SL (Single - Linkage) is a simple agglomerative (bottom-up) hierarchical clustering method. It is also known as nearest neighbour clustering
SL is used for the creation of phenetic trees (phenograms) and in a phylogenetic context, it assumes a constant rate of evolution (molecular clock hypothesis), and is not a well-regarded method for inferring relationships unless this assumption has been tested and justified for the data set being used.
This algorithm constructs a rooted tree (dendrogram) that reflects the structure present in a pairwise dissimilarity matrix. At each step, the nearest two clusters are combined into a higher-level cluster. The distance between any two clusters A and B is the shortest distance between elements of each cluster.
The method is attributed to R. Sibson.