Figure 1: Example of a phylogeny at a particular SNP. In the phylogenetic tree, time moves from past (left) to present (right) across the tree, and the tips of the tree represent observations from the present time. The amount of shared evolutionary history among the two observations with red squares is large, so that a large covariance is expected among their trait values. In contrast, the two observations denoted by blue circles share a smaller portion of their evolutionary history, so that little covariance in their trait values is expected from shared evolutionary history.