24.1 – 1

1. 24.1 – 1 Two bird species in a forest are not known to interbreed. One species feeds and mates in the treetops and the other on the ground. But in captivity, the two species can interbreed and produce viable, fertile offspring. What type of reproductive barrier most likely keeps these species separate? Explain.

2. 24.1 – 1 Since the birds are known to breed successfully in captivity, the reproductive barrier in nature must be prezygotic. Given the species differences in habitat preference, the reproductive barrier is most likely to be habitat isolation.

3. 24.1 – 2 • Which species concept can be used for both sexual and asexual species? • Which can only be applied to sexual species? • Which would be most useful for identifying species in the field?

4. 24.1 – 2 a. All species concepts except the biological species concept can be applied to both asexually and sexually reproducing species because they define species based on characteristics other than the ability to reproduce.

5. 24.1 – 2 b. The biological species concept can be applied only to extant sexual species.

6. 24.1 – 2 c. The easiest species concept to apply in the field would be the morphological species concept because it is based only on the appearance of the organism. Additional information about its ecological habits, evolutionary history, and reproduction are not required.

7. 24.2 – 1 Explain why allopatric speciation would be less likely to occur on an island close to a mainland than on a more isolated island of the same size.

8. 24.2 – 1 Continual gene flow between mainland populations and those on a nearby island reduces the chance that enough genetic divergence will take pace for allopatric speciation to occur.

9. 24.2 – 2 Normal watermelon plants are diploid (2n = 22), but breeders have produced tetraploid (4n = 44) watermelons. If tetraploid plants are hybridized with their diploid relatives, they produce triploid (3n = 33) seeds. These offspring can produce triploid seedless watermelons and can be further propagated by cuttings. Are the diploid and tetraploid watermelon plants different species? Explain.

10. 24.2 – 2 The diploid and tetraploid watermelons are separate species. Their hybrids are triploid and as a result are sterile because of problems carrying out meiosis.

11. 24.2 – 3 In the fossil record, transitional fossils linking newer species to older ones are relatively rare. Suggest an explanation for this observation.

12. 24.2 – 3 According to the model of punctuated equilibrium, in most cases the time during which speciation (that is, the distinguishing evolutionary changes) occurs is relatively short compared with the overall duration of the species’ existence. Thus, on the vast geologic time scale of the fossil record, the transition of one species to another seems abrupt, and instances of gradual change in the fossil are rare. Furthermore, some of the changes that transitional species underwent may not be apparent in fossils.

13. 24.3 – 1 How can the Darwinian concept of descent with modification explain the evolution of such complex structures as the vertebrate eye or heart?

14. 24.3 – 1 Such complex structures do not evolve all at once, but in increments, with natural selection selecting for adaptive variants of the earlier versions.

15. 24.3 – 2 Explain why the concept of exaptation does not mean that a structure evolves in anticipation of some future environmental change.

16. 24.3 – 2 Although an exaptation is co-opted for new or additional functions in a new environment, it existed in the first place because it worked as an adaptation to the original environment.

17. 24.3 – 3 How can heterochrony cause the evolution of different body forms?

18. 24.3 – 3 The timing of different developmental pathways in organisms can change in different ways (heterochrony). This can result in differential growth patterns, such as those producing different patterns of webbing in salamander feet.