Development and evolution of adaptive polyphenisms

SUMMARY Phenotypic plasticity is the primitive characterstate for most if not all traits. Insofar as developmental andphysiological processes obey the laws of chemistry andphysics, they will be sensitive to such environmental vari-ables as temperature, nutrient supply, ionic environment, andthe availability of various macro- and micronutrients. De-pending on the effect this phenotypic plasticity has on fitness,evolution may proceed to select either for mechanisms thatbuffer or canalize the phenotype against relevant environ- mental variation or for a modified plastic response in whichsome ranges of the phenotypic variation are adaptive to partic-ular environments. Phenotypic plasticity can be continuous, in which case it is called a reaction norm, or discontinuous,in which case it is called a polyphenism. Although the mor-phological discontinuity of some polyphenisms is producedby discrete developmental switches, most polyphenisms aredue to discontinuities in the environment that induce only por-tions of what is in reality a continuous reaction norm. In insectpolyphenisms, the environmental variable that induces the al-ternative phenotype is a token stimulus that serves as a pre-dictor of, but is not itself, the environment to which thepolyphenism is an adaptation. In all cases studied so far,the environmental stimulus alters the endocrine mechanismof metamorphosis by altering either the pattern of hormonesecretion or the pattern of hormone sensitivity in different tis-sues. Such changes in the patterns of endocrine interactionsresult in the execution of alternative developmental path-ways. The spatial and temporal compartmentalization ofendocrine interactions has produced a developmental mech-anism that enables substantial localized changes in morphol-ogy that remain well integrated into the structure and functionof the organism.INTRODUCTIONIn most organisms a genotype can produce many differentphenotypes. The exact phenotype that is expressed dependson the environment in which the organism develops. Pheno-typic plasticity can be gradual or discrete. Phenotypes thatchange with small gradual changes in an environmental vari-able are called reaction norms. In addition to these continu-ously variable phenotypes, some organisms can develop twoor more discrete alternative phenotypes, without intermedi-ate forms. This phenomenon is called polyphenism and cancome about in two ways: either when different members of aspecies experience discretely different environments (as inthe case of a bivoltine insect with discrete generations in dif-ferent seasons) or when a continuously variable environmentinduces a discrete threshold-like switch from one develop-mental pathway to another.The mechanisms that mediate these two types of pheno-typic plasticity are beginning to be understood. As we willsee below, the development of alternative phenotypes in re-action norms and polyphenisms can be caused by speciallyevolved mechanisms that are regulated by variation in thepatterns of hormone secretion. Reaction norms can also re-sult from the fact that the rates and timing of developmentalprocesses are affected by such environmental variables astemperature, nutrition, photoperiod, and so on. These envi-ronmental variables affect the underlying chemical and met-abolic processes of development directly, without the inter-vention of a specially evolved mechanism.