The Tree of Life: Metaphor, Model, and Heuristic Device

The tree of life (ToL) is used variously as metaphor, model, and research tool to explore life’s evolution and genealogical relationships. It has broad appeal for multiple scientific communities and the public as a descriptive term covering life’s diversity. However, as biologists identify more entities and events to describe—including species, organisms, cells, genomes, gene families, extra-chromosomal genetic elements, endosymbioses, hybridizations, recombination types, and lateral gene transfer (LGT) events—the ToL strains under the weight of multiple uses and expectations. The multiple uses span levels of biological organization, from genes to clades, and they extend to all life forms despite their disparate mechanisms of reproduction and inheritance. Furthermore, some biologists consider the purpose of the ToL to be communicating patterns of relationship, whereas others consider evolutionary