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How Are Organisms Classified?

  • Taxonomy is the field of biology that classifies living and extinct organisms according to a set of rules.

    • Taxonomy produces a hierarchy of groups of organisms; the organisms are assigned to groups based on similarities and dissimilarities of their characteristics.

    • The classification system begins with three domains that encompass all living and extinct forms of life; the Bacteria and Archaeans are mostly microscopic, but quite widespread.

    • Domain Eukarya contains more complex organisms; it is traditionally divided into four Kingdoms, but now includes "Supergroups" above the Kingdom level (see below).

  • Evolutionary relationships of a species or group of species can be used to build taxonomic groups; the evolutionary history of a species or group of species is called phylogeny.

    • A phylogenetic tree is a hypothesis that depicts the evolutionary relationships among groups of organisms; in detailed phylogenetic trees, branch points indicate when new species diverged from a common ancestor.

    • Species (or groups of species) and their most recent common ancestor form a clade within a phylogenetic tree.

    • Phylogenetic trees constructed by modern methods can depict the relationship between clades and taxonomic groups.


How Are Phylogenetic Trees Constructed?

  • Phylogenetic trees are usually based on morphological or genetic homology.

    • A comparison of anatomical traits can reveal an evolutionary relationship among species.

    • Analysis of genetic differences in among species can identify descent from an ancestral gene.

    • Modern genetic techniques employ "molecular clocks" to assist in the construction of phylogenetic trees.

  • The cladistic approach to phylogenetic trees emphasizes primitive vs. derived characteristics.

    • Shared, derived characteristics are used to construct a tree called a cladogram.

    • Determining the order in which characteristics appeared during evolution can be a challenge.


How is Molecular Systematics Changing Our View of Taxonomy?

  • Taxonomy is a work in progress.

    • As new species are found, taxonomic groups may no longer be monophyletic.

    • Molecular analysis of species has led to a major rearrangement of the traditional four kingdoms.

  • The process of horizontal gene transfer leads to "webs of life" rather than simple phylogenetic trees; the early evolution of prokaryotes may have resembled a web of life.