Quartet Puzzling

Quartet puzzling is a way of constructing a phylogenetic tree by focusing on groups of four lineages in a larger set of n lineages (K Strimmer & A von Haeseler, Mol Biol Evol 13: 964-969, 1996). In the first step, the relationships for each of the possible subsets of four lineages are estimated. For each “quartet,” there are three possible ways to divide the quartet into two pairs, and one of these ways is selected. Next, the quartet relationships are assembled into a larger tree that serves as the estimated phylogeny for all \(n\) lineages.

Which two pairs among this quartet of Italian stone pines are closest? It is a puzzle.

Photo: Noah Rosenberg, September 29, 2021