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Figure 3.7. A tree of life.This particular one is a satire by Matt Groening.The image of evolution as a tree, however; is completely familiar (From the Big Book of Hell © Matt Groening. All Rights Reserved. Reprinted by permission of Pantheon Books, a division of Random House, Inc., New York)

Using cladograms to To reconstruct phylogeny, we need a way to recognize how closely two reconstruct phylogeny creatures are related. Superficially this is very simple: things that are more closely related tend to share specific features. We know this intuitively by simply observing that organisms that we believe are closely related (e.g., a dog and a coyote) share many similarities and because we have seen the results of breeding, in which offspring look, and sometimes act, very much like their parents.

Cladograms were initially described in this chapter without placing them within an evolutionary context. Considered in an evolutionary context, the specific characters that we said characterize groups can be treated as homologous among the groups that they link. Fur in mammals once again (!) provides a convenient example. If all mammals are fur-bearing (and mammals are monophyletic), the implication is that the fur found in bears and that found in horses can in fact be traced back to fur that must have been present in the most recent common ancestor of bears and horses. But this is putting the cart before the horse. It is the distribution of characters that helps us to determine which groups are monophyletic and which are not, and, in the case of mammals, the conclusion that they are monophyletic is in part based upon the fact that mammals all share the specific character of fur (among many other characters). In an evolutionary context, specific characters are termed "derived" or "advanced," and general characters are termed "primitive" or "ancestral." The term "primitive" certainly does not mean worse or inferior, and advanced certainly does not mean better or superior. These refer instead to the timing of evolutionary change; derived characters evolved later than primitive characters. Only derived characters provide evidence of natural (monophyletic) groups because, as newly evolved features, they are potentially transferable from the first organism to acquire them to all its descendants. Primitive characters - those with a much more ancient history - provide no such evidence of unique natural relationships. To illustrate this, we resort for the last time to mammals and fur. Fur, we said, is among the shared, derived characters that unite the mammals as a monophyletic group. On a cladogram, therefore, we look for characters that unite a bifurcation point in the diagram. All organisms characterized by shared, derived characters are linked by the cladogram in monophyletic groups. The idea is that evolutionary history can be recovered (or reconstructed) using shared, derived characters organized on a cladogram. Box 3.1 exemplifies this for a non-biotic group: watches.

Reflecting the hierarchy of character distributions in nature, the cladogram documents monophyletic groups within monophyletic groups. In Figure 3.8, a small part of the hierarchy is shown: humans (a monophyletic group possessing shared, derived characters) are nested within the mammals (another monophyletic group possessing other shared, derived characters). Notice that the character of warm-blooded-ness is primitive for Homo sapiens, but derived for mammals. As we have seen, features can be derived or primitive (but not at the same time), all depending upon what part of the hierarchy one is investigating.

Wristwatches: when is a watch a watch?

Analogue and digital timepieces are commonly called "watches." Implicit in the term "watches" is some kind of evolutionary relationship: that these instruments have a common heritage beyond merely post-dating a sundial. But is this really so? Here we use cladistic techniques to infer the evolutionary history of watches.

Consider three types of watch: a wind-up watch, a digital watch, and a watch with quartz movement (Figure B3.I.I). Six cladograms are possible for these instruments, but it can be seen

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