IGCSE Biology 0610: Natural Selection and Camouflage in a Rodent Population

A species of small, ground-dwelling rodent lives across a region that contains two very different habitats close together: pale, light-coloured sand dunes and outcrops of dark, weathered volcanic rock. In both habitats, individual rodents show natural variation in fur colour, ranging from pale sandy fur to dark, almost black fur. Owls hunt these rodents at night by sight and are their main predator.

Biologists sampling both habitats found that most rodents on the sand dunes have pale fur, with only a small proportion having dark fur, while most rodents on the volcanic rock have dark fur, with only a small proportion having pale fur.

(a) Explain, using the idea of natural selection, why most rodents on the volcanic rock have dark fur while most rodents on the sand dunes have pale fur. In your answer, refer to the variation already present in the rodent population, camouflage against the ground colour, predation by owls, survival to reproduce, and the passing on of alleles. [5]

(b) A farmer suggests that when a pale rodent moves from the sand dunes onto the volcanic rock, its fur gradually darkens during its own lifetime to match its new surroundings. Explain why this suggestion is incorrect. [2]

(c) Suggest what would most likely happen, over many further generations, to the proportion of dark-furred rodents on the volcanic rock if the rock gradually weathered and became covered in pale sand blown from the nearby dunes. [1]

Part (a): Explaining the difference in fur colour between the two habitats

Both rodent populations already contain variation in fur colour, from pale sandy shades to dark, almost black fur, because individuals carry different alleles for this feature. On the pale sand dunes, pale-furred rodents blend in with the ground and dark-furred rodents are easy for night-hunting owls to spot, so owls catch a larger share of dark rodents there. Pale rodents on the dunes therefore survive and reproduce more often, and each surviving pale rodent passes its alleles for pale fur on to its offspring, keeping the dark form rare in that habitat generation after generation.

On the dark volcanic rock, the situation is reversed: dark-furred rodents match the dark background and are much harder for owls to find, while pale-furred rodents stand out sharply against the rock. Dark rodents on the rock are therefore eaten less often, survive longer and produce more offspring, passing their alleles for dark fur on to those offspring. Because this pattern repeats over many generations, an increasing share of the rock population is descended from dark, well-camouflaged ancestors, so most rodents there end up with dark fur, while the sand-dune population remains mostly pale.

Part (b): Why an individual rodent cannot darken during its own lifetime

A rodent’s fur colour is determined by the alleles it inherits from its parents at birth, not by the surface it happens to be living on. Moving from sand to rock does not change which alleles a rodent carries, so its fur colour stays the same throughout its life. What actually changes over time is the proportion of pale and dark rodents in the population, because natural selection affects which individuals survive and reproduce, not because any single rodent’s own body responds to its surroundings and changes colour.

Part (c): Effect of the rock becoming covered in pale sand

If the volcanic rock became covered in pale sand, the ground colour rodents live on would change from dark to pale. Dark fur, which had been an advantage for camouflage on dark rock, would now make rodents conspicuous against the pale sand, while pale fur would become the better camouflage. Owls would then be expected to catch a larger share of dark rodents and a smaller share of pale rodents, so over many further generations the proportion of dark-furred rodents in that population would be expected to fall.

Final answers

(a) Pre-existing variation in fur colour, camouflage against the local ground colour, reduced predation on well-camouflaged rodents, and their greater survival, reproduction and passing on of alleles together explain why dark fur predominates on the rock and pale fur predominates on the sand.
(b) Fur colour is fixed by inherited alleles at birth and cannot change during an individual’s lifetime; only the proportions of pale and dark rodents in the population change, through differential survival and reproduction.
(c) The proportion of dark-furred rodents would be expected to decrease, since dark fur would no longer be camouflaged against the now-pale ground.