Biology of Invertebrates The basics of Cladistics

The value of cladistics is that it provides a systematic, repeatable method for infering phylogenetic relationships and creating a natural classification The immediate goal of a cladistic analysis is to identify sister taxa organized within a monophyletic group by identifying apomorphic characters.

Definitions: Apomorphy - a character derived after splitting from an ancestor - if sister taxa share such a character it is called a synapomorphy Monophyletic group - all descendents and only descendents from a single common ancestor. Paraphyletic group - a group which is missing one or more of the descendents of the common ancestor Pliesiomorphy - an ancestral character if two or more taxa share a pliesiomorphy it is called a sympliesiomorphy. Polyphyletic group - group containing members derived from two or more ancestors. Note: only monophyletic groups are to be considered >natural= groups

A Paraphyletic Group A Polyphyletic Group

Whether a particular character is an apomorphy or a pliesiomorphy depends on what camparisons are being made: Z X Y (1) (2) (1) is an apomorphy of Y (2) is a sympliesiomorphy when comparing X with Y but is a synapomorphy when comparing X,Y with Z

Practical conventions used to make cladograms 1. Dichotomous Branching - all speciation events are to be treated as a dichotomous branching creating two sister taxa. 2. Both of the sister taxa will be given new designations regardless of similarity to the ancestral form. 3. A lineage will retain its designation regardless of change over time if no speciation event has occurred. 4. The degree of change at a speciation event is not relevant

Testability: Ideally, careful camparative anaysis of a particular group of organisms should yield a single cladogram which indicates sister taxa and monophyletic groups. If such a cladogram is based on one suite of characters such as reproductive anatomy, it can be tested by doing a second analysis using a different suite of characters. The second cladogram should match the first. In reality a single analysis will yield several cladograms. Therefore various criteria have been adopted to decide which of the possible cladograms is the most likely (we will not pursue these criteria in this course). Likewise the second analysis often yields a different cladogram from the first and judgements need to be made as to which is most acceptable. Cladograms are in essence competing hypotheses of geneological relationships.