eli5: why do we use a specific epithet in species names? shouldnt the genus be enough?
In: Biology
Sometimes it is, sometimes it isn’t.
The species classification exists because there are often different animals in the same genus that need to be distinguished. If you don’t need to distinguish them, you can still use the generic name only.
Look how many species of Brassica there are, for example. Some of them even need to be distinguished by subspecies.
A species is any group of organisms that can successfully produce fertile offspring with one another. A genus is not descriptive enough, because while many organisms within the same genus may behave or look similar, they cannot reproduce fertile offspring together and thus should not be classified as being the same.
One example is the panthera genus. Panthers, lions, tigers, jaguars, oh my. They may look similar and all be big cats, but there are obviously significant differences between them that make it so just referring to one of them as a “panthera” doesn’t give enough information.
In some cases, it’s because genus, or even species doesn’t give us enough information about an organism.
A fine example is with bacteria.
Take Salmonella. That’s a genus. It has two species. *S. Bongori*, and *S. Enterica*. Generally speaking, humans don’t get sick from *S. Bongori*, when we get salmonella, it’s *S. Enterica*. But then S. Enterica further has 6 subspecies, and almost all human infections are due to *S. Enterica* subspecies *Enterica*
But there are different diseases caused by *S. Enterica* subspecies *Enterica!* Obviously, the salmonella you get from eating a dodgy egg sandwich is one, we call that salmonellosis. Typhoid is also salmonella. Paratyphoid, also salmonella.
These diseases are caused by *S. Enterica* subspecies *Enterica* serovar *Typhi* and *S. Enterica* subspecies *Enterica* serovar *Paratyphi A/B/C.* A **serovar** is a subgroup of a bacterial/viral species that has a different arrangement of structures on the outside of it’s cell. You mount a different immune response to one serovar than you do another, even though its the same species.
In this case, the ability to distinguish not just genus, but all the way down to serovar makes a real-world difference when you get salmonella. Typhoidal salmonella is worse than regular salmonella. You catch it from other people and their contaminated fluids, not food (look up the story of Typhoid Mary if you have the time). We don’t believe that animals get or can carry typhoid, so controlling a typhoid outbreak is different to a salmonella outbreak. We have tests specific to those serovars for diagnosing them. Treatment of typhoid/paratyphoid/salmonellosis is different. We have vaccines against those serovars that don’t cross-immunise you against other serovars. Different serovars vary in their antibiotic susceptibility, so the best antibiotic to give will vary.
tl;dr – Info beyond genus gives you details that you can use to distinguish vital differences between groups within a genus.
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