Biology 9700/42 — February/March 2022

Background Concept

When a species is introduced to a new environment, its population growth is shaped by the same biotic and abiotic factors that act on any population. Growth is rapid when:

• Reproductive rate is high (many offspring per breeding event and a short generation time).
• Mortality is low because natural predators, parasites and pathogens are absent or reduced.
• Resources are abundant (food, water, shelter, mates, breeding sites).
• Competitors are absent, so interspecific competition is minimal.
• The species is pre-adapted to the local climate and habitat.

In the absence of these 'checks', a population grows exponentially until density-dependent factors (food, disease, space) begin to bite.

Understanding the Question

The stem says three pairs (six individuals) of hedgehogs were introduced to Alderney in the 1960s, and by 2017 the population had reached about 600. The question asks the candidate to suggest (i.e. propose plausible ecological) reasons for this large increase. The command word 'suggest' tells you that you do not need proof — a reasonable ecological explanation earns the mark.

Approach

Think about what normally limits a population on the mainland (predators, food, disease, competitors) and then argue that one or more of those limitations are absent or reduced on Alderney. Two clearly distinct points are required.

Step-by-Step Reasoning

1. Few or no predators — islands typically lack large native predators (e.g. badgers, foxes, owls) that eat hedgehogs on the mainland, so fewer individuals are killed and population growth is unchecked.
2. Plentiful food / omnivore — hedgehogs eat slugs, beetles, earthworms and a wide range of other items. With abundant prey on the island and few competitors, food is not a limiting factor.
3. Little competition — other small insectivores are scarce, so the hedgehogs do not have to compete for the same niche.
4. No/fewer diseases — pathogens that maintain hedgehog populations at low density on the mainland may be absent.
5. High reproductive rate — short generation time and several offspring per litter allow rapid multiplication.
6. Well adapted — the climate and habitat of Alderney suit hedgehogs, so selection is not removing individuals.

Any two of these are credited. Two distinct ideas (e.g. predators + food) score two marks.

Key Takeaways

• Population growth in a new environment is determined by how many of the usual limiting factors are removed.
• Islands often lack the predators, competitors and pathogens that limit populations on the mainland.

Common Mistakes

• Giving only one reason (only one mark).
• Vague answers such as 'they survived' or 'there was a lot of space' — these do not name a specific ecological factor.
• Confusing reasons for population increase with reasons for the high frequency of blonde spines (which is part (ii)).

Things to Be Careful About

• The command word is 'suggest', so a reasoned ecological proposal is sufficient — no data are required.
• 'Well adapted' is acceptable only when it is clear what the hedgehogs are adapted to; otherwise the mark scheme prefers a more concrete factor.

Background Concept

Three ideas are needed:

1. Founder effect / genetic drift — when a new population is started by very few individuals, the allele frequencies in that population are determined by which alleles happened to be carried by those founders. Alleles that were rare on the mainland can become common in the new population purely by chance.
2. Inbreeding — in a small, closed population, close relatives mate, increasing homozygosity. A recessive allele is then more likely to be expressed in the phenotype.
3. Directional selection — if one phenotype has a higher fitness than another, the frequency of the advantageous allele rises across generations.

The blonde allele is recessive, so it is only seen in homozygous individuals. A small founder population + inbreeding makes homozygosity much more likely than in a large, randomly-mating mainland population.

Understanding the Question

The stem tells you that the blonde trait is recessive and that 60% of the hedgehogs on Alderney had blonde spines in 2017 — an extraordinarily high proportion. The question asks you to suggest and explain why this proportion is so high. You need to use information from the stem (only six founders) plus evolutionary principles.

Approach

Combine (a) chance effects from a small founder population and inbreeding with (b) any selective advantage the blonde phenotype might have on the island. Three distinct points are required for three marks.

Step-by-Step Reasoning

1. Founder effect / bottleneck / genetic drift — only six founders started the population, so the original gene pool was very small. The frequency of the blonde allele in the founders could already have been much higher than on the mainland simply by chance. As the population grew, this unusual allele frequency was preserved.
2. Inbreeding — with so few founders and a closed island population, related individuals mate, increasing homozygosity. The recessive blonde allele is therefore expressed in the phenotype more often than expected from its allele frequency alone.
3. Directional selection / selective advantage — blonde spines may be advantageous on Alderney (e.g. better camouflage against pale substrates, less visible to the few aerial predators, or thermal advantages). Hedgehogs with blonde spines leave more offspring, so the blonde allele frequency rises over generations.
4. Allele frequency change — the allele frequency for blonde increases (and the frequency of the dominant brown allele falls) over the ~50 years since introduction.

Any three of these ideas, made clear, score three marks.

Key Takeaways

• A high frequency of a recessive trait in a small, isolated population can be explained by founder effect + inbreeding, with or without selection.
• '60% blonde' is a phenotype frequency, not an allele frequency — but the underlying allele frequency for blonde must also be high.

Common Mistakes

• Confusing phenotype frequency (60%) with allele frequency (which would be $\sqrt{0.60} \approx 0.77$ under Hardy–Weinberg, ignoring inbreeding).
• Saying 'natural selection caused the high proportion' without specifying a selective advantage of the blonde phenotype.
• Forgetting the founder/bottleneck effect — this is the central point of the stem ('only three pairs were introduced').

Things to Be Careful About

• The marking scheme credits 'directional selection' as a stand-alone point, but only when accompanied by a clear selective advantage.
• Inbreeding and founder effect are related but distinct — credit both if both are described.