When Tom became intrigued by Ensatina as a graduate student in 2004, he wasn’t sure he’d be able to find an unanswered question to work on: “I knew that people had worked on Ensatina for a long time and so I wasn’t sure that there was enough room or that there was much left to do with it — until I started talking to people and reading more papers.” He realized that there were still gaps in our knowledge and that understanding Ensatina‘s basic story would allow him to ask deeper questions about their evolution.

First, though, Tom and his colleagues wanted to test Ensatina‘s basic evolutionary story against a new line of evidence: the DNA inside the nucleus, which encodes most of an organism’s traits. One might wonder, why bother? After all, Robert Stebbins’ original hypothesis had already been supported by studies of the salamanders’ morphology, proteins, and mitochondrial DNA. But scientists do their best to evaluate their hypotheses against as many different lines of evidence as is possible and practical. These new lines of evidence don’t always overturn our ideas — but they do often help refine and add detail to existing hypotheses.

Besides, in the late 1990s a graduate student at Berkeley had collected some additional mitochondrial DNA sequences and had interpreted their history differently.⁴ Perhaps, he argued, the species got its start south of San Francisco. Another line of evidence might help resolve the issue and clarify the relationships between the subspecies.