Branch lengths

The branch lengths usually represent the evolutionary distances between two consecutive nodes. We tend to split the phylogenetic tree into two parts: its topology (i.e., the pattern of branching) and its associated edge lengths. The expected rate of evolutionary change is assumed constant across all lineages in a phylogeny and the length of a branch is scaled to the expected number of substitutions per site along that branch. These lengths can be integrated in the string representation seen in section 2.1.2; for instance we can write:

$$(Outgroup:0.35,gorilla:0.25,(human:0.3,chimpanzee:0.2));$$