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Lecture 14

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Shawn Lehman

ANT333 Lecture #14 – Cladistics & Phylogenetic Systematics Cladistics/Phylogenetic Systematics  Method classifies taxa into groups called clades, which consist of ancestor organism and all descendants.  All characters shared between taxa can be divided into three types: 1. Homologies & homoplasies, 2. Shared ancestral homologies, and 3. Shared derived homologies. Homoplasies and Homologies  Homoplasy (bad) is a convergent character - one that is shared between species but that was not present in their common ancestor.  Homology (more helpful) is character shared between species that was also present in their common ancestor. Homologies  Shared ancestral homologies (symplesiomorphy - bad): found in ancestor of a group of species but only in some of its descendants. Not what we want.  Shared derived homologies (synapomorphy - good): unique to a particular group of species (and their ancestor). Only shared derived homologies indicate phylogenetic groups. Cladistics/Phylogenetic Systematics  Only shared derived characters gives information about phylogeny (cladistics or phylogenetic systematics.)  Taxa that share many derived characters are grouped more closely together than those that do not.  Relationships are shown in branching hierarchical tree (cladogram).  Want to use as many diverse traits as possible Cladograms  Cladogram: A branching diagram depicting hierarchical arrangement of taxa defined by distribution of shared derived characters.  Cladogram constructed such that number of changes from one character state to next is minimized.  Principle behind this is rule of parsimony - any hypothesis that requires fewer assumptions is a more defensible hypothesis. Definitions  Character: A feature or thing we can examine or label. It is important that feature be heritable.  Homologous Character: Character which is shared by taxa by descent.  Analogous Character: Shared resemblance between characters by other means than descent, such as adaptation. This is often called a homoplastic character.  Shared Character: A character shared by all members of group More Definitions 1  Plesiomorphy: A character which is in state shared by common ancestor of group. (primitive)  Apomorphy: A character in a new state, not primitive one.  Synapomorphy: A character which is in a new state and shared by all member of group. – helps differentiate branches  Autapomorphy: A character found only in that taxon. Is a synapomorphy when discussed at level of members of a taxon. – links members of different taxa Some Other Key Definitions  Out Group: A group outside groups in question which is used to define polarity of character transformations (primitive to derived). VERY IMPORTANT!  Sister Group: A monophyletic group more closely related to group under examination than any other group. (side by side in tree)  Ancestor Problem: All possible ancestors are regarded as sister groups. Monophyletic Group  A group containing an ancestor and all of its decedents. Defined by one or more synapomorphies Paraphyletic Group  A group consisting of an ancestor but not all of its descendants. It is defined by what it does not have. Difficult to deal with. Polyphyletic Group  A group that does not include common ancestor of group. Common ancestor is place in another group. Worst one. It’s All About Tree Topology!  Tree can be flipped or rotated as long as relationships aren’t broken Trees are NOT Ladders!  No direction in phylogeny Systematic Process  Consists of interdependent but distinct steps 1. Taxa to be classified are chosen. 2. Characteristics to provide evidence for relationship are chosen. 3. Characteristics are analyzed to reconstruct relationship among taxa
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