Connecting tree and network edges

I have struggled over the years to try to understand the relationship between trees and networks. In one sense, networks are generalizations of trees, and in another sense a tree is just a simplified network. But it is not always that simple.

For example, not all networks can be created by adding edges to a tree (see Networks vs augmented trees); so the connection between trees and networks is not always obvious. Moreover, it is not always easy to determine which tree edges are present in any given network, or which network edges are present in a given tree.

Nevertheless, this should be basic information in phylogenetics — otherwise, how can we know when a tree is adequate for our purposes, or when a network is needed?

It turns out that I have not been alone in struggling to connect trees and networks. Fortunately, some of these other people decided to actually do something about it, rather than simply struggling on. As a result, a computerized way to relate much of the important information connecting trees with networks now exists.

Klaus Schliep, Alastair J. Potts, David A. Morrison and Guido W. Grimm
Intertwining phylogenetic trees and networks.
Methods in Ecology and Evolution (Early View)

To quote the authors:

Here we provide a framework, implemented in the PHANGORN library in R, to transfer information between trees and networks. This includes: (i) identifying and labelling equivalent tree branches and network edges, (ii) transferring tree branch-support to network edges, and (iii) mapping bipartition support from a sample of trees (e.g. from bootstrapping or Bayesian inference) onto network edges.

These three functions are illustrated in this figure, taken from the paper. It should be self-explanatory to anyone who has tried to relate the edges of trees and networks; but if it is not, then you can read an explanation in the paper.

The R library referred to, including the source code, along with some examples and vignettes, can be accessed on the PHANGORN CRAN page.

Note that PHANGORN (originally created by Klaus Schliep) also contains other functions related to estimating phylogenetic trees and networks, using maximum likelihood, maximum parsimony, distance methods and hadamard conjugation. Specifically, it allows you to: estimate phylogenies, compare trees and models, and explore tree space and visualize phylogenetic trees and split graphs.