What traditional basis was used for the illustration of phylogenetic relationships among organisms?

The traditional basis used for illustrating phylogenetic relationships among organisms is morphology. Morphology refers to the study of the form, structure, and shape of organisms. Historically, scientists have relied on observable physical characteristics, such as skeletal structures, organ arrangements, and overall body plans, to classify and relate different organisms. This approach dates back to the early efforts in taxonomy and systematics, where classifications were primarily made based on similarities and differences in morphology.

The evolutionary relationships derived from morphological traits often led to the development of classification systems that grouped organisms with similar characteristics together, establishing a hierarchy based on these observable traits. Over time, as molecular techniques and genetic information became available, the understanding of phylogenetics evolved, incorporating genetics and biochemistry into the classification process. However, morphology remains a foundational aspect of the initial classification and understanding of the relationships among organisms.