Tom needs to collect more evidence to evaluate his other hypotheses. He knows that his second hypothesis — the idea that eschscholtzii and klauberi are reproductively incompatible — is, at the very least, not completely accurate. After all, he’s found hybrids living in the wild, so the two subspecies can definitely produce offspring together. However, almost all the hybrids he’s studied so far have been from klauberi females and eschscholtzii males. Why does the reverse pairing — an eschscholtzii female and a klauberi male — seem to be so much more rare? That question is still unanswered.
Tom is using satellite images to study his third hypothesis — the idea that the two subspecies don’t interbreed in some places because they never actually meet in those locations. Two salamanders may live in the same general area, but if their preferred habitats aren’t intermixed in that area, they may hardly ever run into one another. If this idea is accurate, we’d expect to observe more intermixing of habitat types in the areas where the subspecies interbreed compared to the areas where they don’t. Tom hopes that satellite images — which often show different habitat types with different colors of vegetation — will reveal these differences if they exist.
Tom’s studies of Ensatina‘s hybrid zones are aimed squarely at understanding the process of speciation: What differences have eschscholtzii and klauberi evolved that keep them distinct? But this has been a tough question to answer. Says Tom, “It would be much easier to just collect the genetic data from the hybrid zone, infer what might be going on, and leave it at that. But to me, the more interesting question is: What are the processes that have led to this pattern? … Getting at the processes is the hard part, but it’s also the most interesting.”