In today's Opinionator, Olivia Judson explains what "sexual tension" means for evolutionary biologists:
Normally, it’s not possible to free one sex from the constraints of the other, because the two are condemned to evolve together. But again, fruit flies are an exception. In the laboratory, it’s possible to let one sex to evolve while holding the other still.
In other words, it is not what was happening between various male and female characters on "Firefly." Yet, fascinating nevertheless.