Cladistics

Willi Hennig (1956, 1966) developed a systematic methodology that sought to emphasize:

• objectiveness and reproducibility (so to not be the product of the subjective approach of individual taxonomists), and
• accordance with "true" evolutionary patterns.

Hennig's system, phylogenetic systematics or cladistics, is now the standard method of phylogenetic inference among evolutionary biologists. Just like phenetic systematics, cladistics takes advantage of the availability of computer software to analyze large data sets. However, the cladistic algorithms "think" about characters in a way that is more consistent with our ideas about how evolution works.

The most important insight of cladistics is that if you take all character states shared by a number of organisms into account (i.e. if you look at overall similarity), you will not necessarily get a classification that reflects actual evolutionary relationships. Instead you need to concentrate on only certain characters-those that provide evolutionary information.

Hennig defined a few terms to describe the distinction between his approach and others. The term apomorphy means a specialized or derived character state; plesiomorphy refers to a primitive or ancestral trait. An same as autapomorphy is a derived trait that is unique to one group, while a same as synapomorphy is a derived trait shared by two or more groups. A same as symplesiomorphy is similarly a shared primitive trait. These terms are defined relative to a particular node (e.g., representing a taxonomic level) on the cladogram. This means that a trait can be a synapomorphy and a symplesiomorphy if different nodes are considered. For example (Figure 2.3): The multicellular sporophyte is an autapomorphy of the land plants, but a synapomorphy of the liverworts and all other land plants, and a symplesiomorphy for the conifers and the angiosperms. Can you think of a synapomorphy for monocots and dicots, respectively?

Figure 2.3:Cladistic vocabulary. At a node A, "multicellular sporophyte" is an automorphy for clade b (land plants). At node B (for liverworts) and clade c (vascular plants), "multicellular sporophyte" is a synapomorphy. At node B, this trait is a symplesiomorphy for conifers and angiosperms.

Since current evolutionary theory says that traits arise (are derived) in lineages through evolution, only synapomorphies can be used to establish relationships. Autapomorphies contain no information about relationships (because they don't group organisms together); symplesiomorphies should not be used to unite taxa. For example, if you are comparing a clubmoss, a fern and a flowering plant, the trait "free-sporing" (as opposed to "non-free-sporing") can not be used to group the clubmoss and the fern into one group, since this feature is ancestral to both, that is, inherited from the ancestor of all land plants. On the other hand, the trait "megaphyll" (as opposed to "microphyll") unites the fern and the flowering plant into a group, excluding the clubmoss.

Organisms that are united by one or more synapomorphies share a common ancestor which possessed these derived traits. They belong to a monophyletic group (Figure 2.4; A) in which all descendants of the common ancestor have to be included. This is also referred to as a "natural" or "evolutionary" group or as a lineage. In modern evolutionary biology, we work hard to recognize only monophyletic groups. If a group does not include all the descendants of a common ancestry, the group is termed paraphyletic (Figure 2.4; B), or a grade. An example of this is the Bryophyta, which includes liverworts, mosses and hornworts, but not the vascular plants. Another example may be "gymnosperms" when used to refer to non-angiosperm seed plants. If the group includes some or all of the descendants, but not the common ancestor, it is called polyphyletic (Figure 2.4; C). For example, a group of all epiphytic plants regardless of their ancestry would be extremely polyphyletic. A sister group (or sister taxon) is defined as the closest relative to a monophyletic group as determined by one or more synamorphies uniting the groups. (Figure 2.5 on next page).

Figure 2.4: More cladistic vocabulary. (A) Monophyletic clades; (B) a paraphyletic clade; (C) a polyphyletic clade.

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