Selection and Evolution: Biology 9700 (Cambridge International AS & A Level)

A biologist studying a large population of adult volunteers records two features.

Trait 1. ABO blood group: each volunteer is classified as group A, B, AB or O, with no volunteers falling between these categories.

Trait 2, hand span: measured to the nearest millimetre, values form a continuous range from about 170 mm to 230 mm, with most volunteers close to the mean and progressively fewer volunteers towards each extreme.

Which row correctly identifies the type of variation shown by each trait and the genetic basis of that type of variation?

The three scenarios below each describe a natural population that experiences a selection pressure acting on a phenotypic characteristic.

(a) In a large hospital's long-term records, babies born with a body mass close to 3.3 kg have the highest survival rate to one year old, while babies born considerably lighter or considerably heavier than this have a higher risk of dying in their first year. State and explain the type of natural selection shown by these data, and describe the effect this type of selection has, over many generations, on the range of birth mass seen in the population. [3]

(b) A population of beetles living among volcanic rocks varies in the colour of its wing cases, from very pale grey to very dark grey. After a nearby eruption coats the surrounding rock surfaces in dark ash, records kept over the next 40 generations show that the average wing case colour of the population becomes progressively darker, and pale-coloured beetles become rare. Identify and explain the type of natural selection shown, and explain how it changes the beetle population's phenotypes, and the underlying allele frequencies, over this time. [3]

(c) On a small island, a population of seed-eating birds feeds on seeds from a single plant species. Following a change in climate, this plant now produces only two seed types (small, soft seeds and large, hard seeds) and no longer produces medium-sized seeds. Over several generations, birds with either a small, narrow beak or a large, powerful beak become more common, while birds with a medium-sized beak become rare, because medium-beaked birds cannot process either seed type efficiently. Identify and explain the type of natural selection shown, and state one likely long-term outcome for this bird population if the two beak-size groups became reproductively isolated from one another. [3]

A species of bacterium infecting hospital patients is treated with a newly introduced antibiotic. Before this antibiotic was ever used on patients, a very small number of bacteria in the population already carried a rare mutant allele that altered a bacterial cell-surface protein, so that the antibiotic could no longer bind to it effectively.

(a) Explain why it is incorrect to say that exposure to the antibiotic caused this mutant allele to appear in the bacterial population. [2]

(b) Explain, in terms of natural selection, how the proportion of the bacterial population carrying the resistance allele increases once the antibiotic is introduced and used repeatedly. [4]

(c) A patient stops taking their course of this antibiotic as soon as their symptoms disappear, rather than completing the full course prescribed. Suggest why this practice increases the risk that a fully antibiotic-resistant bacterial population will develop in that patient. [2]

In a large, randomly mating population of a wildflower species, petal colour is controlled by a single gene with two alleles: the dominant allele R produces red petals, and the recessive allele r produces white petals, but only in plants with the homozygous recessive genotype rr. A survey of a large sample from this population finds that 1% of the plants have white petals.

Assume that this population satisfies all the conditions required for the Hardy-Weinberg principle to apply. You may use the Hardy-Weinberg equations, written here in plain text: "p + q = 1" and "p squared + 2pq + q squared = 1", where p is the frequency of the dominant allele R and q is the frequency of the recessive allele r.

(a) Calculate the frequency of the recessive allele (q) and the frequency of the dominant allele (p) in this population. Show your working. [3]

(b) Calculate the percentage of the population that is heterozygous (genotype Rr) and the percentage that is homozygous dominant (genotype RR). Show your working, and check that your three genotype frequencies add up to 1. [3]

(c) State two conditions (assumptions) that must apply to a population for the Hardy-Weinberg principle to give an accurate prediction of its allele and genotype frequencies. [2]

A population of small mammals living on a mainland has a mixture of allele frequencies at many gene loci. A chance storm event carries a very small number of these mammals (fewer than ten individuals) to a previously uninhabited offshore island, where they establish a new, isolated population that grows over many generations.

(a) Explain why the allele frequencies in the new island population are likely to differ, purely by chance, from those in the original mainland population, and name this phenomenon. [3]

(b) Over many thousands of years, the island population becomes physically unable to interbreed successfully with the mainland population, even when individuals from both populations are brought together. State the term for this type of speciation, and explain how the geographical separation of the two populations allowed it to occur. [3]

(c) Elsewhere on the mainland, a separate research group studies two populations descended from the same original mammal species, living in the same area but active at different times: one population is active only at night, and the other only at dawn and dusk. These two populations have also become separate species, without ever being geographically separated from each other. Name this type of speciation, and suggest how the difference in activity times could have led to genetic isolation between the two populations. [3]

(d) Other than showing that two populations can no longer interbreed, suggest how comparing DNA base sequences between individuals from different populations could provide additional evidence about how closely related those populations are. [2]

A population of freshwater fish living in a single large lake numbers about 50,000 individuals, with a wide range of allele frequencies at many gene loci. A sudden, severe drought dramatically lowers the lake's water level, killing the vast majority of the fish, irrespective of their genotype. Only around 200 fish survive the drought in a few small remaining pools, and this same lake is repopulated by their offspring once the rains return.

Genetic sampling shows that the allele frequencies in the recovered population differ substantially, and apparently at random, from those in the original 50,000-strong population.

Which term best describes the process that has most directly changed the allele frequencies in this fish population, and why?

A dairy farmer keeps a herd of cattle whose average daily milk yield per cow varies continuously, from about 18 to 30 litres per day. The farmer wants to increase the herd's average milk yield over the next several generations and decides to use selective breeding (artificial selection) rather than relying on natural processes alone.

(a) Describe how the farmer could use selective breeding, over several generations, to increase the average milk yield of the herd. [4]

(b) Explain one key difference between artificial selection, as used by the farmer here, and natural selection, in terms of what determines which individuals breed. [2]

(c) Suggest one disadvantage, for the long-term health of the herd, of repeatedly choosing only the highest-yielding cows and bulls as breeding parents over many generations. [2]

A rare recessive metabolic condition is controlled by a single gene with two alleles: the recessive allele c causes the condition only in individuals with genotype cc, while the dominant allele C, present in genotypes CC or Cc, results in an unaffected phenotype (individuals with genotype Cc are unaffected but are carriers of allele c). In a large, randomly mating human population of 10,000 people that satisfies the conditions needed for the Hardy-Weinberg principle to apply, health records show that exactly 25 people have this condition.

You may use the Hardy-Weinberg equations, written here in plain text: "p + q = 1" and "p squared + 2pq + q squared = 1", where p is the frequency of the dominant allele C and q is the frequency of the recessive allele c.

(a) Calculate the frequency of the recessive allele (q) and the frequency of the dominant allele (p) in this population. Show your working. [3]

(b) Calculate the number of people, out of the 10,000, who are expected to be unaffected carriers (genotype Cc), and the number expected to be homozygous dominant (genotype CC). Show that your three genotype numbers add up to 10,000. [4]

(c) A genetic screening programme directly tests a large sample of this population and finds a carrier frequency noticeably different from your prediction in (b). Suggest one reason, other than sampling or measurement error, why the actual carrier frequency in a real population might differ from a Hardy-Weinberg prediction. [2]

A research group is investigating the evolutionary relationships between a reference bird species, R, and four other bird species (A, B, C and D) thought to share a common ancestor with species R. The group sequences the same respiratory transport protein from all five species and counts how many amino acid positions differ from species R.

(a) Using the data in the table, state which of species A to D is most closely related to species R, and which is least closely related. Explain your reasoning. [3]

(b) Explain what is meant by the term "molecular clock", and outline how biologists could use amino acid or DNA sequence differences, together with a molecular clock, to estimate how long ago two species diverged from a common ancestor. [3]

(c) Explain one advantage of using molecular (DNA or amino acid sequence) evidence, rather than relying only on comparing the outward physical (morphological) features of species, when investigating evolutionary relationships. [2]

Two populations of the same plant species are being studied.

Population 1: plants growing in a field with patchy soil fertility. Researchers take cuttings (genetically identical clones) from several plants and grow them in both nutrient-poor and nutrient-rich patches. In every case, the clones grown in nutrient-poor soil develop small leaves, while the genetically identical clones of the same individuals grown in nutrient-rich soil develop much larger leaves. Leaf size in this population is entirely explained by local soil nutrient availability.

Population 2: plants growing in a separate field with uniform soil fertility throughout. When researchers grow offspring from many different parent plants together in identical, controlled conditions (a common-garden experiment), the offspring still show a consistent range of leaf sizes that closely resembles the leaf size of their own parents, regardless of exactly where in the field they are grown.

Both populations are now exposed to a new, sustained environmental pressure that favours plants with larger leaves. Which statement correctly predicts the long-term evolutionary response of each population to this pressure?