Such features are calledsynapomorphies therefore a

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Unformatted text preview: f the derived characters is the most recent?). We will see in appendix 2 how to practically solve this problem when building a cladogram. Monophyletic Groups Figure 2: Monophyletic group Figure 3 : Paraphyletic group A phylogenetic study only has scientific value if the group studied is recognized as being 'natural' or 'monophyletic'. According to Hennig (1950, 1966), the monophyletic status is attained only if the group contains all the descendants, and only the descendants of a common immediate ancestor (Fig. 2). In order to establish this monophyletism, the most convincing approach must demonstrate that the members of this group share one or many derived state(s) that are exclusive to them. Such features are called synapomorphies. Therefore, a hypothesis concerning the monophyletic status of a group of organisms is initially presented. This hypothesis will be refuted (rejected) or corroborated once new data has been obtained and analyzed. If the hypothesis is refuted, the main conclusion will be that the group studied is not monophyletic. This conclusion, valuable per se, may result in a reorientation of research for this group. If the hypothesis is corroborated, it will be concluded that the initial hypothesis is a reasonable approximation of historic reality. On the other hand, a group of species including a common ancestor plus only some of his descendants (Fig. 3) is defined as paraphyletic. Such groups are not informative in term of cladistics and characters that lead to the formation of these groups are not homologous (see below, homoplasies). Only one phylogeny can exist for a given group, but there is no way of knowing if the result of a phylogenetic study is a true 61 Lab3 ‐ Vertebrate phylogeny reflection of historic reality. With this caveat in mind, cladists prefer a methodological approach that aims at invalidating a hypothesis (hypothetico‐deduction), rather than confirming it (induction). This way, the phylogeny deemed to be closest to historical truth will be that which remains intact after multiple attempts to refute it. Graphic representation of phylogenetic hierarchy: the cladogram Cladists incorporate information relative to the phylogeny of groups within a branched diagram: the cladogram. The cladogram is the simplest and most informative graphic expression of the distribution of derived characteristics (apomorphies) within a group of taxa. External branches Present time SG A B C D Living species Branching point or node: common ancestor to C and D last 1(1) Time flow 3(1) 2(1) Transformation of character 1 from state (0) to (1). All species located beyond that point possess the derived character 1(1) Internal branch Distant past Hypothetical ancestor to all species. All characters are in their ancestral state. Figure 4: Example of Cladogram How to read a cladogram? A cladogram represents the distribution and the transmission of characters among species. The vertical axis represents time, with the top of the cladogram being the present time, and the bottom being the most distant time. The horizontal axis presents no information at all, merely a list of species. The or...
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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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