Plant species maintain different levels of variation within and among populations, much of which is genetically determined. As such, variation in form below the taxonomic rank of species, called infraspecific variation, is widely recognized in plants. In contrast to zoological taxonomy, three categories have been applied to recognize this variation in plants. In order of decreasing taxonomic rank, these are: subspecies, variety, and forma. Additionally, the category of cultivar is used to recognize horticultural varieties not typically found in naturally occurring populations. The subspecies, which is the most inclusive of the three categories, is usually applied in recognition of population variation that is correlated with geography. By definition, populations of subspecies differ from other such populations. Furthermore, subspecies are expected to interbreed more freely than species, which may comprise two or more subspecies. The category of variety is similarly applied to recognize variation below the level of subspecies. Unfortunately, the two categories are not distinct, and application of the taxonomic rank of variety is more frequently encountered. Forma, the least-inclusive category, is applied to recognize minor infraspecific variation that is presumably due to variation at a single gene and, as such, may vary within populations. Flower color variants are typically recognized at this level.

See Also

Clines and Ecotypes; Cultivar; Species; Taxonomy.

Bibliography

Stuessy, Tod F. Case Studies in Plant Taxonomy: Exercises in Applied Pattern Recognition. New York: Columbia University Press, 1994.