When gene flow between two populations ceases, the potential for __________ exists.

Neanderthals disappeared about 40,000 years ago when the modern human population increased. This is an example of what outcome(s) of secondary contact (see Table 24.4)? Predict one way this result might have come about.

Cultivated American cotton plants have a total of 52 chromosomes (2n = 52). In each cell, there are 13 pairs of large chromosomes and 13 pairs of smaller chromosomes. Old World cotton plants have 26 chromosomes (2n = 26), all large. Wild American cotton plants have 26 chromosomes, all small. Propose a testable hypothesis to explain how cultivated American cotton probably originated.

SCIENTIFIC THINKING Explain how the murky waters of Lake Victoria may be contributing to the decline in cichlid species. How might these polluted waters affect the formation of new species?

The red wolf, Canis rufus, which was once widespread in the southeastern and south central United States, was declared extinct in the wild by 1980. Saved by a captive breeding program, the red wolf has been reintroduced in areas of eastern North Carolina. The current wild population is estimated to be about 100 individuals. It is presently being threatened with extinction due to hybridization with coyotes, Canis latrans, which have become more numerous in the area. Red wolves and coyotes differ in terms of morphology, DNA, and behavior, although these differences may disappear if interbreeding continues. Although the red wolf has been designated as an endangered species under the Endangered Species Act, some people think that its endangered status should be withdrawn and resources should not be spent to protect what is not a 'pure' species. Do you agree? Why or why not?

Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species that lived side by side in parts of their ranges. However, recent books show them as eastern and western forms of a single species, the yellow-rumped warbler. Apparently, the myrtle warbler and Audubon's warbler __________.

Imagine that part of a population of South American finches is blown by a storm onto an island far offshore and manages to survive and reproduce there for a period of 10,000 years. After that period, a climate change results in lower sea levels and the reconnection of the island with the mainland. Members of the formerly isolated island finch population can now interact freely with members of the original mainland population. Which of the following observations would, by itself, lead you to conclude unequivocally that the island finch population had evolved into a distinct species, according to the biological species concept?

At which point in the adaptation of a population is it clear that speciation has occurred?

Three species of frogs, Rana pipiens, Rana clamitans, and Rana sylvatica, all mate in the same ponds, but they pair off correctly because they have different calls. This is a specific example of a __________ barrier, called __________.