Mate-attracting signals are made up of different properties that influence mating decisions. Analysis of patterns of variation in such properties and patterns of female preference based on such variation can reveal the potential for sexual selection and speciation. I will discuss patterns of variation in four such properties at three levels -within-male, between-male, and between-population - in 35 populations of the North American treefrog (Hyla chrysoscelis). Three properties (pulse number, dominant frequency and pulse rate) show patterns of variation that would allow selection on individual males, and pulse number is under directional selection by females. However, the pattern of selection is non-linear (diminishing returns of adding pulses over and above the mean) and also constrained by energetic costs. Surprisingly, only pulse rate, which is under strong stabilizing selection within populations, shows between-population differentiation sufficient to promote discrimination by females. The pattern of geographical variation in pulse rate is not explained by reproductive character displacement, which predicts a higher pulse rate in areas where a genetically incompatible sibling species is found, but females do show such a pattern in the strength of preferences based on pulse rate. By contrast, pulse rate in the sibling species (H. versicolor), which has arisen three or more times by polyploid speciation, varies little among populations. I will show that part of the explanation is that lower pulse rates are a consequence of cell-size increases that accompany genome duplication. I also propose that two changes in the female-preference criteria of the polyploid species are examples of hidden preferences revealed by changes in male calls.
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