What processes change the genetic make-up of populations over time?
Evolution: sources of genetic variation, sexual versus asexual reproduction and the costs of sex, natural and sexual selection, genetic drift, the bottleneck and founder effects, the Hardy-Weinberg principle, co-evolution and hybridisation.
An SQA Advanced Higher Biology answer on evolution, covering sources of genetic variation, sexual versus asexual reproduction and the costs of sex, natural and sexual selection, genetic drift with the bottleneck and founder effects, the Hardy-Weinberg principle, co-evolution and hybridisation.
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What this key area is asking
The SQA wants you to explain the processes that change populations over time: where genetic variation comes from, the trade-offs of sexual and asexual reproduction, how natural and sexual selection act, how genetic drift and small-population effects work, how to use the Hardy-Weinberg principle, and how co-evolution and hybridisation shape species.
Sources of genetic variation
Sexual and asexual reproduction
Natural and sexual selection
Genetic drift and small populations
Both effects reduce genetic variation and can fix or lose alleles by chance, lowering the population's ability to adapt and raising extinction risk.
The Hardy-Weinberg principle
Co-evolution and hybridisation
Examples in context
Example 1. The northern elephant seal bottleneck. Hunted to about 20 individuals, the species recovered in number but with very low genetic variation, leaving it vulnerable to disease. The example shows a bottleneck cutting variation that has not been restored despite population recovery.
Example 2. Co-evolution of flowers and pollinators. Long-spurred orchids and the long-tongued moths that pollinate them have driven each other to extremes through reciprocal selection. The example shows co-evolution producing matched adaptations in two interacting species.
Try this
Q1. Name the term for random change in allele frequency, strongest in small populations. [1 mark]
- Cue. Genetic drift.
Q2. State one cost and one benefit of sexual reproduction. [2 marks]
- Cue. Cost: only half the population bears young (or finding a mate is costly). Benefit: it generates genetic variation and adaptability.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksExplain the difference between the founder effect and a population bottleneck, and why both reduce genetic variation.Show worked answer →
A 4-mark answer needs both effects defined and the common consequence.
The founder effect occurs when a small number of individuals start a new population, so the new gene pool contains only a fraction of the variation of the original population.
A bottleneck occurs when a population is drastically reduced in size, for example by disease or disaster, so that only a small, possibly unrepresentative sample of alleles survives.
Both reduce genetic variation because allele frequencies are now set by a small number of individuals, an example of genetic drift, and rare alleles may be lost entirely. Reduced variation lowers the population's ability to adapt.
Markers reward (1) founder effect is a new population from few individuals, (2) bottleneck is a drastic reduction in size, (3) both leave a small unrepresentative sample of alleles, and (4) reduced variation lowers adaptability.
SQA AH style3 marksA recessive allele has a frequency of 0.2. Use the Hardy-Weinberg principle to find the proportion of heterozygous carriers.Show worked answer →
A 3-mark answer needs the allele frequencies and the heterozygous term.
Step 1. The recessive allele frequency is q = 0.2, so the dominant allele frequency is p = 1 - 0.2 = 0.8.
Step 2. The Hardy-Weinberg equation is p squared plus 2pq plus q squared equals 1, and the heterozygous proportion is the 2pq term.
Step 3. 2pq = 2 times 0.8 times 0.2 = 0.32, so 32 per cent of the population are heterozygous carriers.
Markers reward finding p from q, identifying 2pq as the heterozygotes, and the correct value of 0.32.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)