Tree Drawing

The direction in which a tree is drawn in mathematical phylogenetics is often chosen for mathematical convenience. The root might be depicted on the left, with time increasing to the right, toward the leaves. Often, time is regarded as “increasing” into the past. Because “up” represents an increasing value on the y-axis, it is most natural for the root, which occurs at the “largest” value in time, to be at the top of a diagram, and for the leaves, at time 0, to be at the bottom. Drawings that place the root at the top perhaps provide the most common orientation for tree diagrams in mathematical phylogenetics.

Although the depiction of tree diagrams downward from root to tips might seem counterintuitive to those only familiar with the typical upward growth of trees from the ground, this Chinese elm shows that real trees do sometimes conform to the mathematical convention.

Photo: Noah Rosenberg, June 18, 2021