Abstract.-. Phylogenetic trees are central to many areas of biology, ranging from population genetics and epidemiology to microbiology, ecology, and macroevolution. The ability to summarize properties of trees, compare different trees, and identify distinct modes of division within trees is essential to all these research areas. But despite wide-ranging applications, there currently exists no common, comprehensive framework for such analyses. Here we present a graph-theoretical approach that provides such a framework. We show how to construct the spectral density profiles of phylogenetic trees from their Laplacian graphs. Using ultrametric simulated trees as well as non-ultrametric empirical trees, we demonstrate that the spectral density successfully identifies various properties of the trees and clusters them into meaningful groups. Finally, we illustrate how the eigengap can identify modes of division within a given tree. As phylogenetic data continue to accumulate and to be integrated into various areas of the life sciences, we expect that this spectral graph-theoretical framework to phylogenetics will have powerful and long-lasting applications.phylogenetics -diversification -Influenza -phylodynamics -biodiversitymicrobiology -Laplacian . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/026476 doi: bioRxiv preprint first posted online Sep. 9, 2015; Phylogenies are essential to many areas of the life sciences. In population genetics and phylogeography, they are used to infer past demography and historical migration events (1). In epidemiology, they are key to understanding how best to control the spread of infectious disease (2). In microbiology, they provide one of the most natural and powerful measures of diversity (3). Phylogenies are also increasingly effective in ecology, where they can inform our understanding of community assembly (4), interspecific interactions (5), and species responses to environmental change (6), as well as guide conservation efforts (7; 8). Finally, phylogenies are essential to comparative phylogenetics (9) and comparative genomics (10) and therefore to our understanding of diversification (11), trait evolution (12), and the genetic underpinnings of both (e.g., (13; 14)).Despite the importance of phylogenetics in the life sciences, the current techniques aimed at extracting information from phylogenies are limited. One of these techniques is built on summary statistics. In microbiology, ecology, and conservation biology, summary statistics based on measures of phylogenetic diversity, such as total phylogenetic branch length, (7; 15) are often used. In diversification analyses, traditional summary statistics quantify either the stem-to-tip (e.g. γ (16) and Lineage-Through-Time plots (17)) or lineage-to-lineage (e.g. β and Colless' index (18)) distribution of branching events across trees. These summary statistics disregard much of the data -an...