As a consequence of there being several ways in which observed patterns of variation in nature can be conveyed in a generic classification, long recognised genera have changed in size over time. The generic rank has its origins in folk taxonomy, where genera were homogenous units of relatively few kinds. In the era of Bentham there was a widespread preference for large genera, many of which were split during the 20th century. In a survey of contemporary (1998–2007) generic delimitation practice we found a significant dichotomy between studies that incorporate molecular data and those that rely exclusively on morphological data. The former lead to delimitation of larger genera whereas the latter in general do not. This finding spurred a broader investigation into what drives changes in overall generic sizes, new data sources or new concepts? Two new data types have been introduced during the course of history: detailed morphology (anatomy, cytology) and chemical data (amino acid and DNA sequence data). Conceptual development has seen several turns: from language and communication, through memory and stability, to evolution and monophyly. We argue that conceptual change has a greater impact than changes in data do, since new data must be interpreted and translated into a classification and since conceptual changes may spur a search for new kinds of data. We conclude that the current trend toward recognising larger genera is a result of a return to study on a broad scale, rather than of incorporation of molecular data.
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