Species Nomenclature: Naming the Wild Irises

What's a Species?

When I was young, I learned that the definition of a species was that two organisms belong to the same species if they can mate and produce fertile offspring. This definition was reiterated, perhaps with an occasional refinement or cautionary exception, in my junior high and high school biology classes. It is probably the most likely definition for a nonspecialist to bring to mind.

Alas, things are not so simple, especially if one is speaking of plants rather than animals. If this definition were applied at face value to the bearded and aril irises, there would be only four species: the tetraploids (all of which can breed with each other and produce fertile offspring) and three diploid species: attica/pseudopumila, all other diploid bearded irises, and all the diploid arils.

There is still no consensus among biologists about the best definition of a species, and in fact some have stated that no definition can be devised to meet all purposes for which the term is needed. Suffice it to say that a central notion behind the concept of species is that each species is a breeding population that has taken an evolutionary course of its own, separate from related populations, and consequently developed a set of distinguishing features.

Although in many instances the differences between species are obvious, there are many situations where distinguishing species is problematic, and subjective assessments come into play. Nature is not obliged to conform to human categories; evolution is a process involving many gradations and complications. As populations evolve, the features of the plants differ and change, but the exact point where the accumulated differences amount to the existence of a new species is quite arbitrary.

A further complication in the naming of species is that the knowledge of botanists is never absolutely complete. Ideally, perhaps, every botanist naming a new species would know all the details of its distribution, ecology, range of forms, genotype, and development through time - and have similar knowledge for all the related species. In reality, a new species may be described on the basis of a few specimens; the species' connection with others may emerge only later, after more observations are made.

Some botanists ("splitters") see significant distinctions everywhere, and divide populations into many species. Others ("lumpers") emphasize the similarities instead, subsuming many different populations under a single broadly defined species. If two botanists independently name the same species, or name two types that are later merged into a single species, the result is synonyms: two or more names that refer to the same species. There are many instances of this in iris nomenclature.

Within a Species...

Because species are populations of plants, not individual plants, they can show a great deal of variation. It can be a surprise, if one is used to growing only named cultivars, to discover that the same species obtained from two different sources may differ in height, bloom color, branching, flower size, or any number of characters. Keeping track of these differences is important to the gardener or hybridizer. Botanical nomenclature is helpful, although it does not address every need.

In botany, there are three ranks of classification below that of species.

The highest is that of subspecies, abbreviated ssp. In principle, subspecies are populations of a species that have become reproductively isolated from each other and are evolving in different directions. Usually, the isolation is the result of a geographical barrier: a body of water, a mountain range, or other terrain where the species cannot grow. The implication is that if the barrier were removed, the subspecies would interbreed again and merge back together.

The next level is that of variety (Latin varietas, abbreviated var.). This should not be confused with the use of the word "variety" in horticulture, which refers to a named cultivar. A botanical variety is a population that has some distinguishing qualities, but is not so isolated as a subspecies. An example might be a population growing at higher elevations with consistently smaller leaves, lighter colored flowers, and more compact growth. There may be continuous variation between the high-altitude variety and its lowland-growing neighbors.

The lowest level is the form (Latin forma, abbreviated f.). A form is any plant or group of plants showing a difference in some character or other, for example, an albino form with white flowers. There is no implication that each form is a breeding population or community - these are just plants that are noticeably different in some way. Botanists, whose interest is usually in the evolution of populations, usually take less interest in forms than do gardeners and breeders, for whom these differences can be intriguing and exciting.

In reality, the distinctions between these three ranks are not maintained very consistently. Again, the problems are lack of complete knowledge about a species' range of variation, and the subjective inclinations of the botanists making use of the terms. It is not unusual to find the same term regarded as a species by one author, a subspecies by another, and a meaningless synonym by a third.

What is the species enthusiast to do, when the names of the plants are so murky and shifting? This is a very different world than that of registered cultivars, where many of us begin our interest in irises. We have been indebted to scholars who, over the years, have tried to sort this all out and present a consistent set of names: Dykes, Lawrence, Rodionenko, Mathew, and others. Most species enthusiasts settle on the most recent such presentation and use it as an authority for proper naming.

Collectors and plant sellers do not all use the latest system of choice, however, and many plants are passed down with names now considered obsolete or incorrect. As a hybridizer, names are useful for me to the extent that they convey the plant's distinctive qualities and expected breeding behavior. Thus, although I. attica is sometimes identified as I. pumila ssp. attica, the former designation is much better suited to my purposes. I. attica is a diploid and can not be used interchangeably with I. pumila in a breeding program. It would also be inconsistent to grow one plant labeled I. attica and a virtually identical one labeled I. pumila ssp. attica, just because I obtained them from people with different nomenclatural preferences. So although one is tempted to retain the names of plants exactly as given in the catalog or listing from which they were obtained, this can create its own kind of confusion.

My own approach, reflected on this web site, is to follow a chosen authority (mine is Mathew) on which species names are valid, and use care to assign each plant to one of these species. In some cases where Mathew is tentative, I have made my own judgments based on my needs as a hybridizer (by separating the diploid I. furcata from the tetraploid I. aphylla, for example). I retain original names from the source of the plants or seeds following the species name, without committing to any particular taxonomic rank. Thus plants purchased as "I. taurica (blue form)", I identify as I. pumila taurica blue. This seems a fair compromise between the need for a consistent set of species names and the need to preserve original identifications attached to the plants or seeds.

This way of writing names lacks the precision of specifically identified botanical taxa (e.g., I. pumila ssp. taurica), but it seems to me that such precision is rather illusory in the horticultural world; there are just too many opportunities for inconsistent terminology and inadequate identification to creep in. Identifying plants in my garden as belonging to a particular botanical subspecies, variety, or form implies a level of scientific rigor that I don't feel is justified by my own level of expertise in such matters. The most important thing is to ensure that the species is correctly identified, and that different plants are noted in my records in a consistent way.

Addendum: What's a cultivar?

"Cultivar" is a term coined from the phrase "cultivated variety". It is a horticultural term, not a botanical one. A cultivar is a unique garden plant, either a clone or a true-breeding seed strain. (The first sense is the one appropriate for garden irises.) A cultivar can be a selected clone of a species, e. g., 'Ostry White', which is a collected albino clone of I. aphylla, or (as is more usually the case), an advanced-generation hybrid with different species in its background, e. g., 'Alpine Lake', an MDB derived from both I. pumila and the germanica, pallida, and variegata ancestors of the tall bearded irises.

The American Iris Society maintains the international registry of all iris cultivars, except for bulb types. This ensures that cultivar names are unique and applied to the correct plants. In recent decades, there has been a growing appreciation of the value of registering widely distributed species clones as named cultivars. If a particular plant is to be circulated and used in breeding, it is helpful to ensure its integrity by having a published description including any pertinent data on collection circumstances, parentage, etc. A registered cultivar name is also immune to shifts of botanical nomenclature which can confuse the identification of species clones over time.

 Tom Waters

January 2011

Unless otherwise noted, all text and illustrations copyright Tom Waters and all photographs copyright Tom or Karen Waters. Please do not reproduce without permission.