Evolution and systematics

Although this family has a small number of genera and species, it still has a good share of controversy when it comes to classification. For example, some systematists over the years have placed the giant panda in a subfamily of Ursidae, as it is in this chapter, or in its own family, called Ailuropodidae. The Malayan sun bear, sloth bear, and polar bear are often grouped under the Ursus genus, but sometimes fall under the genera Helarctos, Melursus, and Thalarctos, respectively. Subspecies of the brown bear (U. arctos) are often listed as separate species, including the Alaskan brown bear (U. middendorffi) and the grizzly bear (U. horribilis). In addition, the red panda (Ailurus fulgens) was once listed with the ursids, but is now considered a to be a member of its own family, the Ailuridae, or a subfamily of the Procyonidae, which includes the raccoons.

This chapter uses the following classification:

• giant panda, Ailuropoda melanoleuca

• Malayan sun bear, Helarctos malayanus

• sloth bear, Melursus ursinus

• spectacled bear, Tremarctos ornatus

• American black bear, Ursus americanus

• Asiatic black bear, U. thibetanus

The family Ursidae is believed to have originated in Asia, and is closely related to the canids (dogs and relatives), pro-cyonids (raccoons and relatives), and ailurids (lesser panda). The giant panda is considered to be the most primitive of the bears. Various evolutionary studies have attempted to determine the relationships of the other bears. Fossil studies seem to indicate that the spectacled bear, which is in the subfamily Tremarctinae, diverged from the remaining bears, which are in the subfamily Ursinae. The fossil record also points to a very close relationship between the Asiatic black and American black bears, and places brown and polar bears close to

A brown bear (Ursus arctos) fishes for salmon. (Photo by Jeff Foott. Bruce Coleman, Inc. Reproduced by permission.)
A polar bear (Ursus maritimus) mother and cub. Polar bears give birth to one or two cubs and wean them at two or three years of age. (Photo by John Swedberg. Bruce Coleman, Inc. Reproduced by permission.)

them evolutionarily. Other studies using mitochondrial DNA and cytochrone-b sequence data have provided clarification, and sometimes challenged, previous conclusions. For example, mtDNA data have indicated that polar bears and spectacled bears are very closely related, and diverged from the ursinids about 2 million years ago. Cytochrome-b data appear to show that the sun bear and American black bear are sister taxa, and are somewhat separated from the Asiatic black bear.

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