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Unformatted text preview: Chapter 26: Phylogeny and the T ree of Life- Phylogeny: the evolutionary history of a species or a group of species o Systematics are used to construct phylogenies; this is a discipline focused on classifying organisms and determining their evolutionary relationships 26.1: Phylogenies show evolutionary relationships- Taxonomy: how organisms are named and classified Binomial Nomenclature- Biologists refer to organisms by their binomial names: the two part format of the scientific name, instituted by Carolus Linnaeus o The first part of the binomial name is its genus o The second part, called the specific epithet is the species Hierarchical Classification- There is a hierarchy of increasingly inclusive categories o Domain, Kingdom, Phylus, Class, Order, Family, Genus, Species- The named taxonomic unit at any level of the hierarchy is called a taxon o Ex: Panthera is a taxon at the genus level, Mammalia is a taxon at the class level Taxa broader than genus are not italicized, but they are capitalized- Larger categories are often not comparable between lineages and the placement of species into orders, classes, and so on do not always reflect evolutionary history Linking Classification and Phylogeny- The evolutionary history of a group of organisms can be represented in a branching diagram called a phylogenetic tree o The branching in some cases matches the hierarchical classification of groups nested within a group of organisms In other situations, certain similarities between organisms may lead taxonomists to place a species within a group of organisms other than the group to which it is most closely related Organisms are often reclassified to accurately reflect its evolutionary history- These difficulties have led some scientists to propose that classification be based solely upon evolutionary relationships o PhyloCode: only names groups that include a common ancestor and all of its descendants Some commonly recognized groups would become part of other groups previously of the same rank Ex: since birds evolved from reptiles, , the birds would be considered a subgroup of the reptiles- A phylogenetic tree represents a hypothesis about evolutionary relationships o These are often depicted as a series of dichotomies, or two way branchpoints Each branchpoint represents the divergence of two evolutionary lineages from a com mon ancestor Tree branches can be rotated around a branch point without changing their evolutionary relationships Sister taxa are groups of organisms that share an im mediate com mon ancestor and are each others closest relatives o Phylogenetic trees are rooted if a branch point within the tree (usually the one farthest to the left) represents the last common ancestor of all taxa in the tree o Polytomies: a branch point from which more than two descendant groups emerge; this indicates that evolutionary relationships among the descendant taxa are not yet clear What We Can and Cannot Learn from Phylogenetic Trees...
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