ST. LOUIS — By day, Dan Chitwood of the Donald Danforth Plant Science Center in St. Louis is a plant morphologist who studies how the form and structure of tomatoes evolves differently as they adapt to new environments. By night, when he needs to think through a problem or take a break, he plays the viola.

Chitwood has now crossed his passions for plants and stringed instruments by publishing a study that documents the evolution of violin shapes using the same methods that he uses for charting the evolving form of leaves. The study published Oct. 8 in PLoS One used these methods to examine the shape of 9,000 instruments built over a period of 400 years.

The morphological analysis Chitwood used enabled him to classify violins according to region and violinmakers’ lineages. He identified four major groups of stringed instrument makers, or luthiers, who each crafted their own signature shapes that were passed down with small changes generation by generation—what Chitwood notes was “descent with modification,” in deference to Darwin’s theory of evolution.

Chitwood gathered images of thousands of violin specimens, culled from the Cozio.com archive of stringed instruments. The archive includes detailed photographs of the front and back sides of valuable violins sold at auction, and Chitwood used open-source statistical computing software to analyze the shapes of their bodies and their f-holes (sound holes). Using linear discriminant analysis, a statistical technique that separates different classes of objects he developed scatterplots that separate individual instruments by luthier. Based on these shape differences, he clustered violins into four main groups of luthiers, whose designs still predominate in most modern instruments. The cluster map resembles an evolutionary tree.

As he studied the clusters, he noticed violins from families of luthiers possess similar qualities — like plants of a common genus, he says. He says many violin-making families included several generations, and a younger apprentice might have made a small change to distinguish his instruments from his forebears. “Different families will go in different shape trajectories than another family. It is like speciation,” Chitwood says. “It’s modification by descent. You mostly recapitulate the design, but you put a little change — a little mark of yourself — and it changes, like a mutation.”

Jim Woodhouse, a structural dynamicist at Cambridge University who provided Chitwood with information about violin acoustics, says the study confirms what luthiers know intuitively. “[This study] reveals some fairly clear trends, and also clear clusters of related shapes,” Luthier says. “This puts some scientific substance behind the anecdotal evidence of makers and expert dealers.”

History also plays a role in shaping violins, according to Chitwood’s analysis: Violins morph in a linear fashion with time, with many of the changes attributable to the rise of Antonio Stradivari. The violin master started producing instruments in the mid-17th century and remained the most prolific luthier of the Renaissance. He introduced a key change around the turn of the 18th century, developing a larger outline shape than other violins of the time. “There had been no other violin until that time that had that particular shape,” Chitwood says. “This shape really foretells the future … because it’s well documented that everyone copied Stradivarius.”

Does the obsession with shape really matter? Modern violinists can’t distinguish the sound of Stradivariuses from brand-new instruments, says Claudia Fritz of the University of Paris. In studies published in 2012 and 2014, she asked accomplished violinists to blindly play “old Italian” violins and newer ones and compare their sound. The players couldn’t tell the instruments apart when they performed in a hotel room and, later, in a concert hall. Slight differences in form—the outline or the shape of the f-holes—have little effect on tone quality compared to the arched shape of the instrument, its varying thickness and its wood.

Chitwood wonders whether some luthiers are just copy cats. A Stradivarius isn’t necessarily the best; it became perceived as the fittest, so it proliferated, he says. “There are a lot of other masters — the Guarneri family, Nicolo Amati — why don’t we copy them as much?

“It was just perceived that Stradivarius instruments were superior,” he continues. “If all the attributes of the violins were made to mimic Stradivarius, maybe one reason we can’t tell the difference is because everybody copied him.”

That the study sets aside differences in sound makes it unique. “This is respectable research, halfway between science and history in terms of academic disciplines,” Woodhouse says, “and revealing things about the story of the violin that were not obvious before reading it.”