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Variation and Natural Selection: Question 10

Syllabus 18.3

Multiple choice Extended 1 mark

A population of a small freshwater fish lives in a lake that was colonised by a predatory fish species several thousand generations ago. Compared with an ancestral population living in a nearby lake that has never had any predatory fish, the present-day population in the colonised lake has much thicker, more extensive bony armour plates along its sides.

Which statement gives the correct natural-selection explanation for this change?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall what a correct natural-selection explanation must include

A correct explanation must state that variation already existed in the population before selection acted, identify a survival or reproductive advantage linked to that variation, and explain that the advantaged individuals reproduce and pass on their alleles, so the population’s characteristics change gradually over many generations because of shifting allele frequencies, not because individuals themselves change and pass on those personal changes.

Step 2: Evaluate option B

Option B states that armour-plate thickness already varied in the ancestral population, that fish with thicker armour were less likely to be killed by the predator and were therefore more likely to survive and reproduce, and that they passed on alleles for thicker armour to their offspring, so average armour thickness increased gradually over many generations. This includes every required step of the reasoning and describes only inherited variation and differential survival and reproduction, not any change acquired during an individual’s lifetime, so it is correct.

Why the other options are wrong

  • A: this describes an acquired characteristic, armour thickened during an individual’s own lifetime in response to attacks, being passed on to offspring. Body changes acquired during a lifetime are not encoded in the alleles passed on in gametes, so they cannot be inherited in this way; this is a classic incorrect (Lamarckian) explanation.
  • C: this wrongly suggests that natural selection can act on an entire population at once, within a single generation, and that individuals “need” and therefore develop thicker armour. In reality, variation must already exist between individuals, and only gradual, generation-by-generation shifts in allele frequency, driven by differential survival and reproduction, produce a change of this kind.
  • D: this wrongly gives the predator a direct, creative role in producing thicker armour, as if the fish’s body responded and grew like a trained muscle. The predator did not cause armour to thicken directly; it only determined which individuals, among those that already varied in armour thickness, were more likely to survive and reproduce.

Final answer

Only option B correctly explains the increase in armour-plate thickness through pre-existing variation, differential survival and reproduction, and the passing on of alleles, option B.