phylogeny

phylogeny (morphological and molecular) ‘Phylogeny’ is the term used to describe the history of evolutionary relationships of organisms. The study of these relationships is termed ‘phylogenetics’. Phylogeny is very much interconnected with taxonomy, the science of the classification of organisms, for taxonomy involves the analysis of characters and the ordering of species in taxonomic units that are usually considered to have evolutionary significance. Because of the imperfections of the fossil record, there are large gaps which make it particularly difficult for palaeontologists to reconstruct evolutionary relationships. Although modern biologists can use hard-part and soft-part morphology, biochemical studies, DNA analysis, and even behavioural information in their analysis, palaeontologists are frequently limited to hard-part morphology only. The two main methods for developing phylogenies in palaeontology are evolutionary systematics and phylogenetic systematics or cladism. Both use morphological criteria as their main source of characters, but the approaches in each case are different.

Evolutionary systematics is a more traditional approach in which all available information is used. This consists primarily of an analysis of the distribution of characters among the species concerned. In this approach the number of characters held in common would indicate the closeness or distance of the evolutionary relationship. There are, however, problems with this approach, for it is clear that not all characters are equally valuable in indicating relationships; some similarities may be due to parallelisms, convergences, or reversals. In the past this method may not have been used rigorously enough, with the result that inappropriate characters have been used to indicate the common ancestry of groups. This method also includes stratophenetics, in which the geological age of species is considered to be extremely important. This is an approach that is widespread in palaeontology as a consequence of the perceived distribution of fossils through the geological record and the fact that they have changed through time. However, once again the incomplete nature of the fossil record causes problems and can result in more primitive members of a taxon being preserved at a higher stratigraphical level than more advanced forms. This approach must therefore be combined with rigorous character analysis. Analyses based on evolutionary systematics are represented graphically as dendrograms (family trees), on which a time scale is commonly superimposed (Fig. 1a).

Phylogenetic systematics relies on morphological characters alone, and emphasizes that characters shared by organisms manifest a hierarchical pattern in nature. This method was developed by Willi Hennig, a German entomologist who published his ideas in 1966, and it is now used extensively both in modern biology and palaeontology. Phylogenetic systematics is generally known as cladistics because of the emphasis on recognizing clades, that is, lineage branches resulting from splits in an earlier lineage. Cladistics is based on the view that recency of common origin can be shown by the distribution of evolutionary novelties or shared derived characters. These synapomorphies allow groups to be distinguished from each other, the resulting hierarchy being expressed as branching diagrams termed ‘cladograms’ (Fig. 1b). It is also accepted that many characters are primitive (symplesiomorphic) and do not contribute to an understanding of close relationships. However, it has to be understood that the system is relative and a character may be derived at one level but primitive at another. For instance, the possession of feathers is a synapomorphy for birds in relation to other reptiles which do not possess this feature. However, it becomes a symplesiomorphy if the relationship of two groups of birds is being considered. In order to decide whether characters are primitive or derived (their polarity), several criteria are used, including comparison with a related group or outgroup, and ontogenetic development of characters. Cladists assume that dichotomous splitting has taken place in each lineage, and they accordingly search for what are called ‘sister-groups’, which can be denoted by the number of synapomorphies that they share.

Three major cladistic groupings are recognized. (1) Monophyletic groups contain the latest common ancestor plus all, and only all, its descendents (Fig. 1b, A+B). Birds, including Archaeopteryx, would be a monophyletic group. (2) A paraphyletic group is one descended from a common ancestor, but not including all the descendents (Fig. 1b, C+D). Thus, although dinosaurs plus birds are monophyletic, dinosaurs alone are paraphyletic. (3) Polyphyletic groups are those containing organisms descended from different ancestors (Fig.1b, B+D). A modern example of this would be seals, sea lions, and walruses, previously classified together as pinnipeds, but now thought to derive from separate groups of carnivorous land mammals. Cladistics traditionally takes no note of time, and cladograms therefore indicate only the sequence of dichotomous splitting. In some instances cladograms are superimposed on time frames to show the relationship to fossil distributions.

In recent years the results of molecular studies have been used to verify phylogenies developed by the more traditional methods already discussed. This involves comparing the structural details of homologous proteins and other molecules in different species. The methods used to develop a phylogeny are similar to those employed when morphological characters are used. The value of these studies in the study of extinct groups is clearly limited.

David K. Elliott

Bibliography

Eldredge, E. and and Cracraft, J. (1980) Phylogenetic patterns and the evolutionary process. Columbia University Press, New York.
Wiley, E. O. (1981) Phylogenetics: the theory and practice of phylogenetic systematics. John Wiley and Sons, New York.