Abstract
This chapter presents the evolutionary mechanisms responsible for physiological adaptations. The considerable evolutionary variation occurs in physiological processes and in the molecular mechanisms used to respond to physiological insults. Significant differences in physiology and biochemistry occur within a species and among closely related taxa. The chapter focuses on the teleost fish Fundulus heteroclitus. Within Fundulus an adaptive difference in glycolytic enzyme expression affects metabolism. The molecular mechanisms responsible for one of these enzymes, LDH-B, are because of recent changes of a few nucleotides in the Ldh-B gene within species, suggesting that changes in gene expression that are biologically important can evolve readily. Acclimation responses among Fundulus are evolutionarily labile, no change in enzyme expression is conserved throughout the genus, and responses unique to one species are observed in four enzymes. These responses are primarily in equilibrium enzymes and are achieved by altering enzyme concentration in a potentially compensatory direction. Surprisingly, the molecular mechanism for these acclimation responses is quite variable: it is not the same in all populations at all temperatures. With the advent of microarray technology and the ability to develop microarrays for many different species, one can examine the quantitative differences in mRNA expression for most genes.