Regular development of morphology is challenged by any environmental influence that draines energy from developing individuals. For half a century biologists have recognized that developmental stability, measured as the capability to regulate development of normal morphological structures, is influenced by genetic variation. This review considers the influence of enzyme polymorphism upon developmental stability. Empirical studies in a wide variety of animals have reported morphological variability and bilateral asymmetry to decrase with the heterozygosity of enzyme polymorphisms. A controversy focuses on the question of whether enzyme polymorphisms directly influence energy budgets and metabolism, or whether the enzymes are neutral markers that either reveal variation in levels of inbreeding or are in linkage disequilibrium with genes directly influencing development. Another controversy focuses on whether the relationships between enzyme heterozygosity and development stability, most commonly reported in poikilotherms, will also be found in homeotherms. These controversies are addressed by considering recent empirical studies of enzyme polymorphism and developmental stability. Kinetic and physiological studies have now established that enzyme polymorphisms can have a major impact upon flux through metabolic pathways and physiological variation. Enzyme heterozygosity is associated with resistance to parasites, which may decrease developmental stability. Enzyme heterozygosity is also related to secondary sexual characters, such as the size and symmetry of horns in white-tailed deer and the size of horns in bighorn sheep. Because symmetry is so important in sexual selection, the development of secondary sexual characters may yield important insights to the relationship between heterozygosity and developmental stability. Many empirical observations are consistent with hypothesis that enzymes have a direct influence on developmental stability. It is not likely that the relationship between heterozygosity and developmental stability will differ substantially between poikilotherms and homeotherms.
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