What is taxonomy?

What should you call that bird? You couldn’t go wrong with “dinosaur,” taxonomists say.

If you saw a feathered, two-footed critter on the lawn, what would you tell people you saw? A robin? A blackbird? How about a dinosaur?

From a taxonomist’s perspective, you couldn’t go wrong with dinosaur. According to taxonomy, the discipline that assigns official scientific names to all known organisms, all birds are dinosaurs. “Robin” and “blackbird” are common names that may mean different things in different places, while the clade “Dinosauria” is a clear scientific designation — and it includes birds, which descended from the ancient giants.

Fundamentally, taxonomy is the science of naming, defining and classifying “biologically, evolutionarily distinct groups of organisms,” said David Baum, a University of Wisconsin-Madison botanist who studies evolution and systematics. Stanford University biologist Paul Ehrlich puts it more simply in an essay on the Birds of Stanford website: “Taxonomy … is the science of classifying organisms.”

The discipline of taxonomy analyzes how creatures should be grouped into different taxa (e.g., these particular birds make up a species distinct from that one); determines what to call those taxa (this bird species is Spinus tristis, the American goldfinch, and that one is Eudyptes robustus, a crested penguin); and lays out how smaller groups nest together into larger ones, such as how different species are grouped under one genus.

This nesting goes from species to genus, then on up through family, order, class, phylum, kingdom and domain. Hence, house cats, the species Felis catus, reside in the genus Felis, nesting within the family Felidae (along with other cats, such as tigers and bobcats), which in turn sits in the order Carnivora (with other carnivores, such as bears and walruses). This order nests inside the class Mammalia, which also includes zebras, whales and humans. Mammals are part of the phylum Chordata, which encompasses all vertebrates and more exotic creatures such as the sea squirt. This phylum lives in the kingdom Animalia, which is part of the domain Eukaryota, which encompasses everything with a nucleus in its cells.

Taxonomy also decides on that two-part, binomial name of genus-plus-species that scientists use to formally designate a specific organism (Homo sapiens for us, Clostridium difficile for one of our unwelcome bacterial guests).

Those definitive names make taxonomy crucial to scientists, Baum told Live Science. “We need to have clear communication. So if I’m talking about a particular evolutionary group and someone else is [too], we know we’re talking about the same thing,” he said. “That’s the fundamental reason we absolutely need taxonomy.”

Taxonomy echoes evolution
Inherent in that usefulness is the way taxonomy groups organisms according to their relationships. In modern taxonomy, that means describing evolutionary links. A taxonomic group must always refer to a set of organisms that descended from the same ancestor, at some point in evolutionary history. Species within the same genus all share a common ancestor. The same goes for each genus within one family and so on.

Taxonomy is so intertwined with evolutionary theory, in fact, that it can be difficult to delineate when a researcher’s “doing taxonomy” and when they’re “doing evolutionary biology,” Baum said.

Classically, a taxonomist engages in taxonomy by examining the various features of an organism or group of organisms, comparing them against known examples, and then, if warranted, reassigning names or assigning new ones. A taxonomist might take a set of specimens and separate potentially different species, as the UN Environment Programme’s Secretariat of the Convention on Biological Diversity describes.

The investigator would then check whether these groups already had names, sometimes by reading centuries-old specimen descriptions, or comparing against samples from museums and herbaria. They’d look at external and internal traits and maybe even analyze DNA. Should those comparisons show no matches, the taxonomist would write up a description and assign a new species name in accordance with the complicated rules of taxonomic nomenclature. Then, the finding would be published.

That work can involve a bit of evolutionary discovery, beyond just naming. In practice, taxonomists are doing evolutionary biology, Baum said. “They’re reconstructing evolutionary history. And so all the time they’re discovering new evolutionary relationships among organisms.”

The field’s interdependence with evolutionary theory also means that taxonomy in turn must respond to evolutionary discoveries. So, groupings and names can change, sometimes dramatically.

Reptiles, for example, originally encompassed lizards, snakes, turtles and crocodiles. Birds were considered distinct. Over time, however, scientists found that crocodiles were more closely related to birds than either of them were to other reptiles. (This was found first via morphological studies but later well-confirmed via molecular analysis, Baum said.) This left taxonomists in a quandary about what the grouping “reptile” should refer to, as one of its core members was now seen to be more closely related to an outsider, Baum said.



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