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Unformatted text preview: Lab 7: Phylogenetic Relationships Phylogeny is the evolutionary history of a group of interrelated species. Phylogenetic methods infer evolutionary relationships from molecular, morphological, and/or behavioral data, presenting these relationships in the form of a tree diagram. A phylogenetic tree is composed of nodes and branches . Terminal nodes represent living taxa, internal nodes represent last common ancestors and inferred ancestral morphotypes, and branches define the relationships between the nodes. You can rotate bifurcating branches at any node without changing the relationships between taxa ! The principles of cladistics , originally called phylogenetic systematics , are based on the belief that one must discriminate between true similarity, homology , and superficial and misleading similarity, analogy , in order to accurately reconstruct phylogenetic relationships. (A classic example of analogy is the similarity between dolphins and sharks a shark is a fish, and a dolphin is a mammal, but they have both adapted to aquatic conditions. They are not very closely related, but one must look beyond superficial similarity to realize this.) To discuss the concepts of homology and analogy, we must first define two essential concepts used in cladistic analysis: character and character state. In fact we have already used these concepts in the above example. A character can be any recognizable feature, for example, a location on a DNA strand; a morphological trait, such as enamel thickness; or a behavioral trait, such as locomotor pattern. A character state is the form each character takes. In the above three examples, character states could be: the presence of Adenine (A) at position three of a sequence of DNA; the presence of thin enamel; or being bipedal. A character must have two or more states. To determine the direction of evolutionary polarity of character states (how they changed through time) we can root the cladogram using the outgroup method. An outgroup is a taxon that is equally distantly related to all members of the group we are interested in, the ingroup . All character states shared between the outgroup and the ingroup are shared primitive characters. Once the characters have been defined, and the character states identified, a tree or cladogram can be constructed. The basic process involves grouping species with similar character states together. In contrast to phenetics (a seldom used method based on evaluation of overall similarity), in cladistics we must partition similarity into three components: (1) that due to analogy (called homoplasy in cladistic analysis); (2) the sharing of primitive homologies ; and, (3) the sharing of derived homologies . Homoplasy...
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