But a third experimental step adds additional nuance to mustard's BCMA variation. The researchers returned to the lab for an experiment with one of B. stricta's closest cousins, Arabidopsis, a small flowering plant. Arabidopsis is biology's best-understood bloomer—particularly on a genetic level. The scientists engineered Arabidopsis to express the BCMA genes, producing either the Colorado or Montana spice variants. Once again, they found the chemical composition of the plant's spice came from genetic variation. When the researchers exposed their spicy Arabidopsis plants to pests, they identified a possible trade-off of BCMA variation. Although the spicy compounds deter certain insects and pathogens, they can increase susceptibility to others. With further analysis, the researchers hope to better understand how this trade-off affected the regional evolution of B. stricta's flavor.

Although many questions remain, Mitchell-Olds and colleagues have presented a first step in understanding how natural selection has shaped a mustard species's variation over evolutionary time. "This paper helps set the standard for studies of adaptation in the wild," says Harvard University zoologist Hopi Hoekstra. "This research demonstrates beautifully how field experiments and molecular techniques can be integrated to tell a more complete story of adaptation than either one alone."

Jack Schultz, who studies the chemical ecology of Arabidopsis at the University of Missouri–Columbia, also praises the methods, observing the need for more studies integrating genomics with field biology. "The approach may be as significant as the result." Combining molecular and fieldwork is a relatively recent effort, but Schultz and others believe it is crucial to furthering the study of ecology.

Mitchell-Olds, for his part, remains amazed at how neatly their findings have demonstrated evolution in action. He says, "We found that environment matters, rapid evolution happens, we can localize changes down to a particular amino acid… The fact that it works, our ideas from evolutionary ecology could be verified—that's delightful."

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