The primary purpose of the evolutionary tree in biology, also known as the phylogenetic tree, is to illustrate the evolutionary relationships among different organisms. This diagram represents hypothesized ancestors and divergences, highlighting how species have evolved over time from common ancestors. By analyzing genetic, morphological, and behavioral traits, scientists can infer how closely related species are and trace the history of evolutionary changes. This understanding aids in classifying organisms and predicting characteristics shared among them based on their evolutionary lineage.