In the genus Paramoryrops (top), electric organ discharges have rapidly evolved, resulting in dramatic differences in signal waveform among closely-related species. In the genus Petrocephalus (bottom), electric organ discharges have evolved much more slowly, resulting in similar waveforms among species. The study by Carlson et al. reveals that this difference in signal diversification rates resulted from evolutionary change in the electro-sensory and electro-motor systems that these fish use to communicate with each other. The duration of each pulse ranges from about 500 microseconds in various Petrocephalus species to nearly 10 milliseconds in Paramormyrops gabonensis.