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Introduction to Biological Classification

Author(s): Deanne Erdmann, MS

Taxonomic Diagrams

Sometimes, biologists group organisms into categories that represent common ancestries, not just physical similarities.  Early naturalists used physical characteristics and later, fossil data, attempting to represent evolutionary relationships among organisms.  Today, modern classification systems use fossil data, physical characteristics and DNA/RNA information to draw increasingly more accurate branching diagrams.

Phylogenetic trees, or phylogenies, represent hypothesized evolutionary relationships among organisms and may include extinct as well as modern species. Cladograms are based only on characteristics observable in existing species. The branching patterns in a cladogram are defined by the presence of unique, evolving innovations (derived characteristics) shared by all members of the group.