Many evolutionary processes have been identified that could lead to signal variability among populations despite stabilizing selection to maintain conspecific recognition. By examining among-population variation in mate recognition signals we may gain insights into the processes behind the evolution of such variation. We documented among-population variation in the advertisement call of Litoria verreauxii in the Australian Snowy Mountains to determine how call structure varied (1) across a broad geographic area, (2) from allopatry to sympatry with northern Litoria ewingii (an undescribed member of the same species complex), and (3) between two subspecies, Litoria verreauxii verreauxii and Litoria verreauxii alpina. Significant variation in all measured call properties was explained by latitude and/or longitude. Most noticeably, a strong east-west trend in introductory note duration and pulse number was detected that coincided with the transition from allopatry to sympatry with northern L. ewingii. The observed variation is discussed within the context of several evolutionary mechanisms including clinal variation and both reproductive and ecological character displacement. We found considerable variation in pulse rate among nearby allopatric populations (based upon current taxonomy). Pulse rate has been implicated in species recognition and reinforcement in L. verreauxii. Some of the among-population variation in pulse rate may reflect the presence of cryptic taxa. However, these results suggest that factors other than interspecific interactions with closely related species can lead to considerable among-population divergence in pulse rate. Finally, there was little difference in call structure between the two subspecies.
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Vol. 2003 • No. 4