The nature of plant species

Abstract

Many botanists doubt the existence of plant species, 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 oaks. 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 wisdom, plant species are more likely than animal species to represent reproductively independent lineages.

Figure 1. Proportion of species taxa that correspond directly to phenotypic clusters compared on the basis of taxon, life history, mating system, polyploidy and contemporary hybridization

Refer also to Table 1. For each comparison, means with different letters are significantly different at P < 0.05 (Tukey’s ‘honestly significant difference’ (HSD) test). Error bars indicate s.e.m.

Figure 2. Fraction of species taxa that represent reproductively independent lineages in major taxonomic groups of plants and animals

Means that do not share any letters are significantly different at P < 0.05 (Tukey’s HSD test). Error bars indicate s.e.m.