Biological Homonyms: Rules of Scientific Naming and Nomenclature
In the complex world of biological classification, precision is everything. However, because thousands of species are described by scientists globally, it is inevitable that the same name is occasionally given to different organisms. In biology, a homonym occurs when a name for a taxon (a group of one or more populations of an organism) is spelled identically to another name belonging to a different taxon.
To prevent confusion, international regulatory bodies have established strict rules to determine which name stays and which must be changed. These rules ensure that every organism has a unique, universally accepted scientific name.
Key Facts
- Senior Homonyms: The first published name of a set of homonyms; it generally takes priority and remains valid.
- Junior Homonyms: Names published later than the senior homonym; these are typically invalid and must be replaced.
- Hemihomonyms: Identical names used for organisms in different kingdoms (e.g., one animal and one plant), both of which remain valid.
- ICZN & ICN: The International Code of Zoological Nomenclature and the International Code of Nomenclature for algae, fungi, and plants govern these rules.
- Tautonyms: A specific case in zoology where the genus and species names are identical (e.g., Gorilla gorilla), which is permitted in animals but prohibited in plants.
The Principle of Priority
The primary mechanism for resolving naming conflicts is the Principle of Priority. Under the International Code of Zoological Nomenclature (ICZN), the first name published is designated as the senior homonym and is considered valid. Any subsequent identical names are junior homonyms and must be replaced.
For example, in 1777, Forster used the name Echidna for a genus of moray eels. Later, in 1797, Cuvier proposed Echidna for the spiny anteater. Because Forster's name was published first, it held priority. Consequently, Cuvier's name became a junior homonym and was replaced by Illiger in 1811 with the name Tachyglossus.
Exceptions: Nomen Protectum and Nomen Oblitum
Priority is not absolute. If a senior homonym has become archaic and is no longer in "prevailing usage," it may be declared a nomen oblitum (forgotten name) and rendered permanently invalid. In such cases, the junior homonym may be designated as a nomen protectum (protected name) to maintain stability in scientific communication.
A notable example involves the name Bucardium. Agassiz proposed it in 1848, and Gray later used it for a genus of cockles in 1853. Despite Agassiz having priority, his name was not used after 1899, while Gray's name became widely accepted. Under ICZN Article 23.9, Gray's junior homonym is protected, and Agassiz's senior homonym is invalid.
Botanical vs. Zoological Rules
While the ICN (for plants, algae, and fungi) and the ICZN (for animals) share similar goals, they have distinct differences:
- Botanical Rules: The ICN considers later homonyms "illegitimate." However, some may be "conserved" or "sanctioned" (specifically for fungi) to avoid widespread confusion. For instance, Myroxylon L.f. (1782) was conserved over the earlier Myroxylon J.R.Forst. & G.Forst. (1775).
- Rank Limitations: In zoology, homonymy only occurs within the same nomenclatural rank (family, genus, or species). A species name can be identical to a genus name without being a homonym.
- Tautonyms: The botanical code strictly prohibits tautonyms, whereas the zoological code allows them.
Parahomonyms
The botanical code is more stringent regarding spelling. Parahomonyms are names so similar that they are likely to be confused; these are treated as homonyms. For example, Astrostemma Benth. (1880) is an illegitimate homonym of Asterostemma Decne. (1838). In contrast, the zoological code generally considers a single letter difference sufficient to distinguish genus-rank names, though certain species-level spelling variations (like caeruleus and coeruleus) are still treated as identical.
Hemihomonyms: Crossing Kingdoms
Because the ICZN and ICN operate independently, they only regulate taxa within their own scope. When an animal and a plant share the same name, they are called hemihomonyms. In these instances, both names are considered valid because they belong to different biological kingdoms.
Common examples include the name Erica, which refers to both a genus of spiders (ICZN) and a genus of heaths (ICN), and Cyanea, which refers to both a jellyfish and a Hawaiian lobelioid.
Hemihomonymy also occurs at the species level, where the same binomial nomenclature is applied to different organisms. The following table highlights several such instances:
| Binomial Name | Animal Taxon | Plant Taxon |
|---|---|---|
| Adesmia muricata | Beetle | Legume |
| Agathis montana | Wasp | Conifer (Mount Panié kauri) |
| Baileya australis | Small baileya moth | Desert marigold |
| Centropogon australis | Waspfish (the fortescue) | Bellflower |
| Cuspidaria cuspidata | Bivalve | Wallflower |
| Ficus variegata | Sea snail (true fig shell) | Common red-stem fig |
| Gaussia princeps | Copepod | Palm |
| Orestias elegans | Pupfish | Orchid |
| Tritonia pallida | Nudibranch | Iris |
Frequently Asked Questions
What happens when two different animals are given the same genus name?
According to the Principle of Priority, the name published first (the senior homonym) is valid. The name published later (the junior homonym) must be replaced with a new, unique name.
Can a junior homonym ever be kept over a senior homonym?
Yes. If the senior homonym is archaic and no longer in use (nomen oblitum), and the junior homonym is widely accepted (nomen protectum), the junior name can be preserved to maintain scientific stability.
Why can a plant and an animal have the exact same name?
These are called hemihomonyms. Because the rules for naming animals (ICZN) and plants (ICN) are managed by different codes, names are only required to be unique within their own respective kingdoms.
What is a tautonym?
A tautonym is a scientific name where the genus and species epithets are identical, such as Gorilla gorilla. This is permitted in zoological nomenclature but is forbidden in botanical nomenclature.
How do botanical rules differ from zoological rules regarding similar spellings?
The botanical code recognizes parahomonyms—names that are similar enough to cause confusion—as homonyms. The zoological code generally requires names to be identical in spelling to be considered homonyms, with some specific exceptions for species-level grammatical variations.