Phylo2Vec: A Vector Representation for Binary Trees

Binary phylogenetic trees inferred from biological data are central to understanding the shared history among evolutionary units. However, inferring the placement of latent nodes in a tree is computationally expensive. State-of-the-art methods rely on carefully designed heuristics for tree search, using different data structures for easy manipulation (e.g., classes in object-oriented programming languages) and readable representation of trees (e.g., Newick-format strings). Here, we present Phylo2Vec, a parsimonious encoding for phylogenetic trees that serves as a unified approach for both manipulating and representing phylogenetic trees. Phylo2Vec maps any binary tree with n leaves to a unique integer vector of length n-1. The advantages of Phylo2Vec are 4-fold: (i) fast tree sampling, (ii) compressed tree representation compared to a Newick string, (iii) quick and unambiguous verification if 2 binary trees are identical topologically, and (iv) systematic ability to traverse tree space in very large or small jumps. As a proof of concept, we use Phylo2Vec for ML inference on 5 real-world datasets and show that a simple hill-climbing-based optimization scheme can efficiently traverse the vastness of tree space from a random to an optimal tree.