Cladogram exercise EXTRA CREDIT

Cladogram exercise EXTRA CREDIT - ] Chapter 9 Shared...

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] Chapter 9 Shared Characters and Classification By now you should be convinced ttrat, taxonomically speaking, we can look at a fish as an assembly of gooi characters to be used for different purposes. In chapteri we used characters in a determinate sense as key characters to identify individuals. yet as we found in chapter g, tle same characters may be used statistically as variibtes to identify populations. Now we return once more to using characters in a determinate sense, but here we do so for classification rather than identifi cation. As best we cffi, we construct a phytogenetic classffication based on how characters differ benveen fish species and groups of species. A phylogenetic classification is an evolutionary hypothesis. It suggests that species or groups of species classified ogether are more closely re- lated genealogically to each other than they are !o any other taxa. Together they form a monophyletic group, all ulti_ mately derived from a single (hypothetical) species called a common ancestor. Various ways exist to construct classifications interpretable as phylogenetic. Some have a more concrete theoretical framework tlan others and so have a more definite set of rules to follow. For this exercise, it is easiest to follow definite rules without exceptions. Therefore, we use a method of phytogenetic systematics (see Wiley 1981) to assemble a cladogram suggesting re- lationships among taxa in a fish family. It is true that many ichthyologists use more ,.Eaditional,, methods for phylo_ genetic classification. Nevertheless, the method of inferring a phylogenetic tree from a cladogram is gaining favor. CLADOGRAM A cladogram is a branching diagram (dendrogram) in which species or other taxa are linked by sharing one or more characters in a unique state or condition. ,,Alternative X', in figure 9.1 exemplifies a cladogram that linls four taxa (A- D) into monophyletic groups by means of ttreir sharing three characters (l-3) in unique states. The uniquaness oi the character state indicates ttrat it is an evolutionrry nou_ elty (i.e., is apomorphic). This means that it has evolved (i.e., was derived) from a more ancesEal (plesiomorphic) condition found in related species outside the monophy'ofo group.l (In fig. 9.1 and thereafter, the plesiomorphic staei indicated by 0, the apomorphic state by 1.) hypothetical species from which the sister groups derived sometime in the past. At that time, the co Each monophyletic group is defined by the presence { the same uniquely derived character states in all its na- bers (synapomorphies) and typicafly consists of two sh. groups or species. A fork in the cladogram repres€nts common ancestor of the nro sister groups. In our e (fig. 9.1), B and CD are sister groups of a group, defined by a synapomorphy in character l, hypothesized common ancestor B'.
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Cladogram exercise EXTRA CREDIT - ] Chapter 9 Shared...

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