Evolutionary Physiology of Closely Related Taxa: Analyses of Enzyme Expression

Comparative biochemistry and physiology offer the advantageof specifically defining the functional parameters or traitsthat affect an organism's performance (e.g., amino acids thataffect K_m, enzymes that affect metabolism). By combining thesefunctional determinations with both intraspecific and phylogeneticallyappropriate analyses, comparative biologists can indicate thata trait is biologically important by demonstrating that it isevolving by natural selection. An evolutionary approach maybenefit from the analysis of variation within and among closelyrelated species. The advantages of analyzing closely relatedspecies are that they allow one to identify more definitivelythe derived conditions and suggest why differences arose. Importantly,there is substantial variation in physiological and biochemicaltraits within and among closely related species. For example,among species within a single genus of teleost, Fundulus, thevariation in enzyme expression is similar to the variation seenamong most superorders of teleost. However, most of the variationwithin the genus Fundulus is most readily explained by evolutionarydistance, and thus there is no compelling reason for furtheradaptive hypotheses. Extending this observation, the greaterthe phylogenetic distance between taxa in a comparative study,the more likely there will be a statistically significant differencethat may only represent evolutionary time. The molecular mechanismsaffecting adaptive variation in enzyme expression appear tobe readily altered and may vary within a species or betweenacclimation conditions. Thus, studies among closely relatedorganisms are more likely to identify the specific molecularor biochemical changes responsible for adaptive variation.