The evolution of a polyphenism by genetic accommodation

An adaptive polyphenism is the ability of a particular genotype to respond to different environments by generating two or more alternative phenotypes, each of which is adapted to the particular environment. This adaptive strategy has evolved in many taxa, yet little is known about the evolutionary origins of this trait. This thesis focuses on how adaptive polyphenisms can evolve through a process called genetic accommodation. Genetic accommodation is a process by which a novel phenotype becomes established in a population through polygenic changes in the genetic background driven by selection on phenotypic variants that are produced by either mutation or environmental change.;A polyphenism was experimentally evolved in a previously monophenic black mutant Manduca sexta larva through selection on genetic variants revealed by heat-shock. A corresponding change in juvenile hormone (JH) titer was observed as determined by a JH bioassay. A cross between a monophenic and a polyphenic individuals revealed that the evolved polyphenism is likely to be controlled polygenically with at least one gene of major effect. The work demonstrates how hormones can act to mask hidden genetic variation and shows how complex traits may evolve. Implications of such changes are discussed along with a more general view of the developmental basis of the process of evolution by genetic accommodation. The theory of genetic accommodation is then discussed in the context of classical theories and studies of adaptive evolution.;The final step of genetic accommodation involves the removal of fitness cost and trade-offs associated with the evolution of the novel trait. Larval coloration and body size was found to be highly correlated and developmentally integrated by JH. Selection to uncouple the developmentally integrated traits revealed that developmental modules that are not under the control of the developmental integrator can evolve to overcome the potential constraint that arises as a consequence of developmental integration. Therefore, the final step of genetic accommodation relies on evolution of developmental modules to overcome potential trade-offs or costs. The ease with which a polyphenism could evolve in the laboratory demonstrates that polyphenisms can evolve from adjustments on pre-existing hormonal regulation. The relationship between the origins of novelties and polyphenisms is discussed.