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Modern Evolutionary Classification Objectives Identify the purpose of cladograms Evolutionary Classification Phylogeny is the study of the evolutionary relationships among organisms Biologists now group organisms into categories that represent lines of evolutionary descent, not just physical similarities Evolutionary classification is the strategy of grouping organisms together based on their evolutionary history Evolutionary Classification Species within a genus are more closely related to one another than to species in another genus because they share a common ancestor The higher the level of the taxon, the further back in time the common ancestor is to all of the organisms in the taxon Classification Using Cladograms Cladistic analysis identifies and considers only those characteristics of organisms that are evolutionary innovations Derived characters are characteristics that appear in recent parts of a lineage but not in its older members Classifying Using Cladograms A cladogram is a diagram that shows evolutionary relationships among a group of organisms It’s like an evolutionary family tree Similarities in DNA and RNA The genes of many organisms show important similarities at the molecular level Similarities in DNA can be used to help determine classification and evolutionary relationships The more similar the DNA sequences of two species, the more recently they shared a common ancestor, and the more closely related they are in evolutionary terms Molecular Clocks A molecular clock is a model that uses DNA comparisons to estimate the length of time that two species have been evolving independently Relies on mutations to mark time The amount of differences in the DNA sequences of two species indicates how long ago they shared a common ancestor Mutations that have no effect on phenotype accumulate at a steady rate By comparing these mutations it can be determined how long the two species shared a common ancestor Notes Review Identify the purpose of cladograms A cladogram is a diagram used to show evolutionary relationships by using derived characters