Variation Basics
Q: Define phenotypic variation.
A: Differences in observable characteristics between individuals of the same species, caused by genetic factors, environmental factors, or a combination of both.
Q: Name the three types of natural selection.
A: Stabilising selection, directional selection, and disruptive selection.
Q: What is meant by allele frequency?
A: The proportion of a particular allele in a population’s gene pool.
Q: What is meant by the struggle for existence?
A: Competition between organisms for limited resources such as food, space, and mates.
Q: State the two Hardy–Weinberg equations.
A:
p + q = 1
p² + 2pq + q² = 1
Q: Distinguish between continuous and discontinuous variation.
A:
Continuous variation shows a range of values with no distinct categories and is usually influenced by many genes and the environment.
Discontinuous variation shows distinct categories and is usually controlled by one or a few genes
Q: Identify which type of selection favours extreme phenotypes at both ends of a range.
A: Disruptive selection.
Q: What is genetic drift, and why is it more significant in small populations?
A: Genetic drift is a random change in allele frequencies due to chance events; it is more significant in small populations because chance has a greater effect on allele survival.
Q: Explain how antibiotic resistance arises in bacteria.
A: Random mutations create resistant bacteria; antibiotics act as a selection pressure killing non-resistant bacteria, allowing resistant individuals to survive, reproduce, and pass on resistance alleles.
Maths & Data Skills
Q: List two conditions required for Hardy–Weinberg equilibrium.
A: Any two of:
Large population size
No mutation
No migration
Random mating
No selection
Q: Explain the genetic basis of continuous variation.
A: Continuous variation is controlled by many genes, each with a small additive effect, often influenced by environmental factors.
Q: Explain how directional selection can lead to a change in mean phenotype.
A: One extreme phenotype is favoured, increasing survival and reproduction, causing the population mean to shift in that direction over generations.
Q: Describe the bottleneck effect and its impact on genetic diversity.
A: A sudden reduction in population size reduces genetic diversity as only a small, unrepresentative sample of alleles remains.
Q: Explain why populations, not individuals, evolve.
A: Individuals do not change their alleles during their lifetime; changes in allele frequencies occur across generations in populations.
Q: If the recessive phenotype frequency is 0.16, calculate q.
A:
q² = 0.16
q = √0.16
q = 0.4
Q: A population shows a bell-shaped curve for height. Identify the type of variation and explain what this suggests about inheritance.
A: Continuous variation; it suggests height is controlled by many genes and influenced by environmental factors.
Q: Describe how stabilising selection affects variation over time.
A: Intermediate phenotypes are favoured while extremes are selected against, reducing variation but maintaining the mean.
Q: Explain how the founder effect can lead to rapid evolutionary change.
A: A small founding population carries limited alleles, causing allele frequencies to differ greatly from the original population.
Q: Describe how selective breeding can improve milk yield in dairy cattle, including one disadvantage.
A: Individuals with high milk yield are selected and bred over generations; disadvantage includes reduced genetic diversity or increased inherited disorders.
In a population, p = 0.7. Calculate q² and state what it represents.
A:
q = 1 − 0.7 = 0.3
q² = 0.09
This represents the frequency of homozygous recessive individuals.
Q: Explain how genetic and environmental factors interact to produce phenotypic variation.
A: Genes determine potential phenotypes, while environmental factors such as diet and climate modify expression; both interact to produce observed variation.
Q: Compare stabilising, directional, and disruptive selection.
A:
Stabilising favours intermediate phenotypes
Directional favours one extreme
Disruptive favours both extremes
Each alters allele frequencies differently.
Q: Explain why Hardy–Weinberg may not apply to natural populations.
A: Natural populations experience selection, mutation, migration, non-random mating, and often have small population sizes.
Q: Outline the steps involved in selective breeding and explain one ethical concern.
A: Selection of desired traits, controlled breeding over generations; ethical concerns include reduced welfare or increased genetic disorders.
Q: A recessive condition affects 1 in 100 individuals. Calculate the carrier frequency.
A:
q² = 0.01
q = 0.1
p = 0.9
2pq = 2(0.9)(0.1) = 0.18
Carrier frequency = 18%