(This exercise is based on Moriarty Lemmon, E., and A. R. Lemmon. 2010. Reinforcement in chorus frogs: lifetime fitness estimates including intrinsic natural selection and sexual selection against hybrids. Evolution 64:1748–1761.)
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Under nearly all circumstances, postzygotic reproductive isolation cannot evolve under the direct action of natural selection. Instead, postzygotic isolation evolves as a by-product of other genetic changes as proto-species diverge. In contrast, natural selection can directly act to enhance prezygotic reproductive isolation. Consider two species with overlapping ranges. If hybrids between the species are less fit than pure species individuals, females that mated with heterospecific males would be at a selective disadvantage, because their resulting offspring had low fitness. Thus, there could be natural selection acting on females to preferentially choose conspecifics and avoid heterospecifics. This process is often called reinforcement or reinforcing selection.
In cases where the species have overlapping (but not coincidental) geographic ranges, reinforcing selection could lead to an interesting pattern: Females from populations that encounter the other species (sympatric populations) are less likely to mate with heterospecifics than are females from populations that do not encounter the other species (allopatric populations). Because of these differences in mating discrimination, individuals from conspecific populations may diverge from allopatric populations in characteristics related to mating and reproduction, such as mating song. This pattern is called reproductive character displacement.
Although the existence of reproductive character displacement is sometimes viewed as evidence for reinforcement, reproductive character displacement can evolve for reasons unrelated to reinforcement. Thus to infer reinforcement, biologists need to dig deeper. Among the criteria needed to infer reinforcement is that the hybrids are indeed maladaptive. Showing that hybrids are maladaptive often requires examining their fitness closely. A key aspect in fitness measurement is that the fitness be measured across the entire life cycle of the organisms.
Emily Moriarty Lemmon and Alan Lemmon have studied the evolution of reproductive isolation between two species of chorus frogs, Pseudacris feriarum and Pseudacris nigrita, that live in the southeastern United States. Although their ranges overlap, P. feriarum tends to live farther north and P. nigrita, farther south.
Use the information in Figure 1 to answer Questions 1 through 3. (Click the image to enlarge figure.)
Question 1. Do populations of P. feriarum from Tennessee come into contact with the other species? Would you classify them as allopatric or sympatric populations?
Question 2. Do populations of P. feriarum from Florida come into contact with the other species? Would you classify them as allopatric or sympatric populations?
Question 3. Based on the pulse rate data, which species exhibits more reproductive character displacement? Explain.
Use the information in Figure 2 to answer Questions 4 through 9.
Question 4. Do hybrid males have significantly lower viability than pure P. feriarum males have?
Question 5. For which life cycle stage, and for which sex, do hybrids fare the worst?
Question 6. Are there any life cycle stages in which the hybrids do significantly better than the pure species?
Question 7. Overall, across all life cycles and both sexes, do hybrids have lower fitness than pure species individuals?
Question 8. How is Question 7 relevant to whether reinforcement is operating in this system?
Question 9. Suppose the researchers only examined the fitness of hybrid females. What conclusions would be different, and how would this affect whether they could claim that reinforcement is occurring?