title: Why should we investigate the morphological disparity of plant clades?
abstract: Disparity refers to the morphological variation in a sample of taxa, and is distinct from diversity or taxonomic richness. Diversity and disparity are fundamentally decoupled; many groups attain high levels of disparity early in their evolution, while diversity is still comparatively low. Diversity may subsequently increase even in the face of static or declining disparity by increasingly fine sub-division of morphological 'design' space (morphospace). Many animal clades reached high levels of disparity early in their evolution, but there have been few comparable studies of plant clades, despite their profound ecological and evolutionary importance. This study offers a prospective and some preliminary macroevolutionary analyses.
  
  
  
  Classical morphometric methods are most suitable when there is reasonable conservation of form, but lose traction where morphological differences become greater (e.g. in comparisons across higher taxa). Discrete character matrices offer one means to compare a greater diversity of forms. This study explores morphospaces derived from eight discrete data sets for major plant clades, and discusses their macroevolutionary implications.
  
  
  
  Most of the plant clades in this study show initial, high levels of disparity that approach or attain the maximum levels reached subsequently. These plant clades are characterized by an initial phase of evolution during which most regions of their empirical morphospaces are colonized. Angiosperms, palms, pines and ferns show remarkably little variation in disparity through time. Conifers furnish the most marked exception, appearing at relatively low disparity in the latest Carboniferous, before expanding incrementally with the radiation of successive, tightly clustered constituent sub-clades.
  
  
  
  Many cladistic data sets can be repurposed for investigating the morphological disparity of plant clades through time, and offer insights that are complementary to more focused morphometric studies. The unique structural and ecological features of plants make them ideally suited to investigating intrinsic and extrinsic constraints on disparity.
authors.affiliation: Milner Centre for Evolution, University of Bath, Bath BA2 7AY, UK.
authors.forename: Jack W
authors.initials: JW
authors.lastname: Oyston
authors.affiliation: Department of Life Sciences, The Natural History Museum, London SW7 5BD, UK and.
authors.forename: Martin
authors.initials: M
authors.lastname: Hughes
authors.affiliation: Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Gerber
authors.affiliation: Milner Centre for Evolution, University of Bath, Bath BA2 7AY, UK, m.a.wills@bath.ac.uk.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Wills
mesh_terms: D044822:Biodiversity; D005075:Biological Evolution; D029624:Ferns / Q000033:anatomy & histology / Q000145:classification / Q000502:physiology*; D019684:Magnoliopsida / Q000033:anatomy & histology / Q000145:classification / Q000502:physiology*; D013045:Species Specificity
keywords: Disparity; Embryophyta; angiosperms; clade shapes; conifers; developmental robustness; ferns; macroevolution; morphological diversity; morphospace; species diversity