solve the CHAPTER MYSTERY

GRIN AND BEAR IT

Most biologists classify the polar bear, Ursus maritimus, as a separate species from the brown bear, Ursus arctos. The teeth, body shape, metabolism, and behavior of polar bears are very different from those of brown bears. But some systematists are now questioning that classification. Are polar bears and brown bears two distinct species? The answer depends on what a species is. The usual definition of species is “a group of similar organisms that can breed and produce fertile offspring.” Polar bears and brown bears can, in fact, mate and produce offspring that are fertile. However, in the natural environment, polar bears and brown bears almost never mate. The question is complicated by DNA analysis. There are different populations of brown bears, and these different populations have somewhat different genetic makeups. DNA analysis has shown that some populations of brown bears are more closely related to polar bears than they are to other populations of brown bears. According to DNA analysis, if polar bears are indeed a separate species, brown bears by themselves do not form a single clade.

1. Classify List the evidence that supports classifying polar bears and brown bears into two different species. Then list the evidence that indicates that polar bears and brown bears belong to the same species.

2. Infer What evidence indicates that different populations of brown bears belong to different clades?

3. Connect to the Do you think that the classic definition of species—“a group of similar organisms that can breed and produce fertile offspring”—is still adequate? Why or why not?