Age of innovation: Progress in systematic botany in the 20th century

Abstract Systematic botany can be regarded as the oldest of the botanical sciences, having originated with the ancient Greeks. Although much progress has been achieved in systematic botany over nearly two millennia, in the 20th century, many significant innovations took place in revealing processes of evolution, principles and methods of classification, and construction of phylogenies. New comparative data were developed in part due to improved instrumentation, especially microscopes and specialized diagnostic chemical equipment. The melding of genetics, chromosomes, and statistical analyses helped reveal mechanisms for the origin of species at the populational level, leading to experimental taxonomy and biosystematics, which had a huge impact on classification at specific and infraspecific levels. Phylogenetic concepts were widely accepted in development of new evolutionary/phylogenetic intuitive classifications, such as those of Bessey, Cronquist, Dahlgren, Hutchinson, Takhtajan, and Thorne. In the second half of the 20th century, quantitative methods were applied to make the process of classification transparent and objective, leading to numerical taxonomy or phenetics, but this initially ignored phylogeny. Phenetics stimulated new algorithms for assessing relationships, especially at the lower levels of the hierarchy, which eventually led to morphometrics, and some phenetic algorithms were also directed toward constructing phylogenies. Independently, Willi Hennig developed an argumentation scheme for phylogeny reconstruction based on shared derived character states, but his method ignored divergence within lineages. Accessibility of DNA in the latter part of the century led to remarkable new insights at all levels of plant diversity, including collaborative classifications of angiosperms, ferns, bryophytes, and many other groups. Although exciting technological developments have taken place in the 21st century, such as new DNA methods (including whole genome analyses), flow cytometry for inferring ploidy levels, statistical modeling in biogeographic reconstructions and dating of molecular trees, ecological niche modeling, and AI methods (especially for automated identification), it is argued here that more conceptual innovation in systematic botany occurred in the 20th century than at any other comparable period in the past 2000 years.