Natural and artificial selection — practice question

Mexican spadefoot toads, Spea multiplicata, spend most of their lives on land, but they go back to ponds to reproduce. Eggs are deposited in water and hatch into tadpoles, which feed in ponds before becoming adults. The tadpoles can be grouped into two broad types: omnivore-type tadpoles and carnivore-type tadpoles. The phenotype differences between these two tadpole types are linked to their different feeding habits. Omnivore-type tadpoles consume tiny fragments of detritus (dead material from plants and animals) and algae (microscopic photosynthetic organisms) at the bottom of ponds. These tadpoles grow slowly. Carnivore-type tadpoles eat small animals in the water, such as fairy shrimp and small omnivore tadpoles. These tadpoles grow rapidly. Fig. 4.1 shows two tadpoles of the same age, one of each type. A fairy shrimp is also shown. All three organisms are the same distance from the camera. Between these two main types of tadpole there is a continuous range of tadpoles with intermediate body phenotypes and feeding habits. For any individual tadpole, regardless of age, a phenotype score can be calculated from the tadpole's features. A tadpole with a phenotype score close to 3 is a typical omnivore type, whereas a tadpole with a phenotype score close to 7 is a typical carnivore type. Phenotype scores were measured for a large number of tadpoles sampled from two ponds. In one pond, detritus and algae were abundant, whereas in the other pond they were scarce. All other conditions in the two ponds were similar. For both ponds, phenotype scores were measured soon after the tadpoles had hatched from eggs and again ten days later.