AQA A-Level Biology: Populations, Evolution and Classification

A hospital records a rapid rise in infections caused by a strain of the bacterium Staphylococcus aureus that is no longer killed by the antibiotic methicillin. Before methicillin was introduced to the ward, almost all cells in the bacterial population were killed by the drug.

Describe and explain how natural selection brought about this change in the bacterial population, from the origin of variation to the change in allele frequency over many generations.

In a species of ornamental plant, seed colour and seed shape are each controlled by a single gene with two alleles. A breeder crossed two plants that were heterozygous for both genes. The dihybrid cross is expected to give offspring in a 9 : 3 : 3 : 1 ratio of the four phenotype classes.

The breeder collected 320 offspring seeds and recorded the phenotypes:

Phenotype class	Observed number
Yellow, round	186
Yellow, wrinkled	54
Green, round	60
Green, wrinkled	20

A chi-squared ($\chi^2$χ2) test was used to compare the observed results with the expected 9 : 3 : 3 : 1 ratio.

(a) State a suitable null hypothesis for this test. (1 mark) (b) Calculate the value of $\chi^2$χ2 using $\chi^2 = \sum \frac{(O - E)^2}{E}$χ2=∑E(O−E)2​. Show your working in a table. (4 marks) (c) The critical value of $\chi^2$χ2 at $p = 0.05$p=0.05 with 3 degrees of freedom is 7.815. State whether you accept or reject the null hypothesis and explain what this tells you about the cross. (1 mark)

In a large population of a species of snail, shell banding is controlled by a single gene with two alleles. The unbanded phenotype is recessive. In a survey of this population, 16% of the snails were unbanded.

Assume the population is in Hardy-Weinberg equilibrium. Use the Hardy-Weinberg equations:

$p + q = 1 \qquad p^2 + 2pq + q^2 = 1$p+q=1p2+2pq+q2=1

where q is the frequency of the recessive allele.

(a) Calculate the frequency of the recessive allele (q) and the frequency of the dominant allele (p). (2 marks) (b) Calculate the percentage of the population expected to be heterozygous (carriers of the recessive allele). (2 marks) (c) State one assumption that must be true for the Hardy-Weinberg principle to apply. (1 mark)

A single species of flightless beetle lived across a low-lying area of forest. Over several thousand years the climate became wetter, and a large permanent river formed that divided the forest into a northern and a southern block. The beetles cannot cross the river. The northern forest became cooler and more shaded, while the southern forest stayed warm and open.

After many thousands of years, scientists found that beetles from the two forests, when brought together, would no longer mate successfully to produce fertile offspring.

Explain how the formation of the river could have led to the evolution of two separate species by allopatric speciation.

Scientists compared the amino-acid sequence of the same blood protein in a reference mammal, species W, with the sequence of that protein in three other mammal species, X, Y and Z. The protein is 146 amino acids long in every species. The table shows the number of amino-acid differences between each species and the reference species W.

Species compared with W	Number of amino-acid differences
Species X	3
Species Y	24
Species Z	9

(a) Using the data, state which species is most closely related to species W. (1 mark) (b) Interpret and explain how comparing the amino-acid sequences of the same protein in different species provides evidence of their evolutionary relationships. (3 marks)

Allele frequencies in a population can change as a result of genetic drift as well as by natural selection.

Explain what is meant by genetic drift and why it has a greater effect on allele frequencies in small populations than in large populations.