Mormyrid fishes are one of six independent lineages of vertebrates to have evolved electric organs from skeletal muscle. The physiological output of the electric organ is an electric signal called the electric organ discharge (EOD). EODs are especially diverse among mormyrids, particularly among a rapidly diverged, geographically restricted species flock Paramormyrops. Recent studies have implicated that EODs may be a contributing factor to the process of species divergence in Paramormyrops. In this dissertation, I have examined the genetic basis of differences between skeletal muscle and electric organ in mormyrids, and the population-level processes that contribute to their diverse physiological outputs of electric organs. In Chapter 1, I provide a general overview of electric organs, with specific attention to the developmental and genetic mechanisms underlying their development. Chapter 1 concludes with a discussion of evolutionary processes responsible for signal diversification. In Chapter 2, I describe the use of suppressive subtractive hybridization to identify genes differentially expressed in the electric organ vs. skeletal muscle in the mormyrid Brienomyrus brachyistius. This work provides a basis for future comparative work with other electric fishes. In Chapter 3, I describe patterns of geographic variation in EOD signaling among a geographically widespread member of the Paramormyrops species flock, P. kingsleyae, and describe morphological correlates of this variation. In Chapter 4, I use a population genetics approach to analyze data concerning signal variation, behavior, and anatomy in Paramormyrops kingsleyae to specifically test the hypothesis that the unique patterns of geographic variation and signal divergence detected in Chapter 3 are the result of geographic barriers and distance that reduce gene flow between local populations.