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Biological Classification Cladistics Cladistics

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Biological Classification Cladistics Cladistics Biological Classification Taxonomy & Linnaean Hierarchy Biological Classification • Levels called taxa (sing., taxon: “classification”) – The more similar two organisms are, the more levels “King Philip Came they have in common “Kings Play Chess On F • Taxonomy: naming & classifying a) Kingdom organisms b) Phylum (Division)* c) Class • Systematics: studying relationships Over F d) Order among taxonomic groups ine Gorreen Good Sand” Soup” e) Family* f) Genus g) Species *not in the original Linnaean hierarchy Carl Linné, 1737 IV. Phylogenetic Systems Problem: Divergence vs. Convergence • Classified based on presumed common ancestry — Homology vs. Analogy • Levels in common suggests a more recent divergence from a common ancestor. • But since we don’t actually know the ancestry above Similarity due to the level of genus or maybe family, dependent upon convergence is degrees of similarity. analogy. – Comparative morphology & anatomy – Comparative embryology (Similar – Comparative biochemistry — proteins & DNA adaptations to • Much disagreement may be debated regarding similar environments; which similarities and which differences are most not shared ancestry.) phylogenetically significant! Cladistics Cladistics More vocabulary: • Clade (“branch”) — replace traditional taxon • A true clade must be monophyletic – Groups of organisms presumed to be derived – must include an ancestor and all of the known from a common ancestor are organized by descendants of that ancestor. bifurcating (two-way splitting) of a branch • A grouping that only includes an ancestor and some of its descendants is paraphyletic. – Each bifurcation is based upon the acquisition • A grouping that includes organisms from different of a new, unique character (apomorphy). ancestries is polyphyletic. • Maximum parsimony: the branch pattern that • Derived apomorphic characters shared by can be created with the fewest required steps is members of a clade are synapomorphic. most likely the most correct. • Ancestral characteristics inherited prior to the branching of a clade are plesiomorphic. Heyer 1 Biological Classification For example: ©Pearson Education, Inc. Assuming this cladogram is correct: ©Pearson Education, Inc. • Retractable claws is a synapomorphic character for the Family Felidae. Hair, Carnivorous teeth, Retractable claws, & Ability to • Hair & Carnivorous teeth are plesiomorphic characters for the purr are all apomorphic characters in the presumed Family Felidae. ancestry of the domestic cat Assuming this cladogram is correct: ©Pearson Education, Inc. Assuming this cladogram is correct: ©Pearson Education, Inc. • Carnivorous teeth is a synapomorphic character for the • Hair is a synapomorphic character (synapomorphy) for the Order Carnivora. Class Mammalia. • Hair is a plesiomorphic character for the Order Carnivora. Tiger Shark Carnivorous (meat-eating) teeth Assuming this cladogram is correct: ©Pearson Education, Inc. ? ©Pearson Education, Inc. • The presence of carnivorous teeth in cats and wolves is a • The presence of carnivorous teeth in cats and sharks is an homology. analogy. • (in cladistics, also called a homoplasy). Heyer 2 Biological Classification Cladogram vs. Taxa Cladogram vs. Taxa Monophyletic Paraphyletic clades grouping Not a true clade! Cladogram vs. Taxa Building Cladograms Assemble a table of character states Polyphyletic grouping Not a true clade! Not phylogenetic! Building Cladograms Building Cladograms Major assumptions: Each bifurcation of the branch is based 1. The group of organisms is upon the state (presence/absence) of monophyletic an apomorphic character 2. The outgroup (used for comparison) is closely related to, but separate from your group 3. You can tell which character states are homologous or analogous. Heyer 3 Biological Classification Building Cladograms Cladograms Cladograms are 1. Arrange your ingroup species in order of 5 4 made by similarity to the outgroup. 3 2 hypothesizing 1 the sequence of evolution of 5 shared 2. Arrange apomorphic 4 derived characters in order of appearance in 3 (apomorphic) species most similar characters to the outgroup. 2 1 Building Cladograms Cladograms Rule of Parsimony: 5 5 4 5 4 3 The simplest explanation is 2 3 4 1 the most likely explanation. 2 3 1 2 1 5 4 3 2 Base change event 1 Cladograms Cladograms Rule of Parsimony: Rule of Parsimony: The simplest explanation is The simplest explanation is the most likely explanation. the most likely explanation. But not always! Heyer 4 .
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