(This exercise is based on Pfennig, D. W. and P. J. Murphy. 2000. Character displacement in polyphenic tadpoles. Evolution 54: 1738–1749.)
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The pressure of competition from phenotypically similar species can cause natural selection to favor the phenotypic divergence of individuals of the different species, with the result that the species resemble each other less. This process is called character displacement.
This exercise is based on a study investigating possible character displacement in two species of spadefoot toad. In both species of toads, tadpoles take two different forms: the carnivorous form or the omnivorous form (see textbook Figure 7.5). This presence of alternative forms, which is known as polyphenism (see Chapter 7), arises mainly from their feeding behavior; all tadpoles are omnivores at birth, but the ingestion of shrimp by a young tadpole will cause it to turn into a carnivore. David Pfennig and Peter Murphy at the University of North Carolina at Chapel Hill have investigated many aspects of this polyphenism, and examined whether the frequencies of these morphs in one of the species have changed due to character displacement brought on by competition with the other species.
The omnivore morph is more common than the carnivore morph in the New Mexico spadefoot toad, Spea multiplicata. In the Plains spadefoot toad, Spea bombifrons, the carnivore morph is more common. In southeastern Arizona, the New Mexico spadefoot toad is the only species of spadefoot toad present in high-elevation ponds, but the Plains spadefoot toad occurs in ponds below about 1500 meters.
To examine the effect of competition on both species, Pfennig and Murphy performed lab experiments in which they placed carnivore morphs of both species in the presence of individuals of the same species (pure-species treatment) and in the presence of the same numbers of individuals from the other species (mixed-species treatment), and monitored the growth of the tadpoles.
Figure 1 Evidence of differential competitive abilities of two species (Spea bombifrons and S. multiplicata, both carnivore morphs) when reared in the laboratory. Shown is mean (± SD) change in snout-vent length of tadpoles when reared with a conspecific (pure-species group) or a heterospecific (mixed-species group). The tadpoles were fed fairy shrimp and detritus.
Question 1
The results are shown in Figure 1. What inferences can be drawn?
Figure 2 Proportion of wild-caught tadpoles of each species expressing the carnivore morphology (height of bar) in nine different natural ponds: five pure-species ponds (i.e., ponds where S. multiplicata was the only Spea species present) and four mixed-species ponds (i.e., ponds where both S. multiplicata and S. bombifrons were present; there were no S. multiplicata carnivores in ponds AP and SH). Number above each bar is the number of tadpoles of each species sampled.
Question 2
Figure 2 shows the frequency of the carnivore morph in the New Mexico spadefoot toad, S. multiplicata, and the Plains spadefoot toad, S. bombifrons, in both the high- and the low-elevation ponds in southeastern Arizona. What evidence supports character displacement operating on S. multiplicata?
Question 3
What are some alternative explanations as to why the New Mexico spadefoot toad has a much lower frequency of the carnivore morph in the ponds with the other species?
Question 4
What type of experiment could you perform to show that the changes in the frequency of morphs of S. multiplicata from the high- and low-elevation ponds were due at least in part to changes in allele frequencies, and were not completely due to environmental differences?
Figure 3 Mean (± SEM) proportion of Spea multiplicata tadpoles from each of nine different ponds expressing the carnivore morphology (height of bar) when reared from fertilization under common conditions. Parents of tadpoles from pure-species ponds were from high-elevation sites where S. bombifrons is absent, whereas parents of tadpoles from mixed-species ponds were from low-elevation sites where both Spea species co-occur.
Question 5
Pfennig and Murphy performed such a common garden experiment, rearing the toads taken from pure-species (high elevation) and mixed-species (low elevation) in the same environment. What inferences can be drawn from their results, shown in Figure 3?
