Fuzzy wuzzy was a bear. Andy Panda, too - giant panda related to bear, not raccoon

All these criteria affirm Davis's arguments (see evolutionary tree, page 44). One limb of the carnivore tree split into a bear and a raccoon lineage more than 40 million years ago. Raccoons and lesser pandas occupy one branch (with a point of division so soon after the bear-raccoon split that we can't be entirely sure about raccoon affinities for the lesser panda). The giant panda, however, didn't split from the bear lineage until much later (no more than 30 million years ago, and as little as 10 million by some estimates). The giant panda lies firmly on the branch of bears, the lesser panda, with less assurance, on the raccoon limb.

A fourth criterion--number and form of chromosomes--uses geneticinformation of a different kind to reach the same conclusion. Giant pandas have 42 chromosomes, most bears 74--a difference that seems, at first glance, to speak against close affinity. But most panda chromosomes are long, with their joining points (or centromeres) in the middle. Each of these long, ''jointed'' chromosomes is the product of fusion between two short chromosomes with joining points at or near the ends. Bear chromosomes are short and acrocentric (joining points at end)--and, you guessed it, two bear chromosomes can often be identified as a pair that produced one of the long panda chromosomes. So pandas probably evolved from bear ancestors by a relatively simple process of chromosomal fusion.

On the other hand, only two chromosomes of the lesser pan- da have recognizable counterparts in either giant pandas or bears. However, 14 lesser panda chromosomes show clear ho- mology (common structure due to descent) between lesser pandas and raccoons--although ten of these can be identified in most non-bear carnivores. Thus, lesser pandas and rac- coons share a general pattern found in most members of their order, while both giant pandas and bears have evolved a strikingly new arrangement, with the adde wrinkle of secondary fusion in giant pandas. The chromosomal evidence supports phylogenies based on genetic similarity: giant pandas are modified bears; less- er pandas may be close to raccoons.

But how do we establish relationships on evolutionary trees, andwhy does the modern evidence of molecules command assent, while classical studies of bones and tissues rarely prevail? It may seem easy at first glance: just tote up the similarities between any two groups; the more alike, the closer the evolutionary relationship. But similarities come in two basic and contrary forms, one reflecting evolutionary relationship, the other confounding it. Hence, our task is more complex: we must learn to distinguish the two kinds of similarity, identify and exclude the confounding style, and base evolutionary trees only on the confirming style.

Similarity due to descent from a common ancestor indicates evolutionary relationship; it's called homology. I am typing with finger bones used by a mouse to run or an aardvark to dig into termite mounds. The bones are simliar in form and position (despite differences in function) because all three species inherited them from the common ancestor of terrestrial vertebrates--and they record our evolutionary affinity (whereas different bones in the fin of a salmon indicate another lineage of descent). Similarity due to independent evolution for common function in separate lineages confounds evolutionary relationship; it's called analogy. The ichthyosaur, a marine descendant of terrestrial reptiles, evolved a dorsal fin and a bilobed tail with uncanny external resemblance to similar features in fishes. Such devices are essential for proper balance and locomotion in water, and several lineages have evolved them independently. But an ichthyosaur's evolutionary affinities are with bronto sauruses, not with sharks. In short, analogy must be identified and eliminated, homology recognized and used in the construction of evolutionary trees. This principle is easy enough to state, but how do we know a homology when we see one? In practice, the fundamental distinction may be very difficult--and the traditional debates of post-Linnaean natural history center upon this dilemma.