How do scientists decide how to group animals into categories? The grouping of organisms into categories in science is known as Taxonomy (for the word nerds, the term originates from the Greek taxis, meaning ‘arrangement’, and nomia, meaning ‘method’ or ‘science’). Essentially, this is a field of biology that sets the rules regarding the names of organisms and what category of organisms they fit into. A taxonomy (as a noun) can therefore tell us whether an organism should be labelled as ‘plant’ or ‘animal’, ‘fish’ or ‘fungus’ (for example). But how do we decide what each category is and what qualifies an organism to fit in that category?
Some of the earliest Western classification systems, building on the obvious animals vs. plants distinction, classified both existing and non-existing organisms by such things as usefulness and awesomeness. For example, in the Great Chain of Being (a philosophy of life that was popular during the Renaissance) wild animals were placed separately on the chain because they were so awesome that they could not be tamed, thus necessitating a separate category from the wimpy domestic animals. And useful working animals like horses were separated from other domesticated animals that were more useless in economic terms (like pets). Granted, the Great Chain of Being also encouraged the notion of “higher” and “lower” beings (in a moral sense, which is heavily value-laden and not very scientific) and it also included nonexistent organisms like angels. But in some ways the Great Chain was onto something.
Despite the drawbacks of the Great Chain of Being, the concept of classifying animals based on usefulness is actually a perfectly logical methodology. In fact, most cultures seem to have converged on similar classification systems focusing heavily on edibility and domestication, generally also with a basic distinction between plants and animals. Other categorical additions include the ways in which the organisms are obtained for food. These kinds of classification systems are called folk taxonomies. Interestingly, within these various folk systems, a single type of organism (i.e. a species) can belong to a number of different categories (i.e. the system is poly-hierarchical). This is very unlike scientific taxonomy in which each organism belongs in one place in the hierarchy, nested within larger groupings of similar organisms.
Folk classification systems are highly contextual, and differ among cultures and individuals. For example, in her study of the folk taxonomies of the Anindiyakwa people of Groote Eyelandt, Julie Anne Waddy  noted that “English speakers would find the folk generic taxon goanna (monitor lizard) sufficient for their purposes without further differentiation among the six or eight scientific species. But to an Aborigine each of these goanna species is a potential food source whose differences in behaviour and habitat one needs to be aware of in order to obtain a meal and therefore each one is assigned to a different folk generic taxon.” In this context, there is an important reason to have different names for each type of goanna that wouldn’t really apply if you weren’t interested in eating them.
For biological science, the development of a comprehensive classification system for all organisms stemmed from a desire to catalogue the natural “order” of the universe, to understand the similarities among all living organisms and to standardise the naming of organisms. Carl Linnaeus is widely regarded as the father of taxonomy; in his major work Systema Naturae (published in 1735), he firmly established the binomial naming system (with every organism being named in a standardised way using a latin genus and species name – still the standard method today!). In addition, Linnaeus characterised the major hierarchical groups: kingdom, class, order, genus and species, to which a few more levels have been added since (domain, phylum and subspecies among others).
The work of Linnaeus provided an excellent foundation, but it was developed before there was an understanding of evolution and genetic inheritance of traits. Before Charles Darwin’s theory of evolution by natural selection (published in 1859), and the discovery of DNA and its role in inheritance, organisms were classified based solely on similarities in visible (i.e. physical or behavioural) characteristics. This meant that the groupings of organisms were often arbitrary,especially considering that the available technology didn’t allow easy observation of subtle differences in form and function. At the time, it was assumed that similarities in the observable physical characteristics of organisms reflected a natural hierarchy of relatedness, with more similar looking organisms being more closely related. Because changes in DNA sequences affect all morphological characteristics in some way, studying the differences in physical features between organisms often achieves similar results to studying the differences in DNA.
However, some of the developments in taxonomy following the discovery of genetics and evolution have been surprising. For example, elephant shrews (pictured below) were originally thought to be most closely related to shrews (a group of mole-like small mammals), despite the recognition of their elephant-like snout (hence the common name). At face value, they do look much more like a shrew than like an elephant, but molecular evidence now suggests that elephant shrews are more closely related to elephants after all!
Nowadays, with our extensive knowledge of the way in which genes interact with environments to produce the shapes and forms of all organisms, the scientific field of taxonomy largely describes the evolutionary and genetic relationships among organisms. Essentially, most organisms are now grouped based on shared ancestry, as determined by DNA sequencing, and these ancestral relationships are displayed in the form of a branching tree. The inevitable culmination of taxonomies based on evolutionary and genetic relationships was a major step in our understanding of the living world. Grouping organisms by their evolutionary relatedness works very well if you’re trying to build a mono-hierarchical taxonomy in which each organism exists in one standard place relative to all other organisms. This system is also great for standardised naming of species.
A mono-hierarchical classification based on evolutionary relatedness works because differences in DNA and in physical features accumulate between different populations of organisms when they no longer interbreed, and the longer the populations are separated, the more differences are likely to accumulate (this is how we get new species, but that’s a story for another day). For example, imagine that two populations of a single species are suddenly separated by a body of water that they are unable to cross (as has happened many times in natural history due to changes in sea level). The two populations can no longer interbreed, and so new DNA mutations that emerge in one population will usually not be present in the other. Over time, the number of DNA changes that differ between the two populations will increase purely by chance, and if those changes affect physical or behavioural characteristics then the two populations will begin to look and act different.
However, other factors can influence the rate and degree of divergence between two populations, through the mechanism of natural selection. For example, if a major predator was on one side of the new body of water, but not on the other, one population will accumulate changes in DNA that affect the organisms’ ability to escape that predator. Perhaps the population over time becomes smaller in size and more cryptic in behaviour, because those individuals that were smaller and tended to hide were more likely to survive and reproduce, and these two traits were inherited by their offspring. Differences in the environment thus influence the degree of similarity among even closely related species that haven’t been separated for a long time.
These two concepts – the order of new species arising over time and the degree of divergence between them – are both necessary for an accurate scientific taxonomy and are now much easier to divulge due to the advancements in DNA technology. But the result is that now taxonomies only tell us about some similarities among organisms – those that are inherited from a common ancestor.
Similar characteristics arising from shared ancestry between two species are termed homologous. But some similarities among organisms arise from similar functions, or similar environments, rather than shared ancestry. These characteristics are termed analogous. This takes us back to the idea of grouping organisms for different functional purposes (i.e. can I eat this or not? Will this animal attack me or will it leave me alone? Is this a fluffy animal or a non-fluffy animal?). In addition to the more practical uses (like not getting attacked, or calculating the probability of a fluffy cuddle), thinking about groups of organisms that share behavioural or morphological characteristics (despite being distant evolutionary relatives) can be very useful for understanding why animals do the things they do.
Biologists essentially do this when studying the evolutionary origins of specific behavioural and morphological characteristics. It is particularly interesting when a similar characteristic appears in two very distantly related species, because it suggests that there is some similar environmental factor shared by those species that favours such a characteristic. This can help to build knowledge about the complex relationships between environments and the species that inhabit them. For instance, many behaviours that were once thought to be unique to humans and some of our closest relatives exist in similar forms in other, often distantly related organisms. Exploring the circumstances in which these behaviours arise in each species can tell us something about why that behaviour is useful, in humans or otherwise.
I will leave you with the example of “babbling”. Babbling is a stage in human language development in which infants imitate sounds that they have heard, and string them together into nonsensical phrases. Babbling is distinct from true language because it doesn’t convey any meaning. During development, the social feedback obtained from parents and other individuals reinforces the use of certain sounds and leads to the acquisition of meaningful language (a process known as vocal learning). Although language is still considered uniquely human, “babbling” has been observed in several other species in which individuals need to develop complex vocal repertoires. These species include many songbirds, pygmy marmosets and greater sac-winged bats. Perhaps other species will be added to the babbling bingo as we discover more about communication in other animals?
Obviously, the reason why we share this characteristic with these species is not because we all inherited it from a common ancestor. This tells us that there’s something very useful about this behaviour for animals with complex and social vocal systems, and that there is much that we can learn from our fellow babbling rellies about the purpose of this particular behaviour. There are so many more examples of this ‘convergent’ evolution, many of which are yet to be discovered.
- Waddy JA. Classification of plants and animals from a Groote Eylandt Aboriginal point of view. The Australian National University; 1988