Systematics the study of the diversity of organisms and their evolutionary relationships Taxonomy – the science of naming, describing, and classifying organisms Classification – ordering organisms into groups based on their similarities or relationships

Carolus Linnaeus developed the modern system of classification Binomial system of nomenclature – Linnaeus’ system of assigning each species a unique two-part name First name is the genus second name is the specific epithet both names must be used to identify the species – scientific name derived from Greek and Latin roots or Latin versions of the names of persons, places or characteristics Scientific names are recognized by all scientists all over the world – common names vary by language and location

Taxonomic Categories Taxon – a formal grouping of organisms at any given level (ex. species, genus, phylum – class Mammalia is a taxon)

Taxonomic categories form a hierarchy closely related species are assigned to the same genus closely related genera are grouped in a single family families are grouped into orders orders are grouped into classes classes are grouped into phyla phyla are grouped into kingdoms these groupings can be separated into subgroupings such as subphyla or superclass

Six Kingdom system of classification –Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia

Some biologists use domains to classify the major groups of living things

Systematics attempts to reconstruct evolutionary relationships phylogeny – reconstruction of evolutionary relationships of organisms once phylogeny is established, classification can then be based on common ancestry Species have various degrees of evolutionary relationship with one another depending on the degree of genetic divergence since their populations branched from a common ancestor

Criteria for classification should be a reflection of evolutionary relationships – based on common ancestry Homologous structures – similarity in structures derived from a common ancestor organisms sharing many homologous structures are considered to be closely related

Molecular taxonomy has emerged from advances in molecular biology focuses on molecular structure used to compare the nucleotide sequences of nucleic acids and the amino acid sequences of proteins – important tool in taxonomy greater similarities in subunit sequences indicate closer evolutionary relationships the number of differences in DNA or RNA nucleotide sequences or in amino acid sequences in two groups or organisms reflect how much time has passed since the groups branched from a common ancestor genes and proteins can be used as molecular clocks

systematists are currently studying: comparisons in ribosomal RNA structure comparisons in mitochondrial DNA

Systematists use two main approaches to classify organisms: 1. Phylogenetic systematics (cladistics) – emphasizes phylogeny (common ancestry) evolutionary relationships are expressed in branching diagrams called cladograms each branch represents the divergence of two or more new groups from a common ancestor (forms a clade) cladogram is constructed from a series of dichotomies

The sequence of branching symbolizes historical chronology. The last ancestor common to both the cat and dog families lived longer ago than the last common ancestor shared by leopards and domestic cats

Classical evolutionary taxonomy – uses a system of phylogenetic classification and presents evolutionary relationships in phylogenetic trees