Linear Trends in Botanical Systematics and the Major Trends of Xylem Evolution

Abstract

For nearly a century the so-called Major Trends of Xylem Evolution have guided thinking regarding wood evolution, but their conceptual foundations have not been examined. I detail and critique nine tenets of Major Trends thinking, including the use of linear schemes to infer phylogeny, the Haeckelian assumption that ontogeny recapitulates phylogeny, the use of homoplasies in phylogeny estimation, and the view of evolution as inexorable progress. In addition, I identify some six meanings of the key term “specialization,” ranging from the notion of division of labor between cell types to the relative position of a taxon in a linear hierarchy. The Trends in their original formulation of 1918–1957 show virtually no overlap with the Trends as currently construed. I suggest that the Trends were based on a conceptual foundation outdated at their outset and that they are unnecessary for any study of plant phylogeny or adaptation.