Selection and Evolution: Question 6
Syllabus 17.2
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?
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Worked solution
Step 1: Identify what actually happened
The population did not move to a new habitat, it stayed in the same lake, and the surviving individuals simply repopulated it. So this is not a case of a small group founding a population somewhere new; it is a case of an existing population’s size crashing drastically before recovering. This distinguishes it from the founder effect (option B), which specifically involves colonisation of a new area.
Step 2: Check whether survival was based on genotype
The question states explicitly that the drought killed fish “irrespective of their genotype”. Survival was determined by chance (for example, which pools happened to retain enough water), not by any fish carrying particular advantageous alleles. This rules out natural selection (option C), because selection requires differential survival/reproduction linked to genotype, not random survival.
Step 3: Check whether mutation could explain the change
Mutation rates are far too low to produce a substantial, population-wide shift in allele frequencies within a single generation (option D). New mutations accumulate gradually over many generations; they do not explain a sudden shift caused by a mass die-off.
Step 4: Confirm the correct term
What remains is genetic drift: because only a small, essentially random sample of the original 50,000 fish survived, the allele frequencies among those ~200 survivors are unlikely, purely by chance, to match the allele frequencies of the original large population. A sudden reduction in the size of an existing population like this (rather than colonisation of a new area) is specifically called a population bottleneck, a type of genetic drift.
Final answer
- Option A, a population bottleneck (genetic drift): the drastic, random reduction in population size, not selection, mutation, or colonisation of a new habitat, explains the shift in allele frequencies.