Ch. 27.1 - 27.4 Phylogeny

Ch. 27.1 - 27.4 Phylogeny - Chapter 27 Phylogeny etc 1...

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Chapter 27: Phylogeny, etc. 1 Phylogeny * (pp. 543-548; B3–B5) Phylogeny is the evolutionary history of a group of organisms Based on the idea that organisms are related by evolution Understanding these relationships is critical to our understanding of both evolution, and how biological processes work Phylogenetic trees A phylogenetic tree is a model of how a group of organisms descended from a common ancestor The model consists of nodes , where groups split, branches where evolution occurs, and tips representing observed taxa which are the endpoints of the process we are trying to model. A point where more than two branches diverge is called a polytomy . Polytomies mean we don’t know which event happened Frst. ±igures BS2.1, BS2.2 Monophyletic group A monophyletic group is a group defned by a single common ances- tor All descendants of the ancestor must be in the group Monophyletic groups can also be called clades or taxa . As biologists, we should try to think in terms of clades Are ²ying vertebrates a clade? What are some prominent groups that are not clades? * 1
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Reading phylogenetic trees The tree indicates the pattern of branching of lineages (evolving lines) Tips are assumed by the model to be monophyletic A tree is a model of how evolution occurred Trees that correspond to the same model are considered equivalent (Figure BS2.3) How do we judge how closely related two organisms are, according to a tree? 1.1 Constructing phylogenetic trees First, measure characteristics (or characters ) of the taxa of interest – i.e., anything that seems useful to measure Can be morphological (i.e., physical) or genetic Then infer (make an educated guess about) the phylogenetic relation- ships. – Phenetic approaches use measures of distance between organisms – Cladistic approaches are based on modeling how evolution occurs on the tree Morphological vs. genetic characteristics Should we use morphological or genetic characteristics to infer phylo- genies? We usually have more information from genetic characteristics,
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Ch. 27.1 - 27.4 Phylogeny - Chapter 27 Phylogeny etc 1...

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