So how does this apply to evolution? Using character hierarchies portrayed on cladograms, we establish clades or monophyletic groups2: groups that have evolutionary significance because the members of each group are more closely related - by genealogy - to each other than they are to any other creature. If a group is monophyletic, it also implies that all members of that group share a more recent common ancestor with each other than with any other organism. The cladogram in Figure 3.6 suggests that the cat and the dog are more closely related to each other than either is to the monkey.
Now that we're speaking in terms of organic evolution, the specific characters that we said characterize groups can now be treated as homologous among the groups that they link. Mammalian fur once again (!) provides a convenient example. We conclude that mammals are monophyletic based upon the fact that mammals all share a unique type of fur (among many other characters). 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.
Because now we're working with the evolution of organisms, the word "specific" (or "diagnostic") is generally replaced by the terms derived, or advanced, and "general" ("nondiagnostic") characters are termed primitive or ancestral. "Primitive" certainly does not mean worse or inferior, just as "advanced" certainly does not mean better or superior; these refer only to how much the character has been changed by evolution. Primitive specifies the condition of a particular feature in the ancestor; advanced specifies an evolved condition of that character in its descendant.
2. To add to the nomenclature, these are sometimes termed "natural groups."
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