QUESTION 27 (4 marks)
In a freshwater lake in Africa, a species of crab and its snail prey species both exhibit specialisations that are unusual for freshwater species. The crabs possess strong claws with characteristics of marine crabs that crush and peel shells, and the snail prey have thick, strong shells that resist crushing.
a) Identify which pattern of evolution this example represents.
[1 mark]
b) Describe how the two species may have evolved these characteristics.
[3 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
27 | a) | Coevolution | • states coevolution [1 mark] | |
27 | b) | The increased ability for crabs to crush shells acts as a selection pressure on the prey and the stronger shells on the snails acts as a selection pressure on the crabs. Within the crab population, some will have stronger claws and be more successful in preying on the snails, and within the snail population, some will have stronger shells and resist predation — these are both selection advantages. This means that the population will gain a higher proportion of the genes that lead to claw strength or shell strength and, therefore, the population gains phenotypic resistance. | • states the selection pressures [1 mark] • describes variation in claw strength and shell strength occurring in both populations [1 mark] • describes the individuals with the selection advantage surviving and passing on the genes/traits to the next generation [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.