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Unformatted text preview: ust as parsimonious as others can be generated from the same character matrix. In such a case, the systematician must produce all the cladograms and discuss the differences. Homoplasies: convergence and reversal Underlying causes in a conflictive transformation series may be of two types. The first type is related to interpretation errors. Since there is no method in existence that can rectify human error, there is no point in wasting time on this issue. The second type, clearly more interesting from an evolutionary science standpoint, concerns nature's whims. Some characters are said to be convergent, which means that they are structurally similar even though they appeared independently in taxa that don't have an immediate common ancestor. The ability to fly in birds and bats is a character that is said to be convergent for both groups, since there’s no direct common ancestor to both bats and bird that possessed the ability to fly. Other characters show reversals. A reversal happens when a character that is unique to a taxon group reverts to more ancestral level in a taxon within this group. For example, the presence of wings is generally considered as a unique (apomorphic) character for insects. But many species of insects do not have wings (e.g. fleas, lice, etc.). These species have winged ancestors. Even though this character reverts to a more ancestral state, winglessness is apomorphic for these species. A convergence hypothesis cannot be established beforehand, but is rather the result of a phylogenetic analysis. For instance, in the case of bats and birds, many unique characters (synapomorphies) phylogenetically link each of these groups to different non‐winged ancestors. Therefore, it is from this information that we can postulate that wings are the result of a convergence in both groups. The same reasoning is used in cases of reversal. Previous knowledge of phylogeny of the groups involved is therefore needed before a homoplasic hypothesis can be constructed. As a result, based on the cladistic method, all characters are given equal value at the time of cladistic analysis. The quality of a phylogenetic hypothesis is rather derived from the addition of consistencies in the distribution of many apomorphic characters at each level of the phylogenetic hierarchy, than on the subjective interpretation of the adaptive or historical value of some characters. Following this approach, it is therefore possible that similarities initially considered as being apomorphic become non‐homologous characters or homoplasies, after all the characters have been analysed globally. 64 Lab3 ‐ Vertebrate phylogeny Appendix 2 An Example in Phylogenetic Research The following table contains observations (fictitious) that were carried out on 7 species of fish and one species that was chosen as the sister group (SG) to the group that we want to study. Table1: Observation Matrix
chamber Reproduction Scales Number
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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.
- Fall '08