Image Credit: billp1969, Pixabay licence, Image Cropped

You might have come across the word “subspecies” when reading about biodiversity, but what does the term actually mean? And do we really need a more precise classification beyond species? There is unfortunately no consensus about this. Ask 5 biologist and you’ll get at least 10 different answers. So let’s have a look at why it’s such a complicated issue.

If we’re going to talk about subspecies, we should probably define species first. And that’s where the problem starts. There are more than 20 different ways to define a species. Some just taking morphology into account, others using genetics or ecology or a combination of all three. The biological species concept says a species is a group of individuals that can interbreed with each other and produce fertile offspring, but can’t breed with other species. Horses and donkeys can produce offspring (a mule or a hinny), but because that offspring is usually sterile, horses and donkeys are two different species. Sounds like an easy definition, right? Too bad it doesn’t always work. Some plants and bacteria can reproduce asexually by cloning. So it’s a bit arbitrary as far as definitions go. Morphology is even more confusing. Look at a chihuahua and a wolf. Same species.

With the onset of genetics, there have been attempts to make the definition of a species more objective and base it on genetic principles. The phylogenetic approach defines a species as a group having a shared and unique evolutionary history and are therefore sharing genetic similarities. You’ve probably (either knowingly or unknowingly) seen a phylogenetic tree before. We’ve pictured one below, which shows the evolutionary relationships between different species of some species of primate. You can see us (Homo sapiens) down the bottom, and our common ancestor with the chimpanzee denoted by the photo of a fossilised skull. We are sufficiently separated from the two types of chimpanzee to be considered a different species. Unlike the biological species concept, this one allows for gene flow between species (although in this example I wouldn’t try it). But how similar does the group need to be to be considered as one species?

Here you can see the evolutionary relationships between a group of primates, including us. Our common ancestor existed between 23.5 and 33.5 million years ago, and our common ancestor with chimpanzees and bonobos existed between 10 and 5.7 million years ago. (Image Credit: PLOS ONE PHYLOGENY, CC BY 2.0)

Back to the main question: what is a subspecies? Subspecies are groups below the species level, allowing one species to be divided into subspecies (you can see that in the above example with the two subspecies of Pan troglodytes, the chimpanzee). A species either has no subspecies at all or at least two, but some (or all) subspecies can already be extinct. Subspecies are usually defined based on morphological differences between populations. The populations should occupy a distinct breeding range. That means they are isolated either geographically (there might be a mountain range or the sea between them or just a wide area where the species is not distributed) or due to sexual selection (members of one group might not breed with members of the other group because they look different). If they happen to meet and breed, they can produce fertile offspring, so they can’t be classified as two different species based on that earlier biological definition.

Subspecies should also differ genetically, so they should have their own branches in the phylogenetic tree of the species. Subspecies can be seen as an intermediate state in the speciation process where the two new species that derived from a common ancestor are not fully separated yet (see the grey zone in the figure below). Since there is no agreed definition of subspecies, not all are described based on the same criteria. Some were defined before the usage of genetic methods and are therefore only based on morphology. And as we mentioned before, differences in morphology don’t always have a genetic basis, even when selective human breeding isn’t involved. “Phenotypic plasticity” can cause individuals of the same species that live in different habitats to look wildly different. This is a result of the organism changing its behavior, physiology or morphology in response to changes in the environment without changing its genetic code. It’s more common in plants, but it happens in animals too. For example, the color of a flamingo depends on what they eat. The more crustaceans they eat, the deeper pink they are. In the Caribbean, they find lots of crustaceans, so they are deep pink. In drier habitats such as Lake Nakuru in Kenya, they find less crustaceans and are pale pink.

Here you can many of the processes that cause a species to diverge into two separate species (or subspecies)

Are subspecies useful?

Yes and no. In the context of conservation biology, subspecies might be a useful tool. They might be populations on the way to speciation and have unique genetic diversity that should be preserved. But because there is no real definition and taxonomists often don’t agree with each other if two populations should be one species, two subspecies, or two different species, it makes it hard for conservation agencies to handle. Scientists want to preserve distinct genetic groups, as preserving genetic variance is a key aspect of preserving biodiversity, not to mention the information we gain regarding evolutionary history. But many descriptions of subspecies are from before the onset of genetic methods and might therefore not describe distinct genetic groups. Additionally, some groups which are more morphologically distinct and easily recognisable may not be that genetically dissimilar from other subspecies.

It makes life hard for conservationists. Prioritisation of resources is essential when funding is hard to come by, so choosing whether conversation effort should be divided between the different subspecies or rather focus on a single subspecies is crucial. Prioritising one subspecies might not be ideal, then again it’s always easier to rustle up conservation funding if the existence of an entire ‘animal’ is on the line.

How to name a subspecies?

When species are described, they get a latin name that consists of two words: one for the genus and one for the species (e.g. Panthera pardus). For naming a subspecies, there are different rules in different fields. In zoology, there is one word added to the end of the species name to describe the subspecies (e.g. Panthera pardus fusca). You sometimes also see the species name in parentheses like Larus (argentatus) smithsonianus. This means that some scientist would see it as its own species (Larus smithsonianus) and others as a subspecies of Larus argentatus, Larus argentatus smithsonianus. The author who uses the notation with parentheses doesn’t want to take position. In botany, there are several ranks below species (e.g. variety), so we need to add subspecies (or the abbreviation “subsp.” or “ssp.”), e.g. Schoenoplectus californicus subsp. tatora.