherited by offspring/next generation Reject “pass on phenotypes”.
c. a. divided species/gene pool / part of species/gene pool becomes separated / species splits into separate populations
b. reproductive isolation / lack of interbreeding
Mark point \(b\) refers to a lack of interbreeding between separated populations in a species, not the lack of interbreeding after speciation.
c. may be due temporal/behavioural/geographic isolation
d. different natural selection/different selective pressures