A cladogram fig 1 represents a natural hierarchy of

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Unformatted text preview: s the common ancestor of all endotherms?... 59 Lab3 ‐ Vertebrate phylogeny Appendix 1 Phylogenetic research: the theory Cladistics: a Simple Method Figure 1: A Cladogram. (Illustration: Erin Chambers). The main purpose of phylogenetic research is to reconstruct the evolutionary history of living beings based on similarities and differences which characterize them. Over the last few decades, systematics has found a new purpose in science by adopting the cladistic methodology. This appendix explains how to express phylogenetic hypotheses by means of a branching diagram called a cladogram. A cladogram (Fig. 1) represents a natural hierarchy of monophyletic groups. These are defined on the basis of consistency in the distribution of derived (or apomorphic) characters. The objective of this lab session is to acquaint you with the basic principles of phylogenetic research. Characters: Terminology The foundation for any phylogenetic hypothesis lies in the possibility that a researcher finds a sufficient number of characters to construct it. Such characters may be ontogenic, structural, behavioural, biochemical or hormonal elements that are consistent and that can be applied in comparative studies (Nelson & Platnick, 1981). This latter aspect is fundamental because the polarity of this character must be determined. Polarity implies a distinction, for a given character, between an ancestral state* (also identified as plesiomorphic) and a derived state* (also identified as apomorphic). This step is crucial because only a group of organisms displaying an exclusive (apomorphic) state of a character can eventually be considered as monophyletic, based on this character. Wiley (1981) stated a number of criteria that allow one to determine whether a character is homologous or not (in which case it is called homoplastic). For instance, a character observed in two or more taxa is deemed homologous for these taxa if it is also observed in one of their common ancestors. If the same character is present in different states in two taxa, these different states of the same character will nevertheless be considered homologous, if it can be shown that one is derived from the other. If this is the case, the original or ancestral 60 *by extension, we will call a character in its ancestral and derived states ‘ancestral character’ and ‘derived character’, respectively. Lab3 ‐ Vertebrate phylogeny state is considered as the plesiomorphic state whereas, the derived state is considered to be the apomorphic state of the character. Once the apomorphic state of a character is shared by a group of species, this state is identified as a synapomorphy for these species. Hence, a group sharing a synapomorphy is monophyletic. Characters with more than one derived state: Sometimes characters possess several derived states: a character may have evolved several times during time (for instance, the number of digits on vertebrate hind limbs can be none, four or five). This may be problematic, since the sequence of transformation will have to be determined (which o...
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This note was uploaded on 02/04/2014 for the course BIO 1130 taught by Professor Fenwick during the Fall '08 term at University of Ottawa.

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