Unit Four. The Evolution and Diversity of Life


15. How We Name Living Things



In 1799, the skin of a most unusual animal was sent to England by Captain John Hunter, governor of the British penal colony in New South Wales (Australia). Covered in soft fur, it was less than 2 feet long. As it had mammary glands with which to suckle its young, it was clearly a mammal, but in other ways it seemed much like a reptile. Males have internal testes, and females have a shared urinary and reproductive tract opening called a cloaca, lay eggs as reptiles do, and like reptilian eggs, the yolk of the fertilized egg does not divide.

It thus seemed a confusing mixture of mammalian and reptilian traits. Adding to this impression was its appearance: It has a tail not unlike that of a beaver, a bill not unlike that of a duck, and webbed feet! It was as if a child had mixed together body parts at random—a most unusual animal. Individuals like the one pictured here are abundant in freshwater streams of eastern Australia today. What does one call such a beast? In its original 1799 description, it was named Platypus anatinus (flatfooted, ducklike animal), which was later changed to Ornithorhynchus anatinus (ducklike animal with a bird’s snout) — informally, the duckbill platypus. How biologists assign names to the organisms they discover is the subject of this chapter. You will be surprised at how much information is crammed into the two words of a scientific name.


15.1. Invention of the Linnaean System


It is estimated that our world is populated by some 10 to 100 million different kinds of organisms. To talk about them and study them, it is necessary to give them names, just as it is necessary that people have names. Of course, no one can remember the name of every kind of organism, so biologists use a kind of multilevel grouping of individuals called classification.

Organisms were first classified more than 2,000 years ago by the Greek philosopher Aristotle, who categorized living things as either plants or animals. He classified animals as either land, water, or air dwellers, and he divided plants into three kinds based on stem differences. This simple classification system was expanded by the Greeks and Romans, who grouped animals and plants into basic units such as cats, horses, and oaks. Eventually, these units began to be called genera (singular, genus), the Latin word for “group.” Starting in the Middle Ages, these names began to be systematically written down, using Latin, the language used by scholars at that time. Thus, cats were assigned to the genus Felis, horses to Equus, and oaks to Quercus—names that the Romans had applied to these groups.

The classification system of the Middle Ages, called the polynomial system, was used virtually unchanged for hundreds of years, until it was replaced about 250 years ago by the binomial system introduced by Linnaeus.


The Polynomial System

Until the mid-1700s, biologists usually added a series of descriptive terms to the name of the genus when they wanted to refer to a particular kind of organism, which they called a species. These phrases, starting with the name of the genus, came to be known as polynomials (poly, many, and nomial, name), strings of Latin words and phrases consisting of up to 12 or more words. For example, the common wild briar rose was called Rosa sylvestris inodora seu canina by some and Rosa sylvestris alba cum rubore, folio glabro by others. This would be like the mayor of New York referring to a particular citizen as “Brooklyn resident: Democrat, male, Caucasian, middle income, Protestant, elderly, likely voter, short, bald, heavyset, wears glasses, works in the Bronx selling shoes.” As you can imagine, these polynomial names were cumbersome. Even more worrisome, the names were altered at will by later authors, so that a given organism really did not have a single name that was its alone, as was the case with the briar rose.


The Binomial System

A much simpler system of naming animals, plants, and other organisms stems from the work of the Swedish biologist Carolus Linnaeus (1707-1778). Linnaeus devoted his life to a challenge that had defeated many biologists before him— cataloging all the different kinds of organisms. Linnaeus, a botanist studying the plants of Sweden and from around the world, developed a plant classification system that grouped plants based on their reproductive structures. This system resulted in some seemingly unnatural groupings and therefore was never universally accepted. However, in the 1750s he produced several major works that, like his earlier books, employed the polynomial system. But as a kind of shorthand, Linnaeus also included in these books a two-part name for each species (others had also occasionally done this, but Linnaeus used these shorthand names consistently). These two-part names, or binomials (bi is the Latin prefix for “two”), have become our standard way of designating species. For example, he designated the willow oak (shown in figure 15.1a with its smaller, unlobed leaves) Quercus phel- los and the red oak (with the larger, deeply lobed leaves in figure 15.1b) Quercus rubra, even though he also included the polynomial name for these species. We also use binomial names for ourselves, our so-called given and family names. So, this naming system is like the mayor of New York calling the Brooklyn resident Sylvester Kingston.



Figure 15.1. How Linnaeus named two species of oaks.

(a) Willow oak, Quercus phellos. (b) Red oak, Quercus rubra. Although they are clearly oaks (members of the genus Quercus), these two species differ sharply in the shapes and sizes of their leaves and in many other features, including their overall geographical distributions.


Linnaeus took the naming of organisms a step further, grouping similar organisms into higher-level categories based on similar characteristics (discussed later). Although not intended to show evolutionary connections between different organisms, this hierarchical system acknowledged that there were broad similarities shared by groups of species that distinguished them from other groups.


Key Learning Outcome 15.1. Two-part (binomial) Latin names, first used by Linnaeus, are now universally employed by biologists to name organisms.