Edexcel A-Level Biology: Classification and Biodiversity

On a sheltered shore, a population of a marine snail shows a range of shell colours from pale yellow to dark brown. A new predatory crab arrives that hunts by sight and feeds far more easily on snails that stand out against the dark rocks. Over several years the proportion of dark-brown snails in the population rises steadily.

Describe and explain how natural selection brings about this change in the population, and explain how, over a much longer period, the same process could lead to evolution of the snail population. (6 marks)

Ecologists surveyed the ground beetles in a patch of ancient woodland using pitfall traps. The numbers of each species caught are shown below.

Beetle species	Number of individuals (n)
Species P	18
Species Q	12
Species R	6
Species S	4
Total (N)	40

The team uses the index of diversity in the form

$D = \frac{N(N-1)}{\sum n(n-1)}$D=∑n(n−1)N(N−1)​

where $N$N is the total number of organisms of all species and $n$n is the number of organisms of each species.

(a) Calculate the index of diversity, $D$D, for this woodland. Show your working and give your answer to two decimal places. (4 marks)

(b) A nearby plantation of a single conifer species, surveyed in the same way, gave an index of diversity of 2.10. Explain what the difference in the two values tells you about the biodiversity of the two habitats. (2 marks)

Biologists compared the amino acid sequence of the same respiratory protein from four mammal species. A reference species, the mouse, was compared with each of the others. The protein is 120 amino acids long. The table shows the number of amino acid positions that differ from the mouse sequence.

Species compared with mouse	Number of differing amino acids
Rat	6
Rabbit	18
Cow	30
Whale	36

(a) For the rabbit, calculate the percentage similarity of the amino acid sequence to that of the mouse. Show your working. (2 marks)

(b) Using the data, identify which species is the most distantly related to the mouse, and explain your reasoning. (3 marks)

A conservation team is breeding a rare flightless bird in captivity. Only eight birds were originally rescued, and all the chicks bred since have descended from these few founders. Genetic testing shows the captive flock has a low proportion of polymorphic gene loci and low heterozygosity compared with a wild population of a related bird.

The team plans to release birds into a reserve where a new fungal disease of the species has recently appeared.

Suggest and explain why the low genetic diversity of the captive flock could threaten the success of this release programme. (5 marks)

On a single island, two populations of a fruit-eating insect feed on two different fruit trees that ripen at different times of year. Insects on the early-ripening tree breed in spring; insects on the late-ripening tree breed in autumn. The two populations are not separated by any physical barrier, yet over time they have become so genetically different that they can no longer interbreed.

Explain how this difference in breeding season could have led to the formation of two species without any geographical separation. (4 marks)

The three-domain system places all living organisms into three large groups.

(a) Name the three domains. (2 marks)

(b) State one feature of the cells of organisms in the domain Eukarya that distinguishes them from organisms in the other two domains. (1 mark)