The purpose of using phylogenetic trees in evolutionary biology is to illustrate the evolutionary relationships among different species. These diagrams represent the branching patterns of evolution, showing how species have diverged from common ancestors over time. Phylogenetic trees are constructed using genetic, morphological, and behavioral data to trace the lineage and evolutionary history of organisms. They are crucial for understanding biodiversity, evolutionary processes, and the traits that define different species. By analyzing these trees, researchers can make inferences about the processes that drive speciation and adaptation.