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Unformatted text preview: der of species on the X axis is totally subjective (for instance, people like to place the outgroup on the left end, so that it is left out of the rest of the species). Therefore, only the relation between groups on the Y axis is important, and no conclusion about the relationship between species can be made based on their proximity on the X axis. 62 Lab3 ‐ Vertebrate phylogeny Time is displayed on the Y axis in a non‐quantitative way. You cannot measure time on the Y axis and you cannot determine the sequence of events that are located on different branches based on their position on the Y axis. Character transformation events are symbolised by tick marks on branches and are labelled X(Y), where X is the character number and Y the derived state of this character (usually coded as “1” or “2”). The derived state is transmitted to all species located beyond (=above) the transformation point, until present day, or until a subsequent transformation occurs. A cladogram represents groups that share a common derived character (a synapomorphy). Those groups are called monophyletic groups. On figure 4, groups [SG ABCD], [ABCD], [BCD] and [CD] are monophyletic groups. Species that share a recent common ancestor are more closely related than species with an older common ancestor: C and D are more closely related than A and D. A Method for Determining Polarity: the Outgroup Comparison How do we establish if a character state is apomorphic or plesiomorphic? One way is to use a method called the outgroup comparison. The outgroup comparison uses a comparison of the character state within the study group (or ingroup) with the state observed in a taxa outside of this group (outgroup). Thus, when the state of a character is identical to the state observed in the outgroup, it is defined as ancestral (plesiomorphic), whereas any state different from that of the outgroup constitutes a derived (or apomorphic) state. A good outgroup should be closely related to the ingroup otherwise it would be difficult to find homologous characters between taxa of these groups. Ideally, the outgroup should be a monophyletic group sharing a direct common ancestor with the ingroup. It is also referred to as the Sister Group (SG). Conflictive genealogy: The principle of parsimony More often than not, the phylogeneticist will have to face conflictive sets of character transformation (thus conflictive phylogenies). This is when the principle of parsimony is invoked. This principle, also known as Occam's razor, indicates that the solution which requires the minimum number of character states or transformations (evolutionary events) must be selected when different hypotheses are in conflict. In phylogenetic terms, this does not imply that evolution necessarily follows a parsimonious path but that the most acceptable cladogram, from a scientific standpoint, is the one that contains the lowest number of transformations. 63 Lab3 ‐ Vertebrate phylogeny One of the consequences of parsimony principle is that sometimes several cladograms that are j...
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- Fall '08