Natural selection and speciationEdexcel A-Level Biology B: Subtopic test

1. 1
   Rock pocket mice live in the Arizona desert. On pale sandy ground most of the mice have pale fur, but on areas of dark volcanic rock most have dark fur. Owls hunt the mice by sight at night. Fur colour is controlled by a single gene with a dark allele and a pale allele, and mice of both fur colours are born in both types of area.

   (a)

   Which factor is the main selection pressure acting on fur colour in these mice?

   [1 mark]

   • AMutation of the fur-colour gene
   • BPredation by owls
   • CThe number of alleles in the gene pool
   • DThe temperature of the desert at night

   (b)

   Which statement correctly describes the variation in fur colour in these mice?

   [1 mark]

   • AIt is environmental variation caused by the colour of the ground
   • BIt is acquired during the mouse's life and passed to its offspring
   • CIt only appears after natural selection has taken place
   • DIt is genetic variation caused by different alleles of one gene

   (c)

   Explain how natural selection has led to a higher proportion of dark-furred mice on the dark rock.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The kangaroo rat lives in North American deserts. It has long, powerful hind legs and a long tail, spends the hot day in a cool burrow, forages for seeds at night, and produces very small volumes of highly concentrated urine.

   (a)

   Which feature of the kangaroo rat is a physiological adaptation?

   [1 mark]

   • AProducing small volumes of highly concentrated urine
   • BLong, powerful hind legs
   • CForaging for seeds at night
   • DA long tail

   (b)

   Which statement best describes the niche of the kangaroo rat?

   [1 mark]

   • AThe area of desert in which it lives
   • BThe number of kangaroo rats in the population
   • CIts role in the ecosystem, including where and when it feeds, determined by its adaptations
   • DIts position in the food chain only

   (c)

   Explain how the kangaroo rat's behaviour of spending the day in a burrow increases its chance of survival in the desert.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   About 10 000 years ago a river changed course and became a wide, fast-flowing barrier that ground squirrels cannot cross, splitting one population into two. The environment differs on the two sides: one side is dry scrubland and the other is woodland. The two populations now differ in body size and breeding season, and in captivity their members do not produce fertile offspring.

   (a)

   Explain how the change in the course of the river could have led to the formation of two species of ground squirrel.

   [3 marks]

   (b)

   In a species of insect, two groups living on the same host plant have started to breed at different times of year. Explain how this could lead to sympatric speciation, and state one way it differs from the speciation of the ground squirrels.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A hospital ward has an outbreak of Staphylococcus aureus infection. Most of the bacteria are killed by the antibiotic methicillin, but a few carry an allele that makes them resistant (MRSA). The bacteria divide about every 30 minutes, and patients on the ward are frequently given antibiotics.

   (a)

   Explain how MRSA could become the dominant strain of bacteria on the ward.

   [6 marks]

   (b)

   The hospital's infection control team argues that developing new antibiotics will never be enough on its own. Evaluate this view.

   [6 marks]

   Total for question 4: 12 marks

End of questions