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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.,
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