Many botanists doubt the existence of plant species1,2,3,4,5, viewing them as arbitrary constructs of the human mind, as opposed to discrete, objective entities that represent reproductively independent lineages or ‘units of evolution’. However, the discreteness of plant species and their correspondence with reproductive communities have not been tested quantitatively, allowing zoologists to argue that botanists have been overly influenced by a few ‘botanical horror stories’, such as dandelions, blackberries and oaks6,7. Here we analyse phenetic and/or crossing relationships in over 400 genera of plants and animals. We show that although discrete phenotypic clusters exist in most genera (> 80%), the correspondence of taxonomic species to these clusters is poor (< 60%) and no different between plants and animals. Lack of congruence is caused by polyploidy, asexual reproduction and over-differentiation by taxonomists, but not by contemporary hybridization. Nonetheless, crossability data indicate that 70% of taxonomic species and 75% of phenotypic clusters in plants correspond to reproductively independent lineages (as measured by postmating isolation), and thus represent biologically real entities. Contrary to conventional wisdom8, plant species are more likely than animal species to represent reproductively independent lineages.
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Funding for this work was provided by the US National Institutes of Health, the US National Science Foundation, and the Guggenheim Foundation (to L.H.R.). We thank M. Barker, S. Bickford, A. Buerkle, J. Burke, B. Gross, M. Moody, J. Strasburg, Y. Supir, M. Tseng and K. Whitney for helpful comments on an earlier version of the manuscript, and the Centre for Plant Biodiversity Research at CSIRO Plant Industry, Australia for providing office space and library resources for this project. W. Hastie, M. Hearn and M. Thornton, from the CSIRO Black Mountain Library, provided extensive bibliographic assistance. Author Contributions L.H.R. conceived of the literature surveys, T.E.W. and L.H.R. collected the data, and E.J.B. and L.H.R. conducted the data analyses. The manuscript was written by L.H.R. with comments and assistance from T.E.W. and E.J.B.
This file contains the Supplementary Methods and additional results.
Supplementary Table 1 nature04402-s2.doc Phenetic studies of plant species.
Phenetic studies of animal species.
Correspondence between crossability indices (CI), species taxa, and phenetic clusters.
Correspondence between species taxa and crossability indices (CIs) in plants.
Crossability indices (CIs) in plants
Correspondence between species taxa and crossability indices (CIs) in animals.
This file contains additional references.