Spea multiplicata is one of several species of American spadefoot toad. (a) Young spadefoot toads are called tadpoles and live in water in ponds. S. multiplicata tadpoles show three different phenotypes due to genetic variation. The three phenotypes are: detritus feeder, intermediate and carnivore. Detritus feeders are small, and carnivores are large. Intermediates vary in size between the two extremes. A detritus feeder and a carnivore are shown in Fig. 7.1. [Figure 7.1] Detritus feeders: • eat detritus (small pieces of dead organic matter) and algae (photosynthetic protoctists) • have smooth mouthparts, small jaw muscles and long intestines. Intermediates: : can eat all available food (detritus, algae and fairy shrimps) have teeth-like mouthparts, medium-sized jaw muscles and medium-sized intestines. Carnivores: • eat fairy shrimps and other small animals • have teeth-like mouthparts, large jaw muscles and short intestines. Scientists counted the number of each type of tadpole in two different ponds: pond 1 and pond 2.

1. (a)(i)In pond 1, the scientists observed: • a high density of tadpoles • a low abundance of food • that most of the tadpoles they counted were either detritus feeders or carnivores, with very few intermediates present. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 1.[3]
2. (a)(ii)In pond 2, the scientists observed: • a low density of tadpoles • sufficient food availability for all tadpoles • that most of the tadpoles they counted were intermediates, with fewer detritus feeders or carnivores. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 2.[3]
3. (a)(iii)The intestine length of S. multiplicata tadpoles shows continuous variation. Sketch a curve on Fig. 7.2 to show how intestine length varies in the tadpole population in pond 2. [Figure 7.2][1]
4. (a)(iv)A student suggested that the variation in S. multiplicata tadpoles could lead to sympatric speciation in some populations. Outline the features of sympatric speciation.[2]
5. (b)Fig. 7.3 shows the evolutionary relationships between three species of American spadefoot toad. [Figure 7.3] Explain how analysis of DNA allowed the evolutionary relationships shown in Fig. 7.3 to be determined.[3]

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