The Coral of Life

Abstract

The Tree of Life (ToL) has been of central importance in the biological sciences, usually understood as a model or a metaphor, and portrayed in various graphical forms to summarize the history of life as a single diagram. If it is seen as a mathematical construct-a rooted graph theoretical tree or, as more recently viewed, a directed network [Network of Life (NoL)]-then its proper visualization is not feasible, for both epistemological and technical reasons. As an overview included in this study demonstrates, published ToLs and NoLs are extremely diverse in appearance and content, and they suffer from inevitable bias towards particular groups, or are restricted to a single major taxon. Metaphorical trees are even less useful for the purpose, because ramification is the only property of botanical trees that may be interpreted in an evolutionary or phylogenetic context. This paper argues that corals, as suggested by Darwin in his early notebooks, are superior to trees as metaphors, and may also be used as mathematical models. A coral diagram is useful for portraying past and present life because it is suitable: (1) to illustrate bifurcations and anastomoses, (2) to depict species richness of taxa proportionately, (3) to show chronology, extinct taxa and major evolutionary innovations, (4) to express taxonomic continuity, (5) to expand particulars due to its self-similarity, and (6) to accommodate a genealogy-based, rank-free classification. This paper is supplemented with a figure, The Coral of Life (CoL), which is, to the author's knowledge, the first attempt to combine all of the above features in a single diagram for the entirety of life, thus serving as a prototype for further analysis and improvement. The discussion is partly historical: references to classical and modern writings help the reader to understand how biological thinking and methods of visualization have evolved to reach this achievement.

The Coral of Plants

“…The weakness of cladograms, and of graph theoretical trees in general, is that the number of terminal nodes (taxa) cannot be increased beyond a certain limit without risking interpretability and readability. Mandalas and spiral trees hardly solve this problem, even though they may contain thousands of endpoints (Podani, 2019).…”

“…Lamarck and Darwin warned us about the latter long time ago. If the purpose of a diagram is to summarize the past and present plant life together, ranks do not work at all: Groups delineated at present cannot be projected back into the past in a meaningful way (see Podani, 2010Podani, , 2019, and references therein) -another topic elaborated in the present article. The solution is to switch to rankfree groups and phylogenetic nomenclature, which is a painful step for many of us so firmly accustomed to the Linnaean tradition.…”

“…For visualizing hybridization or other types of relationships between different lineages, interconnections (anastomoses) between branches 4 are also allowed ("fan corals"). For more formal definitions, see Podani (2017Podani ( , 2019.…”

The ‘Holy Grail’ in Phylogenetic Reconstruction: Seeing the Forest for the Trees?

“…Although overlooked/ignored by 20 th century Aristotelian 'evolutionary' systematists, Darwin conceived that taxa and their traits evolve integrally/synchronously and diversify cladogenetically. He diagrammed macroevolution as a tree (but see Podani, 2019), or cladogram.…”

“…This diagram idealized anagenesis unidimensionally (lines) and diversification events nondimensionally (bifurcation points). Terminal taxa, whether species or 'genera' (Podani, 2019), also were idealized nondimensionally. It is critical to appreciate that this clado-anagenetic model is essential to NS theory.…”