A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Genetic Diversity and Natural Selection — mark scheme explained
The short answer
Understanding genetic diversity and natural selection is fundamental to grasping the mechanisms of evolution. This section delves into how genetic variation within a population enables natural selection, leading to the adaptation and survival of species over time.
The question
A population of bacteria is exposed to an antibiotic. Initially, the frequency of the resistant allele (R) is 0.1. After several generations, the frequency increases to 0.8. Explain how this change can be attributed to natural selection. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the initial and final frequencies of the resistant allele (R).
- S2
Initial frequency of R = 0.1
- S3
Final frequency of R = 0.8
- S4
Step 2: Note that the resistant allele arose by random mutation before antibiotic exposure, and explain how the antibiotic affects the population.", "The resistant allele (R) was already present in the population due to a random mutation that occurred before exposure; the antibiotic does not create or direct resistance but acts as a selection pressure.
- S5
The antibiotic kills bacteria that do not have the resistant allele, reducing their reproductive success.
- S6
Step 3: Describe the impact on the resistant allele.
- S7
Bacteria with the resistant allele (R) survive and reproduce more successfully, passing the allele to the next generation.
- S8
Step 4: Conclude how natural selection leads to an increase in the frequency of the resistant allele.
- S9
Over several generations, the frequency of the resistant allele increases from 0.1 to 0.8 due to directional selection favoring antibiotic resistance.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the initial and final frequencies of the resistant allele (R).
- S2
Initial frequency of R = 0.1
- S3
Final frequency of R = 0.8
- S4
Step 2: Note that the resistant allele arose by random mutation before antibiotic exposure, and explain how the antibiotic affects the population.", "The resistant allele (R) was already present in the population due to a random mutation that occurred before exposure; the antibiotic does not create or direct resistance but acts as a selection pressure.
- S5
The antibiotic kills bacteria that do not have the resistant allele, reducing their reproductive success.
- S6
Step 3: Describe the impact on the resistant allele.
- S7
Bacteria with the resistant allele (R) survive and reproduce more successfully, passing the allele to the next generation.
- S8
Step 4: Conclude how natural selection leads to an increase in the frequency of the resistant allele.
- S9
Over several generations, the frequency of the resistant allele increases from 0.1 to 0.8 due to directional selection favoring antibiotic resistance.
Final answer: The frequency of the resistant allele (R) increased from 0.1 to 0.8 due to natural selection favoring antibiotic resistance.
Common mistakes
- Confusing genetic diversity with genetic drift. — Remember that genetic diversity refers to the variety of alleles in a population, while genetic drift is a random change in allele frequencies due to chance events.
- Forgetting that mutations are random and not directed by need. — Always emphasize that mutations are random changes in DNA and do not occur to meet specific needs of the organism.
- Confusing directional selection with stabilizing selection. — Practice distinguishing between directional selection (favoring one extreme) and stabilizing selection (favoring intermediate forms). Use examples to reinforce the differences.
- Failing to link genetic diversity to evolutionary outcomes. — Practice explaining the importance of genetic diversity in providing a range of traits that can be selected for or against, leading to adaptation and evolution.
- Incorrectly interpreting data on allele frequencies. — Practice analyzing data sets and graphs that show allele frequency changes. Relate these changes to the principles of natural selection and explain how they indicate evolutionary processes.
- Failing to provide specific examples when explaining types of selection. — Practice providing specific examples for each type of selection. For instance, use antibiotic resistance in bacteria for directional selection and human birth weights for stabilizing selection.
Where the marks go
- Full worked solution (all marking points)4 marks