Jorge, Plants

What is the Difference Between Genus, Species, Variety and Cultivar?

Darwin’s illustration of the tree of life from On the Origin of Species.

The difference between genus, species, variety and cultivar is that each are different taxonomical ranks, containing populations of organisms with genetic similarities. These ranks reflect the ultimate goal of taxonomy, which is to lay out the tree of life, accurately documenting the relationships between organisms, both living and dead, tracing life back to a single ancestor. In this article, we seek to explain both why populations have been placed in certain ranks and the naming conventions used, which allow the easy identification of organisms.

Scientific name or species name – Prunus incisa

Comprised of genus Prunus and specific epithet incisa. Epithets usually refer to a feature of the plant (serrulata – little-saw, which refers to the shape of the leaves), but sometimes its origin (nipponica – Japan) or discoverer (sargentii – discovered by Charles Sargent). Genus is capitalised while its specific epithet is lower case italicized, just like its variety. Often genus is abbreviated to save time (P. incisa).  

Genus is the highest taxonomic rank you’d likely come across when browsing for plants. Genera are easy to learn. Prunus, for example, contains plums, cherries, peaches, nectarines, apricots and almonds.

Genera are hotly debated and sometimes revised. Taxons – a population of organisms – can be monophyletic, paraphyletic or polyphyletic. In monophyletic groups all species are descended from a common ancestor; paraphyletic groups contain all the descendents of a common ancestor minus one or more monophyletic groups; and finally polyphyletic groups are characterised by convergent features or habits of scientific interest. Today, taxonomists seek to avoid polyphyletic groups, believing taxons should reflect evolutionary relationships. Despite this, polyphyletic groupings persist, because of their usefulness to researchers studying similarities spread across evolutionary groups.

One recent study found that Prunus is monophyletic with all species descending from a single eurasian ancestor. Prunus, however, can be divided further into several subgenera. Historically these taxons would be based on morphology, although today they are often based on genetics. Thus subgenera are also disputed. An example of a subgenus is the Prunus subg. Padus that includes Prunus padus – a species of cherry native to the UK. As with genus, subgenus is also capitalised.

The scientific epithet completes the species name, distinguishing the plant from others in the genus. But what is a species? One definition states a species is a group of similar individuals which can reproduce successfully with each other while at the same time being reproductively isolated from other similar species. This definition leaves it up to scientists to decide when a group of individuals is distinct with some placing greater weight on genetics, others more obvious characteristics such as their morphology.

When a group of individuals becomes geographically isolated, it will begin to develop unique traits, making it distinct from the rest of the species. These distinct groups are known as varieties. Over time, they may become so different from the parent group that they are unable to breed, leading to the creation of a new species. Often, however, a variety come into contact with its parent group, resulting in an influx of genes that erodes their distinct features, reintegrating it into the greater species group.

Variety – P. nipponica var. kurilensis

The example in question, var. kurilensis is from the Kuril Islands – an island chain North of Japan, which is significantly colder than the Japanese mainland. It is extremely hardy and one of the few ornamental cherries suitable for the Nordic countries’ climate. Varieties are true to type as their seeds produce offspring with the same unique characteristics of the parent plant. Generally, plants aren’t advertised by their variety with nurseries preferring cultivars.

Cultivars are distinct from varieties in that they do not occur naturally in the wild. Cultivars are selected by humans for specific characteristics and are propagated through vegetative cuttings i.e. cloning. Propagation by seed will often lead to something different from the parent plant and as such they aren’t true to type.

Cultivars can be created through mutation breeding and hybridisation. Sometimes hybridisation programs can take years involving multiple crosses that each add a desirable trait as in the case of the Malus ‘Evereste’ – a cultivar resistant to fire blight, apple scab and powdery mildew. Mutation breeding involves bombarding plants with radiation as to induce mutations (new traits). An example of this is the ‘Rio Star’ grapefruit that is red in colour and produces more flesh and juice than varieties found in the wild. Cultivated varieties are more expensive than natural varieties due to the cost involved in development.

Cultivar – Prunus x incam ‘Okame’ / Prunus x incam cv. Okame

Cultivars are often capitalised and placed in single quote brackets, although sometimes they are written formally and preceded by an abbreviation. In the case of hybrids an x is placed before the second epithet as in the case ‘Okame’ that is a cross between the incisa and campanulatus.

If you are interested in learning more about taxonomy, please read our article: Plant Taxonomy: a History.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

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