Biodiversity, classification and evolution

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   Students surveyed fish in a lagoon in the Maldives and recorded the number of individuals of each species.

   Species	Number of individuals, $n$
   A	10
   B	6
   C	3
   D	1

   Calculate the index of diversity, $D$, using $D = \frac{N(N-1)}{\sum n(n-1)}$ where $N$ is the total number of individuals.

   Mark scheme

   • $N = 20$, so $N(N-1) = 380$ (1)
   • $\sum n(n-1) = 90 + 30 + 6 + 0 = 126$ (1)
2. Question 25 marks

   A population of crabs living on pale coral sand includes individuals with pale shells and individuals with dark shells. Shell colour is genetically determined. Birds feed on the crabs.

   Explain how natural selection could lead to an increase in the proportion of pale-shelled crabs over many generations.

   Mark scheme

   • Variation in shell colour exists in the population, caused by different alleles (originally from mutation) (1)
   • Pale crabs are camouflaged against the sand, so they are less likely to be eaten by birds (selection pressure) (1)
   • Pale crabs are more likely to survive and reproduce (1)
   • They pass on the allele for pale shells to their offspring (1)
   • So the frequency of the pale allele increases in the population over many generations (1)