Testing hypotheses of neural evolution in gymnotiform electric fishes using phylogenetic character data

In this paper, we propose a method to test alternative hypotheses of phenot ypic evolution. The method compares patterns observed in phylogenetic chara cter data with patterns expected by explicit models of evolutionary process . Observed patterns of character-state diversity are assessed from four pro perties of character-state change derived from a phylogenetic analysis: the sequence and correlation of transformations on a cladogram and the spatial and functional localization of these transformations to parts of an organi sm. Patterns expressed in terms of the localization of transformations are compared with the expectations of null models that the number of transforma tions is proportional to measures of size or complexity. Deviations from th e values expected by the null models are then compared with qualitative exp ectations of the models. The method is applied to characters in the nervous system of gymnotiform el ectric fishes. Patterns in the diversity of 63 reconstructed character-stat e changes are compared with the expectations of 10 published models of neur al evolution. A total of 63 expectations are reviewed, of which 33 (52%) ar e found to be consistent with the gymnotiform neural data. In general, the models reviewed are not successful at making global predictions, in part be cause they have been cast in excessively general terms. The data support th e conclusion that evolution in the nervous system of gymnotiforms has invol ved a mosaic of processes, each operating differentially on functional and developmental systems and at different spatial and temporal scales. The res ults also indicate that more refined models are required, each making more explicit predictions.