| Cladistics |
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A horizontal cladogram that shows the relationship among various insect groups.
A vertical or tree-like cladogram.
A cladogram is a type of tree diagram that is generated through cladistic analysis to show the postulated relationships between different groups of organisms. More precisely (according to O'Keefe and Sander) a cladogram is a branching diagram depicting the pattern of shared similarities thought to be evolutionary novelties among a series of (previously defined) taxa.
In a cladogram, all organisms lie at the end branches, which are connected by nodes, which in turn are on branches linked by even more fundamental nodes. Each node should ideally be dichotomous, meaning have only two branches, although in practice ambiguious results mean three or more connections, resulting in trichotomy and so on.
The two taxa on either side of a branch are called sister taxa or sister groups. Each node forms the base of a subtree, except for the most basal node, which is the base of the entire tree. All the organisms included in all the branches that are derived back toa single node - regardless of whether it contains one item or a hundred thousand branches - is called a clade.
A natural group consist of all the organisms in any one clade. These share a unique ancestor, the most recent common ancestor or "MRCA", which they do not share with any other organisms on the diagram.
Each clade is defined by a series of unique shared characteristics called "synapomorphies" that characterise all its members down to and including the MRCA, but not the other clades (or forms from which it diverged). For instance, hardened front wings (elytra) are a synapomorphy of beetles, a radula or rasping toorth-likeorgan is the synapomorphy of molluscs, and circinate vernation, or the unrolling of new fronds, is a synapomorphy of ferns. In practice each node is defined by a number of synapomorphies.
Cladograms today are generally compiled from large amounts of data which are fed into a computer that is running a special cladistic program. The computer calculates all the variables and comes up with a cladogram, or rather a whole lot of possible cladograms, of which the most parsimonious (the one which has the fewest reversals (loss of characteristics)) is usally chosen by researchers.
In a cladogram a node defined by a large number of synapomorphies, or which appears in a number of alternate trees, is more likely to represent an actual natural group then one supported by opnly a few synapomorphies, or which disaapears when a different computer analysis of the data is used.
Not every phylogenetic tree is a cladogram. Only the outcomes of cladistic analyses are cladograms. The trees that are used here in Palaeos will for the most part be hand-made supertrees made of several cladograms plus sometimes some extra information.