Skewed Distribution of Species Number in Grass Genera: Is It a Taxonomic Artefact?

The grass family (Poaceae) comprises about 10,000 species distributed in some 785 genera, seven large subfamilies and a few small ones. The distribution of species in genera appears skewed toward monotypic genera and those with few species. This pattern follows the hollow curve distribution documented by Willis 1 . Explanations of the pattern have been attributed to statistical, biological and taxonomic factors. This study explores potential biological and statistical explanations for species distribution in Poaceae. Patterns of species distribution in the family and its major subfamilies were investigated, and the influence of age, habit and habitat on these patterns was assessed. Results showed that species distribution is not only skewed for the number of small genera but also for the total number of species in larger genera. Phylogenetic position does not appear to explain species distribution in the family and in fact refutes the age and area theory proposed by Willis. Genus size appears to be correlated with habit where larger genera are predominantly perennial. Genera with mixed annual and perennial species do not reflect the hollow curve pattern. These patterns of species distribution may be explained by polyploidy and hybridisation, two prominent features in the evolution of the family. 11.