Ever since Darwin, biologists have been interested in the mechanisms of speciation. For many years zoolgists have suggested that speciation occurs only after populations diverge in allopatry (i.e. when they are separated from each other geographically), but recent studies on African cichlid fishes has demonstrated the possibility of sympatric speciation -- divergence of populations without geographic separation. The discovery of a case of stable signal polymorphism within a species of electric fish is therefore of great interest to evolutionary biologists. It suggests one possible way these fish might distinguish among potential mates on the basis of a species recognition signal. These fish may be at an early stage of speciation, and that the signal divergence we see may be an essential first step in sympatric speciation. Signal polymorphism in electric fish is especially interesting because of the novelty of the electrosensory modality of communication.
The original discovery is described in:
Arnegard, M. E. Bogdanowicz, S. M. and Hopkins, C. D. (2005). Multiple cases of striking genetic similarity between sympatric alternate electric signal forms of mormyrid fish. Evolution 59, 324-343.
The most recent analysis of behavioral and sensory factors in signal recognition is described in:
Arnegard, M. E. Jackson, B. S. and Hopkins, C. D. (2006). Time-domain signal divergence and discrimination without receptor modification in sympatric morphs of electric fishes. Journal of Experimental Biology 208, 2182-2198.
Since the discovery, several newspapers and radio broadcasts have explained the discovery in popular terms.
The discovery results after more than 20 years of field work on Mormyrid electric fishes of Africa exploring species, populations, signals, population genetics, behavior, and sensory biology. We recently demonstratde for a species of mormyrid electric fish from Gabon, tentatively named Brienomyrus magnostipes, that Electric Organ Discharges (EODs) fall into discrete, non-overlapping signal types (usually two) which are unrelated to size or sex of the individual. Individuals from different signal types have different habitat preferences, but they are overlapping. All of the individuals belong to the same species and the DNA suggests they are interbreeding. Although their signals diverge within populations, the sensory receptors, which in other species are finely tuned to the EOD signal characteristics, are identical in the two signal types. Behaviorally, one of the two morphs prefer`s its own signal type compared to the heteromorphic signal in a choice experiment between the two morphs. Examples of signal polymorphisms are of great interest to evolutionary biologists because they might help explain the mechanism of sympatric speciation, which is know to occur in other groups of fishes but is poorly understood in terms of evolutionary mechanism. This case is especially interesting because it is the first case for the electrosensory signaling modality.
The impact of this work is the world-wide community of biologists who study speciation, animal communication, and evolution. It is more broadly interesting to non-scientists who follow the evolution debate in the popular press.