Phylogenetic tree

A phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the descendants, and the edge lengths in some trees correspond to time estimates. Each node is called a taxonomic unit. Internal nodes are generally called hypothetical taxonomic units (HTUs) as they cannot be directly observed.

History
Although the idea of a "tree of life" arose from ancient notions of a ladder-like progression from lower to higher forms of life (such as in the Great Chain of Being), Charles Darwin (1859) first illustrated and popularized the notion of an evolutionary "tree" in his seminal book The Origin of Species. Over a century later, evolutionary biologists still use tree diagrams to depict evolution because the floral analogy effectively conveys the concept that speciation occurs through the adaptive and random splitting of lineages. Over time, species classification has become less static and more dynamic.

Types
A dendrogram is a broad term for the diagrammatic representation of a phylogenetic tree.

A cladogram is a tree formed using cladistic methods. This type of tree only represents a branching pattern, i.e., its branch lengths do not represent time.

A phylogram is a phylogenetic tree that explicitly represents number of character changes through its branch lengths.

An ultrametric tree or chronogram is a phylogenetic tree that explicitly represents evolutionary time through its branch lengths.

Fields of study

 * Evolutionary biology
 * Phylogenetics
 * Comparative phylogenetics
 * Cladistics

Images

 * Phylogenetic Trees Based on 16s rDNA
 * Poster-sized tree of life illustration
 * A 3D View
 * Human Y-Chromosome 2002 Phylogenetic Tree
 * In 2003, the Science journal dedicated a special issue to the tree of life, including an online version of a tree of life.
 * iTOL: Interactive Tree Of Life
 * Phylogenetic Tree of Artificial Organisms Evolved on Computers

General

 * Discover Life An interactive tree based on the U.S. National Science Foundation's Assembling the Tree of Life Project
 * PhyloCode
 * A Multiple Alignment of 139 Myosin Sequences and a Phylogenetic Tree
 * Tree of Life Web Project
 * http://www.aisee.com/graph_of_the_month/jura.htm  The most detailed and comprehensive family tree of dinosaurs yet available
 * Phylogenetic inferring on the T-REX server
 * NCBI's Taxonomy Database