Violins are kind of like leaves. They've changed over time, driven in part by their designers' tastes. Violins fall into distinct lineages, recognizable by their shapes, just as leaves from one or another plant would be. And they show signs of a sort of natural selection: Violins look more and more like the ones first created by Antonio Stradivari.
This is according to Dan Chitwood, a biologist who normally studies leaves. Specifically, he studies how leaf shapes have evolved over time and the genetic basis of that evolution. Doing that research means quantifying and tracking shape changes over time, something just as easily applied to Chitwood's other avocation, the viola.
Chitwood compares violins to living, evolving organisms, complete with mutations and a sort of survival of the fittest.
Chitwood's first question was whether he could tell the difference between different kinds of string instruments based on their shape while taking overall size out of the equation. To do that, he drew on an auction house's database of more than 9,000 instruments in the violin family—the viola, cello, bass, and the violin itself—from prominent luthiers over a range of 400 years. Chitwood used those to construct instrument outlines, which he could then compare using a method called linear discriminant analysis.
To understand the idea, imagine constructing a shadow puppet. You could cut the puppet out of a single piece of cardboard or wood, or you could assemble it using a set of elementary shapes. In a similar way, you could describe a violin's shape as a whole, or you could break it down into more abstract shapes. A double bass looks like an especially broad-bottomed pear, while a lute looks like a squash with some triangle thrown in. While Chitwood's study uses a more sophisticated set of basic shapes, the goal is the same. By quantifying how much pear, squash, and triangle a shape has, you can quantify the similarities and differences between different instruments' shapes.
Violas and violins, it turns out, are hard to discriminate, but cellos and double basses are distinct. While all four are pear-shaped to some extent, the double bass takes that to another level, complete with a bit of stem where the instrument's neck attaches to the main body. When Chitwood turned his attention specifically to the roughly 7,000 instruments in his database, he found that violins fell into four families, each represented by an archetype designed by an actual human family—Maggini, Amati, Stainer, and, of course, Stradivari, whose violins were slightly more bass-like in shape. What's more, other violins became more like these four over time, and especially more like Strads.
In that respect, Chitwood compares violins to living, evolving organisms, complete with mutations and a sort of survival of the fittest. "Despite using molds, Antonio Stradivari nonetheless innovated new shapes, using a method both faithful to the previous outlines but with the potential to change," he wrote in the paper. Meanwhile, luthier Jean-Baptiste Vuillaume purposely copied Stradivari's designs, because those were the ones customers selected.