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Phylogenetic tree
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<p>A phylogenetic tree or phylogeny is a graphical representation which shows the <a href="/facts/Evolution/qE5amP6a">evolutionary</a> history between a set of <a href="/facts/Species/oDg6b96r">species</a> or <a href="/facts/Taxon/caQC1e87">taxa</a> during a specific time. In other words, it is a branching <a href="/facts/Diagram/H0PKEUrI">diagram</a> or a <a href="/facts/Tree_(graph_theory)/TCWk4Joy">tree</a> showing the evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical or genetic characteristics. In evolutionary biology, all life on Earth is theoretically part of a single phylogenetic tree, indicating <a href="/facts/Common_ancestry/bwEgMu7F">common ancestry</a>. <a href="/facts/Phylogenetics/bw4rsovt">Phylogenetics</a> is the study of phylogenetic trees. The main challenge is to find a phylogenetic tree representing optimal evolutionary ancestry between a set of species or taxa. <a href="/facts/Computational_phylogenetics/MTyNUhHg">Computational phylogenetics (also phylogeny inference)</a> focuses on the algorithms involved in finding optimal phylogenetic tree in the phylogenetic landscape. </p><p>Phylogenetic trees may be rooted or unrooted. In a <i>rooted</i> phylogenetic tree, each node with descendants represents the inferred <a href="/facts/Most_recent_common_ancestor/Cbg1YLCV">most recent common ancestor</a> of those descendants, and the edge lengths in some trees may be interpreted as time estimates. Each node is called a taxonomic unit. Internal nodes are generally called hypothetical taxonomic units, as they cannot be directly observed. Trees are useful in fields of biology such as <a href="/facts/Bioinformatics/D5x2L8ee">bioinformatics</a>, <a href="/facts/Systematics/fagPXIkh">systematics</a>, and <a href="/facts/Phylogenetics/bw4rsovt">phylogenetics</a>. <i>Unrooted</i> trees illustrate only the relatedness of the <a href="/facts/Leaf_nodes/RcXz9noF">leaf nodes</a> and do not require the ancestral root to be known or inferred. </p>