Concept 24.2

Q1: 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.

A1: Continued gene flow between mainland populations and those on a nearby island reduces the chance that enough genetic divergence will take place for allopatric speciation to occur.

Q2: Normal watermelon plants are diploid (2n=22) but breeders have produce 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.

A2: The diploid and tetraploid watermelons are separate species. Their hybrids are triploid and as a result are sterile because of problems carrying out meiosis.

Q3: In the fossil record, transitional fossils linking newer species to older ones are relatively rare. Suggest an explanation for his observation.

A3: According to the model of punctuated equilibrium, in most cases the time during which speciation 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 record are rare. Furthermore, some of the changes that transitional species underwent may not be apparent in fossils.