Cladistics

Cladistics is a method of reconstructing phylogeny without necessary recourse to either fossils or molecular distances. It is required to trace the evolutionary changes within a group of organisms (a series of taxa), and the following stages are followed.

First, one or more outgroups are selected; these are taxa that are known to be outside the group under study (the ingroup), but not too distantly related to it. Second, the characters that vary among taxa of the ingroup are listed. Third, the character states (the ways in which the ingroup taxa differ) are listed as objectively as possible, and the corresponding states of the outgroup(s) are also listed. The states shared by the outgroup with members of the ingroup are deemed to be most plausibly inherited from their common ancestor: They are called symplesiomorph, or shared primitive. States that occur only in one or more members of the ingroup and not with the outgroup(s) are deemed to have evolved since the ingroup came into being: They are called apomorph, or derived. Some of these apomorph states are confined to a single member of the ingroup:These are called autapomorph, or uniquely derived. Other apomorph states are shared between different members of the ingroup: These are called synapomorph, or shared derived. It is the synapomorph states, of course, that are evidence for phylogenetic relatedness.

The simplest case reconstructs the phylogeny of just three ingroup members (the three-taxon problem). An example in anthropology might be the reconstruction of the phylogenetic relationships of human, gorilla, and baboon. We might take the lemur as a sensible outgroup; it is acknowledged that lemurs are phylogenetically more distant from the members of the ingroup than they are from each other. We can list a few characters that vary in the ingroup, and their states in the different taxa, as follows:

In Characters 2 and 4, human is different from the outgroup, but uniquely so: The states are autapomorph (uniquely derived). These two characters therefore show how unusual human is, but not which of the other two ingroup members it more resembles. But if we take Characters 1, 5, and6, we see that though human is again different from the outgroup, it shares the derived states with gorilla; these states are therefore synapomorph (shared derived) and are evidence that human and gorilla are phylogenetically closer to each other than to baboon. Finally, Character3 is a special case; it has three states, not just two, and logically, the gorilla state (sparse) is intermediate between that of baboon and the outgroup (dense) and that of human (very sparse).

The deduced primitiveness or derivedness of the character states is called their polarities. The character states are typically coded; in the above case, all the characters have States 1 and2, except for Character 3, which has States 1, 2, and 3. One then has the option of whether State 2 is to be treated as intermediate between 1 and 3 (ordered) or whether all three states are equidistant.

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