Courtship choreography is stabilized among genetically isolated populations.

Sexual selection has sculpted diverse and intricate courtship displays throughout the animal kingdom, where failure to achieve the choreographic standards of a potential partner can be highly costly for reproductive success. Yet this raises a paradox: if there is strong selection for optimal display choreography within species, how do courtship displays begin to diverge? To address this, we measure how the choreography of courtship changes among three populations of the dancing dune fly-Apotropina ornatipennis Malloch (Diptera: Chloropidae)-a species in which males and females spend their days cavorting on Australia's hot sandy beaches. Merging population genetics with detailed quantification of the courtship display, we explore which elements of the display are the first to diverge between isolated populations, whether new behaviors arise rapidly, and whether sequence rearrangements occur in the modular structure of the display. We find that these tiny flies express courtship repertoires that rival some of the most sophisticated displays in the animal kingdom. Yet despite clear genetic and geographic isolation, the complex choreography of courtship is largely stable-with the frequency of "wing sweep" emerging as the only display element showing evidence of divergence among populations. In contrast to the notion that courtship behavior should be highly evolvable and rapidly diverge among allopatric populations, our findings suggest that complex courtship displays can remain remarkably stable over short evolutionary timescales.