Many native bees lay their eggs in hollow stems and in holes that beetles have made in wood. Suppose a group of scientists collected data on the body length of bees and the diameter of their nest cells. They hypothesized that the diameter of the nest cell is correlated with body length, but they only made measurements on a group of closely related bees. To tease apart the effects of evolution and ecology on these traits, the scientists conducted a series of independent phylogenetic contrasts. The scientists measured body length and nest cell diameter in species A, B, C, and D. Species E is the common ancestor of species A and B, and species F is the common ancestor of species C and D. A phylogenetic tree has four terminal branches labeled A, B, C, and D. The common ancestor of A and B is labeled E. The common ancestor of C and D is labeled F. D one refers to a comparison of A and B. D two refers to a comparison of C and D. D three refers to a comparison of E and F.