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.

What does the term punctuated equilibria describe?

If you sequenced the DNA of all your classmates, you would probably observe that the percentage of Neanderthal genes in their genomes varies from 0–4%. How would you know whether there is a significant difference in percentage between students with blue eyes and students with brown eyes?

Can factors that cause sympatric speciation also cause allopatric speciation? Explain.

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.

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as an endangered species under the Endangered Species Act, some people think that its endangered status should be withdrawn and resources should not be spent to protect what is not a 'pure' species.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?

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

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 __________.