On the Shapley value of unrooted phylogenetic trees

TL;DR

This paper investigates the properties of the Shapley value in unrooted phylogenetic trees, revealing limitations in reconstructing tree topology and its effectiveness for biodiversity prioritization.

Contribution

It demonstrates that different trees can have equivalent Shapley transformation matrices and null spaces, and shows the Shapley value may not always be effective for prioritizing species in conservation.

Findings

Non-isomorphic trees can have permutation-equivalent Shapley matrices.

Estimating split counts or Shapley values alone does not reconstruct tree topology.

Shapley value may fail as a biodiversity prioritization criterion.

Abstract

The Shapley value, a solution concept from cooperative game theory, has recently been considered for both unrooted and rooted phylogenetic trees. Here, we focus on the Shapley value of unrooted trees and first revisit the so-called split counts of a phylogenetic tree and the Shapley transformation matrix that allows for the calculation of the Shapley value from the edge lengths of a tree. We show that non-isomorphic trees may have permutation-equivalent Shapley transformation matrices and permutation-equivalent null spaces. This implies that estimating the split counts associated with a tree or the Shapley values of its leaves does not suffice to reconstruct the correct tree topology. We then turn to the use of the Shapley value as a prioritization criterion in biodiversity conservation and compare it to a greedy solution concept. Here, we show that for certain phylogenetic trees, the Shapley value may fail as a prioritization criterion, meaning that the diversity spanned by the top $k$ species (ranked by their Shapley values) cannot approximate the total diversity of all $n$ species.