Annals of Botany

Why should we investigate the morphological disparity of plant clades?

Macroevolution of major clades is often studied by plotting their taxonomic diversity through time. It is equally informative, but less commonplace, to investigate how clades explore anatomical ‘design’ space by quantifying their morphological disparity through time. Counterintuitively, diversity and disparity are usually decoupled. Metazoan clades often reach their highest disparity relatively early in their evolution, while diversity is still low.

 Simplified models of the pattern of morphological disparity through the Phanerozoic.
Simplified models of the pattern of morphological disparity through the Phanerozoic. The ‘traditional’ model assumes that patterns of disparity loosely track diversity, which increases (albeit irregularly) through time. Gould (1989) espoused an inversion of this model, derived largely from his own interpretation of the significance of fossils from the Middle Cambrian Burgess Shale. Cambrian genera were believed to represent numerous, highly distinct bodyplans, between which there were morphological differences comparable with those distinguishing the living phyla. Most of these Cambrian bodyplans were lost arbitrarily in the early Palaeozoic, resulting in a marked reduction in disparity (‘decimation’). Subsequent evolution entailed increasing diversity within this more limited number of themes, but disparity was belived to persist unchanged. Fortey et al. (1996) summarized findings from the then-published empirical studies of disparity, which revealed comparable levels of disparity amongst Cambrian invertebrate groups and their living counterparts. Subsequent studies have largely confirmed the validity of the latter picture. For more details see Oyston et al. (2016)

Oyston et al. show that this macroevolutionary rule of thumb holds true in a sample of major plant clades, and discuss the types of data and methodological approaches that will facilitate future work on plant disparity.