Cladistics

Originally conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966 (Lewin 1997). Today, cladistics is the most popular method for constructing phylogenies not only from morphological data but also from molecular. Unlike phenetics, cladistics is specifically aimed at reconstructing evolutionary histories.

By categorising shared-derived characters (synapomorphies), cladists attempt to identify sister groups. A shared-derived character is a trait that is shared by two groups (making them sister groups) but not by a distant common ancestor. Traits shared with a distant common ancestor are termed primitive, and are to be avoided in cladistics. Taxa with the most synapomorhpies are deemed to have diverged most recently, followed by the taxon with the next most synapomorphies, and so on.

Cladistics can be applied to many different levels of taxonomy, with sister groups being species, genera, orders, etc. To know whether a trait is primitive or derived an outgroup comparison is used. This involves taking a species or group that is distantly related to the species in question, and making comparisons of characters under examination. For example, to discern whether the tabulars (a pair of bones in the occipital region of the skull) in golden moles (Chrysochloridae) are a shared-derived character only of this family, we examine other families of Insectivora for their presence. The presence of tabulars in any other insectivoran family would indicate that it is not a shared-derived character for the golden moles, but is a primitive feature shared by a common ancestor of the families. In this example, the tabular bones of chrysochlorids are indeed a shared derived character unique to that family.

This methodology leads cladists to claim objectivity for cladistics because it removes subjective judgement of relationships based on similarity. As with phenetics, the process of cladistics involves a character matrix. Unlike phenetics, the cladogram that is based on the matrix predicts explicit phylogenetic reconstruction. Cladistics works well at higher taxonomic levels. At lower levels however, it often has to be interpreted statistically since the actual primitive state of a character is unknown. There are such a vast number of possible trees available, even when only considering a handful of species. For example, for 50 species there are 2.8 x 10 to the power of 74, possible trees, which is 10,000 times as many trees as there are atoms in the universe (Lewin 1997). Analytical methods are therefore required to select the tree that involves the smallest number of mutations to produce the observed diversity of traits. In other words, the simplest solution is reached, in the hope that it is the most likely. This is known as maximum parsimony.