An edge is a direct connection between two in a graph. It is often represented as a line, either straight or curved. The meaning of an edge depends on the context. For instance, in a phylogenetic tree, an edge typically indicates direct descent but it , but if two edges end in a node, they may also indicate represent reticulation events such as horizontal gene transfer or contamination. An edge may be either undirected or directed. In the latter case, the direction is often shown by drawing the line as an arrow.
An edge may also have additional properties. For instance, the weight of an edge may correspond to the importance or support of the edge so that a large weight suggests that the edge is very important or that it has high statistical support whereas a low weight suggests the opposite. The length of an edge in a phylogenetic tree or network, on the other hand, often represents temporal difference or a mutation rate (or a combination of them) so that any two taxa at opposite ends of a long edge tend to be more different from one another than two other taxa at the opposite ends of a short edge.
Fig. 1. Example of a graph depicting the names of important parts of a graph or tree.
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