page_title: EDGE species
section_title: ED
hierarchical_section_title: EDGE species / Calculating EDGE Scores / ED
caption_attribution_description: English: ZSL EDGE of Existence programme phylogenetic tree
context_page_description: Evolutionarily Distinct and Globally Endangered species are animal species which have a high EDGE score, a metric combining endangered conservation status with distinctiveness of taxon. Distinctive species have few closely related species, and EDGE species are often the only surviving member of their genus or even higher taxonomic rank. The extinction of such species would therefore represent a disproportionate loss of unique evolutionary history, biodiversity and potential for future evolution.
  Some EDGE species, such as elephants and pandas, are well known and already receive considerable conservation attention, but many others, such as the Vaquita the bumblebee bat and the egg-laying long-beaked echidnas are highly threatened yet remain poorly understood and are frequently overlooked by existing conservation frameworks. Recent research indicates that 70% of the world's most threatened and evolutionarily distinct mammal species are currently receiving little or no conservation attention.
  The Zoological Society of London launched a global conservation initiative, the EDGE of Existence Programme in 2007 to raise awareness and funds for the conservation of these species.
context_section_description: Some species are more distinct than others because they represent a larger amount of unique evolution.  Species like the aardvark have few close relatives and have been evolving independently for many millions of years.  Others like the domestic dog originated only recently and have many close relatives.  Species uniqueness’ can be measured as an 'Evolutionary Distinctiveness' (ED) score, using a phylogeny, or evolutionary tree. ED scores are calculated relative to a clade of species descended from a common ancestor. The three clades for which the EDGE of Existence Programme has calculated scores are all classes, namely mammals, amphibians, and corals. 
  The phylogenetic tree has the most recent common ancestor at the root, all the current species as the leaves, and intermediate nodes at each point of branching divergence. The branches are divided into segments (between one node and another node, a leaf, or the root). Each segment is assigned an ED score defined as the timespan it covers (in millions of years) divided by the number of species at the end of the subtree it forms.  The ED of a species is the sum of the ED of the segments connecting it to the root. Thus, a long branch which produces few species will have a high ED, as the corresponding species are relatively distinctive, with few close relatives. ED metrics are not exact, because of uncertainties in both the ordering of nodes and the length of segments.
caption: In this phylogenetic tree, species A has a higher ED score than either species B or C — it represents a branch rather than a twig on the tree of life.  If species A were to go extinct, there would be no similar species left on the planet and a disproportionate amount of unique evolutionary history would be lost forever.
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