CATEGORISING LIFE
Classifying a group of organisms as a separate species has significant implications for conservation. But the science behind such decisions is far from settled
Classifying a group of organisms as a separate species has significant implications for conservation. But the science behind such decisions is far from settled. Katie Burton delves into the arcane world of taxonomy.
It could happen out in the field, perhaps in a steamy jungle or a parched desert or deep beneath the ocean surface where few people have been before. Or it might happen in a dusty, dimly lit archive. What matters is spotting something a little bit different, something you haven’t seen before. The discovery of a new species is always an exciting event for the person whose job it is to make such finds: the taxonomist. It goes to the heart of the scientific endeavour – uncovering some previously hidden aspect of our world. But when it comes to species, we might reasonably ask: what’s really being discovered? What makes two different moths, which may look identical to an untrained eye, separate species, when a poodle ( Canis lupus) and a great dane (also Canis lupus) are most definitely just the one?
The answer has repercussions beyond the closeted world of taxonomy. Most notably, it has implications for conservation. We define some organisms today as ‘endangered species’ and put them onto lists. But if our concept of what a species is shifts, or if a population originally deemed to be one species with lots of individuals turns out to be two species, one of which has far fewer individuals, these lists can, and indeed do, shift.
‘If you want to formulate species-specific conservation programmes, then clearly you need to know what your species are,’ says Wolfgang Wuster, a herpetologist (reptile and amphibian specialist) and taxonomist at Bangor University. ‘If you think you’ve got a very widespread species that’s found all over the place in many different habitats, then you might not worry about its conservation status. If you find out that it’s actually several species, then you might suddenly realise that there’s a completely different lineage about to go extinct.’ Given this, the answer to the question ‘what is a species’ is surprisingly difficult to pin down. Different people favour different definitions, a huge range of criteria come into play and it’s not uncommon to hear people argue that it’s actually a man-made concept – that species don’t really exist in nature, but are defined as such by humans as a useful form of categorisation.
Of course, all of this is taking place during a period of mass extinction, in which many species and subspecies are disappearing before they can even be identified. ‘We estimate that there are somewhere between ten million species on Earth and, at the very highest end, 100 million species. The 20 million range is the number I often use,’ says Richard Pyle, a Hawaii-based naturalist who spends his days scuba diving, sometimes uncovering new species of fish. ‘We’ve only named about two million of them. In other words, 250 years after this process began, we’re only a tiny bit of the way there. And we have lots of reasons to believe that the sixth great extinction is upon us. We’re losing biodiversity faster than we can even know that it ever existed. That’s the context around how we play this game.’ There was no concept of biological species in natural history before the 17th century. Modern taxonomy stems from the work of Carolus Linnaeus, the Swedish botanical taxonomist who, in the mid-1700s, formulated the binomial system of nomenclature, in which each specimen is identified by a generic name (genus) and a specific name (species) – Homo sapiens, for instance.
Linnaeus defined different species according to the natural characters of individual organisms, or morphological descriptions – something that still remains important in modern taxonomy. Today, morphology (essentially, the features that can be examined by the human eye) is used in combination with examination of DNA and of cells and behaviour to identify new species, which are then named according to a strict set of rules. For Linnaeus, operating a century before Charles Darwin formulated his theory of evolution, two organisms were the same species because they came from the same parents, as created by God. Modern taxonomists are interested in identifying organisms that are the same species because they have followed the same distinct evolutionary path, usually as a result of geographical isolation from others.
But none of this really defines a species. When does an organism become different enough from another one that it counts? The traditional definition, often encountered in school textbooks, is known as the ‘biological species concept’ and holds that two organisms are the same species if they can breed and produce fertile offspring. A donkey and a horse can breed but their offspring (a mule) will be infertile and they are therefore different species. All dogs, on the other hand, can produce fertile offspring (although human breeding has made this practically difficult in some cases) and are therefore the same species. They are, in fact, the same species as wolves, too ( Canis lupus), although they’re considered to be a subspecies ( Canis lupus familiaris).
But this is really just one method of identification, not a definition. For starters, there are exceptions. Ligers, the offspring of tigers and lions, can sometimes produce fertile offspring, but tigers and lions are considered to be different species. Then there’s a whole host of animals, plants and bacteria that reproduce asexually and, in any case, the margins between species are often blurred – most individuals don’t interbreed, but some do and their offspring are sometimes fertile.
‘Science has no clear definition for what a species is. It always has been, and probably always will be, a matter of subjective decision,’ says Pyle. ‘The one that
I use is the one that’s been around for the entire time and which we will always use, which is that a species is what a community of taxonomists says it is. That’s a little bit snarky, but it also underpins another debate in our world: are species actual entities in nature that exist independently of humans?’
Pyle’s feeling is that they don’t, but that doesn’t mean the exercise of categorisation is worthless. ‘My school of thought comes from the idea that evolution doesn’t produce species,’ he says, ‘evolution produces populations of organisms that have different levels of interbreeding with each other over time. Those populations change over time because of climate change and other circumstances. It’s just a kind of amorphous mix of things that definitely cluster but don’t necessarily have hard boundaries between species.’
Mostly, none of this a problem. Taxonomists are okay with uncertainty. Science is the gradual accumulation of knowledge, followed by correction – in other words, tinkering. And taxonomists like to tinker. But having no fixed definition does leave things open for debate and the debate can heat up. One of the hot topics today is the increasing use of genetic analysis to identify species. This concept defines a species as the smallest group that can be genetically distinguished from another species but still share a clear ancestor.
Differences in the genetic make-up of organisms are certainly a useful way of distinguishing between populations, but more traditional taxonomists balk at the idea that a species can be defined solely based on DNA – something that’s becoming much more common in the scientific literature and that’s contributing to what has been described as a ‘new age of discoveries’. This new age has seen more than 400 species of mammal discovered since 1993, an acceleration that has led some taxonomists to point to ‘taxonomic inflation’.
Sometimes, these new discoveries result from exploration of previously neglected regions (the deep sea, for example), but often they are made in the lab and result in what was once considered one species being divided in two, or what was considered a subspecies being elevated to species level. In early 2020, for example, a research paper announced the discovery of a new species of primate called the Popa langur. Found exclusively in Myanmar, near the extinct volcano Mount Popa, these animals weren’t unknown to humans before the paper came out, but were considered members of a wider species called Phayre’s langur (or leaf monkey), found right across Southeast Asia. The new categorisation meant that the Popa langur was critically endangered from the moment it was ‘discovered’ – there are just 200–250 individuals – although the categorisation is so new it doesn’t yet appear on the International Union for Conservation of Nature’s (IUCN) Red List (Phayre’s langur is listed as ‘endangered’). Like most other recently described primates, the Popa langur was first identified in a lab using genetic information, in this case taken from a 100-year-old skin and skull housed at the Natural History Museum in London. The genetic information was then supplemented by very subtle physical differences. Similarly, scientists used to recognise two different subspecies of the red panda ( Ailurus fulgens): the Chinese red panda and the Himalayan red panda. However, another 2020 study by researchers from the Chinese Academy of Sciences pointed out that the pandas fall into two clear genetic clusters, which they argue are distinct enough to classify them as separate species. (Again, just the one species appears on the Red List.) But the question remains: what is distinct enough? ‘It has been a hot topic now for about ten years and it has been heating up a little bit more recently,’ says Pyle. ‘Now that we have relatively inexpensive ability to access the DNA of specimens, it’s almost like the perfect characteristic to determine whether or not two species are the same or different. I say almost because we don’t quite understand how to translate the genetic information into the context of how species have historically been named. There’s a little bit of tension in the sense that people who only look at the genetics sometimes find different patterns than people who look at it in a more traditional way. And, if the names are changing every year, it gets really hard to put things into evolutionary context or biodiversity context.’
Les Christidis, an ornithologist at Southern Cross University in New South Wales, is more outspoken. ‘The problem with just relying on genetic information or DNA data is, what is the threshold of differentiation that qualifies for species recognition? Is it two per cent, five per cent, 20 per cent? There’s no consistency in the application of genetic distances as a species boundary. There’s a tendency to over-inflate the significance of the genetic differences in order to define a new species. Your paper is more cited if you’re describing a new species as opposed to a subspecies and there are conservation-funding implications. The primates are
a classic example of where every slightly differentiated population is now classified as a separate species.’ By and large, taxonomists have a high tolerance for this type of healthy academic debate, but it can prove frustrating for conservation professionals. For them, a clear list of species is important in order to carry out conservation work. It was partly this divide that led to an intellectual sparring match, only recently resolved, in which the question ‘what is a species’ came to the fore. It all kicked off with an article written by Christidis and Stephen Garnett, a professor of conservation and sustainable livelihoods at Charles Darwin University in Darwin, Australia. They argued that the lack of concrete definitions and the lack of oversight when it comes to identifying new species contribute to a chaotic system. ‘The assumption that species are fixed entities underpins every international agreement on biodiversity conservation, all national environmental legislation and the efforts of many individuals and organisations to safeguard plants and animals. Yet for a discipline aiming to impose order on the natural world, taxonomy... is remarkably anarchic,’ they wrote. ‘“Species” are often