Classifying Organisms

Taxonomy: science of describing, naming, and classifying living and extinct organisms and viruses
Systematics: study of biological diversity and the evolutionary relationships among organisms, both extinct and modern * Systematists use fossil, molecular, and genetic data to infer evolutionary relationships
Hierarchical system involving successive levels
Taxon: each group at any level
Domain: the highest level * All of life belongs to one of 3 domains: bacteria, archaea, and eukarya
Phylogenetics: reconstructing the evolutionary relationships among organisms

Cladistic approach: compares homologous traits, also called characters, which may exist in two or more character states
Shared primitive character or symplesio-morphy: shared by two or more different taxa and inherited from ancestors older than their last common ancestor
Shared derived character or synapo-morpy: shared by two or more species or taxa and has originated in their most recent common ancestor * Basis of the cladistic approach is to analyze many shared derived characters to deduce the pathway that gave rise to those species

Choose species
Choose characters
Determine polarity of character states * Primitive or derived?
Analyze cladogram * All species (or higher taxa) are placed on tips in the phylogenetic tree, not at branch points * Each cladogram branch point should have a list of one or more shared derived characters that are common to all species above the branch point unless the character is later modified * All shared derived characters appear together only once in a cladogram unless they arose independently during evolution more than once

Preferred hypothesis is the one that is the simplest for all the characters and their states
Challenge in a cladistic approach is to determine the correct polarity of events * It may not always be obvious which traits are primitive (came earlier) and which are derived (came later in evolution) * Fossils may be analyzed to help resolve

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