3 - The Emergence of Lineage Thinking

Summary

In this chapter I outline the transformation of systematics into phylogenetics by tracing the emergence of lineage thinking. One of the routes to a realist interpretation of the natural system of systematic relationships was to temporalize it. Lineage thinking emerged when the previously atemporal and symmetrical affinity relationships between contemporaneous taxa were replaced by asymmetrical ancestor-descendant relationships that tracked the arrow of time. This transition was accompanied by a rapid decrease in the diversity of shapes of affinity diagrams published in the systematic literature, and it marked a shift from predominantly reticulating or web-like systems to tree-like figures soon after the publication of Darwin’s On the origin of species in 1859. I argue that this graphic revolution largely records the influence of evolutionary expectations, as biologists redrew their diagrams to fit the theoretical dictates of Darwinian descent with modification. The current swell of enthusiasm for evolutionary networks has driven several recent authors to the peculiar argument that even Darwin disliked the tree of life as an evolutionary metaphor, an argument I will refute. Reconceiving the systematic relationships between taxa as phylogenetic pathways along which body plans evolve had an epistemic corollary. Speculation became a necessary tool for the evolutionary storyteller.