The nature of plant species

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Abstract
Some botanists think that plant species don't really exist: they are a product of the human imagination. Claims of extensive hybridization among the likes of oaks, blackberries and dandelions have encouraged the idea that individual, discrete species are a nonsense. Zoologists in the main seem content with the concept of animal species. The idea of a species has now been tested by measuring phenotypic and reproductive distinctness of over 400 genera of plants and animals. The results show that difficulty in pinning species names to vague clusters of individuals, when it does occur, is more likely to result from asexual reproduction, polyploidy (multiple chromosomes) and over-differentiation by taxonomists than it is to be due to hybridization between distinct, sexual species. In fact, plant species are more likely than animal species to represent reproductively independent lines. Analysis of phenetic and/or crossing relationships in over 400 genera of plants and animals shows that although discrete phenotypic clusters exist in most genera (more than 80%), the correspondence of taxonomic species to these clusters is poor (less than 60%) and no different between plants and animals. However, crossability data indicate that 70% of taxonomic species and 75% of phenotypic clusters in plants correspond to reproductively independent lineages. 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.