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Description: The idea of what constitutes an animal species has been a topic of longstanding interest in the biological sciences. Therefore, the analysis of eight molecular datasets involving human and teleost examples along with morphological samples from several groups of Neotropical electric fish (Order: Gymnotiformes) were used in this thesis to test the dynamics of both intraspecific variation and interspecific diversity. Three questions concerning both intraspecific and interspecific subdivision were addressed in this thesis. Meaningful intraspecific divisions were assessed by testing whether or not modern human variation can be subdivided into geographically distinct groups as was shown among two fish species. This was further tested by observing trends in a worldwide distribution of three craniometric traits within Homo sapiens. In terms of investigating molecular interspecific diversity among humans, two experimental exercises were performed. A cladistic exchange experiment tested for the extent of discontinuity and interbreeding between H. sapiens and neanderthal populations. As part of the same question, another experimental exercise tested the amount of molecular variance resulting from simulations which treated neanderthals as being either a local population of modern humans or as a distinct subspecies. Finally, comparisons of hominid populations over time with fish species helped to define what constitutes taxonomically relevant differences between morphological populations as expressed among both trait size ranges and through growth patterns that begin during ontogeny. Compared to the subdivision found within selected teleost species, H. sapiens molecular data exhibited little variation and discontinuity between geographical regions. This conclusion was based on the results of phylogenetic analyses and molecular variance measures. Results of the two experimental exercises concluded that neanderthals exhibit taxonomic distance from modern H. sapiens. However, this distance was not so great as to exclude the possibility of interbreeding between the two subspecific groups. Finally, a series of characters were analyzed among species of Neotropical electric fish. These analyses were compared with hominid examples to determine what constituted taxonomically relevant differences between populations as expressed among specific morphometric traits that develop during the juvenile phase. Such examples were ultimately useful for purposes of objectively defining subdivisions among populations of H. sapiens.