Evolutionary Physiology of Closely Related Taxa: Analyses of Enzyme Expression

Comparative biochemistry and physiology offer the advantage of specifically defining the functional parameters or traits that affect an organism's performance (e.g., amino acids that affect Km, enzymes that affect metabolism). By combining these functional determinations with both intraspecific and phylogenetically appropriate analyses, comparative biologists can indicate that a trait is biologically important by demonstrating that it is evolving by natural selection. An evolutionary approach may benefit from the analysis of variation within and among closely related species. The advantages of analyzing closely related species are that they allow one to identify more definitively the derived conditions and suggest why differences arose. Importantly, there is substantial variation in physiological and biochemical traits within and among closely related species. For example, among species within a single genus of teleost, Fundulus , the variation in enzyme expression is similar to the variation seen among most superorders of teleost. However, most of the variation within the genus Fundulus is most readily explained by evolutionary distance, and thus there is no compelling reason for further adaptive hypotheses. Extending this observation, the greater the phylogenetic distance between taxa in a comparative study, the more likely there will be a statistically significant difference that may only represent evolutionary time. The molecular mechanisms affecting adaptive variation in enzyme expression appear to be readily altered and may vary within a species or between acclimation conditions. Thus, studies among closely related organisms are more likely to identify the specific molecular or biochemical changes responsible for adaptive variation.