Classifying Organisms

• Systematics is the scientific study of the diversity of organisms and their evolutionary relationships.

• Taxonomy is the branch of systematics devoted to naming, describing, and classifying organisms.

• Classification is the process of arranging organisms into groups based on similarities that reflect evolutionary relationships.

• Scientific names allow biologists from different countries with different languages to communicate about organisms.

• Biologists in distant locations must know with certainty that they are studying the same (or different) organisms.

• Classifications help biologists organize their knowledge.

• In the binomial system of nomenclature, the basic unit of classification is the species.

• The name of each species has two parts: the genus name followed by the specific epithet.

• The hierarchical system of classification includes domain, kingdom, phylum, class, order, family, genus, and species.

• Each formal grouping at any given level is a taxon.

Determining the Major Branches in the Tree of Life

• The three-domain classification system assigns organisms to domains Archaea, Bacteria, or Eukarya.

• The domain classification is based on molecular data.

• Domain Eukarya includes the fungi, plants, animals, and protists.

• Some systematists recognize the kingdoms Archaea, Bacteria,
  Fungi, Plantae, and Animalia.

• Several “supergroups” composed of mainly unicellular, mainly aquatic eukaryotic organisms were formerly classified as kingdom protista.

• Members of Archaea and Bacteria are prokaryotes.

• The fungi, which include molds, yeasts, and mushrooms, absorb nutrients produced by other organisms.

• Kingdoms plantae and Animalia both consist of multicellular eukaryotes.

• As new data are interpreted, organisms assigned to these kingdoms often must be reclassified.

• In a cladogram each branch represents a clade, a group of organisms with a common ancestor.

• Each node, or branching point, represents the splitting of two or more new groups from a common ancestor.

• The node represents the most recent common ancestor of the clade represented by the branches.

• The root represents the most recent common ancestor of all the clades shown in the tree.

• We can determine the relationships among taxa by tracing along the branches back to the nodes.

• The cladogram indicates which taxa shared a recent common ancestor and how recently they shared that ancestor compared to other groups.

Reconstructing Evolutionary History

• Systematists seek to determine evolutionary relationships, or phylogeny, based on shared characteristics.

• Homology, the presence in two or more species of a trait derived from a common ancestor, implies evolution from a common ancestor.

• Some seemingly homologous characters are acquired independently by convergent evolution, independent evolution of similar structures in distantly related organisms, or by reversal, reversion of a trait to its ancestral state.

• The term homoplasy refers to superficially similar characters that are not homologous.

• Shared ancestral characters suggest a distant common ancestor.

• Shared derived characters (synapomorphies) indicate a more recent common ancestor and can be used as evidence for constructing cladograms.

• Molecular systematics depends on molecular structure to clarify phylogeny.

• Comparisons of nucleotide sequences in DNA and RNA, and of amino acid sequences in proteins, provide important information about how closely organisms are related.

• A monophyletic group, or clade, includes all the descendants of the most recent common ancestor.

• A paraphyletic group consists of a common ancestor and some of, but not all, its descendants.

• A polyphyletic group consists of organisms that evolved from different recent ancestors.

Constructing Phylogenetic Trees

• Modern systematists agree that taxa must be monophyletic.

• Each monophyletic group consists of a common ancestor and all its descendants.

• Phylogenetic systematists (cladists) use shared derived characters to determine relationships among groups of organisms.

• Contemporary systematists classify reptiles and birds in a single clade.

• Cladists use shared derived characters to reconstruct evolutionary relationships and diagram these relationships in cladograms.

• They use outgroup analysis to determine which characters in a given group of taxa are ancestral and which are derived.

• An outgroup is a taxon that diverged earlier than any of the other taxa being investigated.

• Systematists choose the simplest explanation to interpret the data—the principle of parsimony.

Applying Phylogenetic Information

• Understanding how species are related can help scientists answer questions and solve problems in other disciplines.

• For example, phylogenetic information has helped us better understand the origin and transmission of HIV.