Tree thinking, time and topology: comments on the interpretation of tree diagrams in evolutionary/phylogenetic systematics

This paper presents a graph theoretical overview of tree diagrams applied extensively in systematic biology. Simple evolutionary models involving three speciation processes (splitting, budding and anagenesis) are used for evaluating the ability of different rooted trees to demonstrate temporal and ancestor–descendant relationships within or among species. On this basis, they are classified into four types: (i) diachronous trees depict evolutionary history faithfully because the order of nodes along any path agrees with the temporal sequence of respective populations or species, (ii) achronous trees show ancestor–descendant relationships for species or higher taxa such that the time aspect is disregarded, (iii) synchronous trees attempt to reveal evolutionary pathways and/or distributional patterns of apomorphic characters for organisms living at the same point of time, and (iv) asynchronous trees may do the same regardless the time of origin (e.g. when extant and extinct species are evaluated together). Trees of the last two types are cladograms, the synchronous ones emphasizing predominantly—but not exclusively—the evolutionary process within a group, while asynchronous cladograms are usually focused on pattern and infrequently on process. Historical comments and the examples demonstrate that each of these tree types is useful on its own right in evolutionary biology and systematics. In practice, separation among them is not sharp, and their features are often combined into eclectic tree forms whose interpretation is not entirely free from problems.