Evolution of Life-History Characters and Phenotypic Plasticity in Mimulus guttatus

DOCTORATE DISSERTATION: Phenotypic variation results from genetic differentiation and plastic response to the environment. Patterns of both characters and character plasticities may be adaptive, selected to increase fitness under common conditions. I investigated the potential for evolution of life-history characters and phenotypic plasticity in Mimulus guttatus (Scrophulariaceae) by studying genetic variation among and within populations, natural selection, and evolutionary response to selection. In a controlled environment I found genetic differentiation among populations consistent with local adaptation to water persistence for life-history characters and their plasticities. Annual populations from seasonally wet habitats had greater reproductive effort, less vegetative allocation, and more, smaller flowers than perennial populations from persistently wet habitats, These differences were not found under all water conditions, indicating that plasticity to water availability differed among populations in parallel with habitat water persistence. Heritability estimates were used to measure genetic variation for life-history characters and their plasticities in two populations. Heritabilities for both characters and plasticities were comparable in magnitude, but varied with water availability. Heritabilities wer least in the annual population and greatest in the perennial population under moderate water conditions. For each population, heritability was greatest for plasticity to water levels uncommon in their natural habitat. When experimental plants were grown in a natural M. guttatus population, there was phenotypic selection to decrease flowering date and increase shoot weight, and stabilizing quadratic selection on life span. Selection on plasticities acted to decrease variation in flowering date and increase variation in life span. Genetic variation in characters and plasticities, measured on the same individuals, combined with selection reveals the potential for evolution by natural selection. The evolution of characters and plasticities was tested with replicate experimental populations planted into nature. Plants from populations that flowered earlier, had shorter life spans and were less productive, produced offspring of longer lived plants from high productivity areas. Evolutionary change in life-history characters and their plasticities in response to microenvironmental variation revealed that both are important adaptations to heterogeneous natural environments.