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Figure 12.3 For any four taxa (A, B, C, D; the tips of the branches), there are three different
unrooted trees (I, II and III at left of figure). Each unrooted tree can be rooted on any of its five
branches; two of the possible five rooted trees for unrooted tree II are shown at the right (IIA, IIB).
Rooted trees IIC, IID, and IIE are not shown. (Redrawn from Gibson and Muse 2002 .)
to other organisms are called characters . Characters can be based on morphol-
ogy, physiology, ecology, behavior, biochemistry, or genetics.
There are several approaches to developing classifications using these char-
acters. Unfortunately, debates remain over which approach is more objective,
appropriate, or practical. Phenetic systematics focuses on overall similarities
among organisms, involves all possible characters, and calculates average simi-
larities with all characters assumed to be equally useful. In some cases, classifica-
tions based on phenetic similarities may reflect the phylogeny of taxa because
those that are most similar may well have shared a most-recent ancestor, but
this need not be so because of convergent evolution.
Cladistic ( phylogenetic ) systematics uses only cladistic relationships as a basis
for constructing classifications ( Hennig 1966 ). The rate or amount of change is
not considered and only monophyletic taxa are allowed. This approach focuses
on the order of origin of lineages and also takes into account the amount and
nature of evolutionary change that occurs after cladogenesis. Characters are not
assumed to be equal, and are weighted accordingly. One of the major difficul-
ties in any reconstruction of phylogeny is to determine which character is primi-
tive or ancestral ( plesiomorphic ), and which is derived ( apomorphic ).
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