Do you want to write about species or other groups of organisms without sounding wrong? Follow this simple semi-dichotomous guide to success!

Step 1 - What is it?

What is the group you're writing about? Is it a species, is it smaller than a species, or is it bigger than a species?

Step 2a - It's a species

Okay, great. Species are easy to write about, even if almost every biologist has a different opinion on what a species actually is, and even if species from one part of the Tree of Life act totally different than species from another part.

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Species are always written with their accompanying genus. The genus and species are always italicized, and the genus is capitalized while the species is not.

Correct usage examples = Tyrannosaurus rex, Papaver somniferum, Escherichia coli.

Incorrect usage examples = T-rex, rex, Trex

It is correct usage to abbreviate the genus if the genus has already been mentioned previously. Example:

  • Tyrannosaurus rex is a very large non-avian theropod dinosaur found in late Cretaceous rocks of western North America. Many scientists have studied T. rex, and it is unlikely to lose any popularity contests.

In situations wherein more than one genus is being discussed, it's okay to use abbreviations for both genera:

  • It is unlikely that Tyrannosaurus rex ever injected any narcotic compounds derived from Papaver somniferum. Many millions of years separated T. rex and P. somniferum.

In situations wherein more than one genus is being discussed, and they start with the same letter, one can still abbreviate as long as slightly different abbreviations are used:

  • Tyrannosaurus rex is sometimes confused for its East Asian relative, Tarbosaurus bataar. One of the distinguishing features of Ta. bataar from Ty. rex is that the former is much less popular than the latter.

There are some organisms wherein these abbreviation rules are somewhat ignored. This usually happens when a species has been studied so much that "everyone" knows what you mean. This happens with E. coli, it happens with C. elegans, it happens with T. rex. It's not really correct usage, but if lots of people make a mistake, then English language tradition (and, in most cases, taxonomic nomenclature) basically says that using the popular mistake is a better idea than using the less popular correct usage.

Step 2b - It's smaller than a species

Particularly if you are writing about an issue concerning regional or local conservation, you are going to discuss groups smaller than species. These could be recognized subspecies, other recognized groups, or they could be something else.

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If it's a subspecies then, just like a species, it is not used alone (it's used in combination with a genus and species) and it is italicized.

  • The two subspecies of Western gorilla (Gorilla gorilla) are both critically endangered. Scientists are still trying to determine whether the western lowland gorilla (G. g. gorilla) or the Cross River gorilla (G. g. diehli) is at greater threat of extinction.

If it's a recognized group, but not a subspecies, then it follows the general naming trend of:

  • Genus species grouptype groupname

This is particularly common with domesticated plants and animals, wherein different "breeds" might be distinguished in a way similar to that. When trying to write about domesticated plants and animals in a very accurate way, do your homework and read more than this.

If it's not a recognized group, then it does not get italicized, even if future studies might support it being distinguished as one.

  • The population genetics of Panthera leo are complicated, as could be expected from an animal with a widespread range and fairly recent extirpations across parts of its pre-human range. The Addis Ababa population may be genetically distinct enough to be a distinct subspecies.

Step 2c - It's bigger than a species

Okay. Is it a genus or something bigger than a genus?

Step 3a - It's a genus

Genera are capitalized and italicized. Sometimes individual organisms can only be identified to generic level: this is most common with fossil organisms. In that case, then the correct usage is to refer to the genus (capitalized and italicized) and then follow it with the abbreviation sp.

  • This calvarium is too large to be from an australopithecine, but it is difficult to assign it to anything more precise than as a member of our genus. Thus, this specimen is Homo sp., until further fragments from this locality are studied.

Step 3b - It's bigger than a genus

Any taxonomic group larger than a genus is capitalized but not italicized.

  • Homo sapiens is the most abundant member of Primates.

Depending on who you ask, taxonomic groups larger than a genus do not have ranks, or they have ranks. I try to not use ranks, but I do not speak for the entirety of any profession. If you do use ranks, follow this correct usage:

  • Homo sapiens is the most abundant member of the Order Primates.

Singular and plurals

Because some of the ranks of taxonomic nomenclature are derived from Greco-Roman roots, their pluralization is sometimes not normal for English writing. Here are some examples:

  • Subspecies, subspecies
  • Species, species
  • Genus, genera.
  • Phylum, phyla

Referring to an individual within a group

This depends a lot on how precise our ability to identify the individual is.

  • Through our binoculars we were able to sight an animal. Getting closer to the animal, we were able to determine it was a mammal, probably a felid. Getting very close to the felid, we were able to determine it was a vibrant specimen of Panthera leo.

Referring to multiple individuals within a group

Similarly, this depends on our level of precision.

  • They first sighted the group of animals from a far away distance. As our travelers approached them, they were able to distinguish feathers and wings on these birds. The overall size of the birds suggested ratites, and once close enough the travelers were able to determine that they had happened upon a small gathering of several Struthio camelus.

What happens if something has two names?

Due to the nature of scientific research, particularly before most human knowledge was shoved online, sometimes groups of organisms have more than one name. In these cases of synonymy, usually the winner is either the older name and/or the more prevalent name.

  • Different ceratopsian researchers have different thoughts, but if Triceratops and Torosaurus are the same genus, then Triceratops would remain while Torosaurus would become the junior synonym.

Does all life follow these general rules?

For the most part, yes, but in some situations no. Virii, prokaryotes (of both eubacterial and archaeobacterial affiliations), plants, animals, fungi, and other eukaryotes officially have different organizations which create, maintain, and modify the rules of their taxonomic nomenclature.

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As a person who's only published on animals, I recognize my biases and admit freely that any mistakes above were not made in my attempt to force other life to be named in the same way as animals.

Some Kinja wrong usage examples?

5 Important Articles That Were Lost-And Found-In Museum Storage

A genus (Barosaurus) isn't italicized. A individual organism is capitalized when it shouldn't be (sauropod). A genus and species aren't italicized (Electrotettix attenboroughi). Nit-picking grade: B+.

Hold Onto Your Butts (and Dying Triceratops): Jurassic Park Is Back in Theaters

Genera are sometimes not capitalized or italicized (Brachiosaurus, Gallimimus, Triceratops, Dilophosaurus). Genera and species are corrupted ("T. Rex") and then misspelled and corrupted ("Tyrannosaurs Rex"). Nit-picking grade: C-.

Less time separates us from T-Rex than T-Rex from Stegosaurus

A genus and species are corrupted beyond recognition ("T-Rex"). A genus isn't italicized (Stegosaurus). Nit-picking grade: D-, see me after class.

Jurassic Park Lied to You: T-Rex Had Great Eyesight Really

Genus and species are corrupted beyond recognition ("T-Rex"). Members of a genus are improperly referred to ("Velociraptors"). Genus and species are corrupted and misspelled ("Tyrannous Rex"). Genus and species are not italicized (Tyrannosaurus Rex, Velociraptor). Nit-picking grade: F——-, todayifoundout.com please study this guide before writing more things.

Some Kinja correct usage examples?

The Dinosaur Waited Patiently on Its Wing Legs, Ready for Flight

Refers to a genus and species correctly.

New Evidence Suggests Tyrannosaurs Hunted In Packs

Refers to groups of animals correctly. Capitalizes and italicizes a generic name.

Tiny Thumb-Sized Hedgehog Lived in Canada 52 Million Years Ago

Refers to genera and species correctly.

Both Halves Of A Giant Sea Turtle Fossil Found 160 Years Apart

Refers to a genus and species correctly.

Suggestions/comments/questions

If any of the above doesn't sound correct, please let me know, I do aim to be as accurate (but as general) as is possible. Edited on 26 August 2014 because I had some formatting errors, including not italicizing a genus and species. FML

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Top image is a slightly modified image of Alexander Roslin's 1774 painting of Carl von Linné, uploaded onto Wikipedia by AlphaZeta.