Great praise for categories, and seeing beyond them

Classification is inevitable. It’s a widespread human tendency and a bedrock of scientific study. From rocks to stars to the stinkbug buzzing against my window, from parts of speech to diseases to the fundamental forces of nature, if an object or phenomenon can be described, it will be grouped with others like it and distinguished from those that differ.

One of the best-known scientific classifiers was, of course, Carl Linnaeus, the father of taxonomy. In his Systema Naturae, published in the 18th century, Linnaeus attempted to develop a scheme that could reveal the divine order in creation. Though he was not the first to label organisms by species and genus, Linnaeus consistently used binomial nomenclature and grouped genera into higher taxa according to shared features. Scientists still use some of the basics of this system today.

The benefits of this classification system are enormous. The scheme guides researchers in giving newly discovered organisms names. Those names provide a shared language so scientists can communicate what they are studying and what they’ve learned, allowing knowledge to build over time. Grouping organisms also helps researchers recognize patterns. Biologists can thus study not only how a single lineage evolved, but also the fundamentals of evolution itself. They can ask how organisms are related, how environments shape organisms and other grand questions, such as what are the origins of life? Similar praise can be showered on other classification schemes. By arranging the elements according to their properties on a periodic table, for example, Dmitri Mendeleev and others were able to predict undiscovered elements.

Yet categorical perfection is challenging, and pitfalls abound — including in the most modern systems. In this issue, Tina Hesman Saey describes how species don’t always stay in the boxes that scientists assign. There’s a fair amount of interbreeding, especially within microbes and plants, and we know that human ancestors shared DNA across species lines. In an accompanying essay, Susan Milius directly tackles the troubles with defining the term “species,” one of biology’s richest categories. The term even frustrated Charles Darwin, who in On the Origin of Species wrote, “I was much struck how entirely vague and arbitrary is the distinction between species and varieties.”

Classification can also impede our understanding. In the 18th century, French astronomer Charles Messier began compiling a list of “nebulae.” His original goal was to distinguish these fuzzy blobs of light in the sky from comets that come and go. But by putting these blobs all in one box, astronomers failed to realize that the blobs aren’t all the same kind of thing. As data came in, opinion swung back and forth between nebulae as clouds of gas or groupings of stars. It wasn’t until the 20th century that Edwin Hubble confirmed that, though some nebulae are gas clouds and some are star groups within our galaxy, others are galaxies outside the Milky Way — their own “island universes.” (For the origin of that phrase, check out “An American astronomical evangelist coined the phrase ‘island universe.’”)

There’s a lesson here that spans the sciences, and perhaps the rest of life: Boxes are helpful, but they can also blind us.