Though Tom continues hunting for telltale stretches of DNA that provide clues to Ensatina‘s evolutionary history, he is most intrigued by a question about Ensatina‘s evolutionary future: Why doesn’t Ensatina‘s ring join up fully? In the 1960s, one of Robert Stebbins’ graduate students, Charles W. Brown, discovered a few locations in Southern California where the muted western form (eschscholtzii) and the blotchy eastern form (klauberi) live together and actually do interbreed, producing blurrily blotched hybrids.⁵ It was this observation that piqued Tom’s interest. Why do the two forms interbreed in some places and not others, and — since they do sometimes interbreed — what’s keeping the two forms distinct? Why don’t these two subspecies blend into one another, as the forms around the rest of the ring do?

Tom considered many possible hypotheses for why eschscholtzii and klauberi don’t interbreed more than they do:

1. Perhaps they rarely recognize each other as potential mates. Many animals use particular clues to help them determine who would make an appropriate mate. Those clues may come in the form of a smell (e.g., a pheromone), a physical trait (e.g., a color pattern), or a behavior (e.g., a particular mating call or dance). Maybe eschscholtzii and klauberi have evolved such that they are attracted to different cues and so now avoid each other in the salamander singles scene.
2. Perhaps they are reproductively incompatible. The two subspecies might have no qualms about mating with one another but rarely produce healthy offspring because of basic biological differences that have evolved as the two lineages moved south.
3. Perhaps they rarely mate because they rarely meet. For example, the two might prefer different habitats or have such different lifestyles that they rarely even run into one another — let alone get together and mate.

And of course, some combination of these factors might come into play — but to figure out which, Tom would need to collect some evidence…