Date of Graduation

1994

Document Type

Thesis

Abstract

A central question in evolutionary biology has been, how is within-population genetic diversity maintained in the face of selection and drift. I examined the potential contribution of environmental seasonality to the maintenance of diversity within plant populations by asking: Is demography of a natural population season-specific? Does season-specific demography vary among biotypes within the population? Are season-specific responses influenced by the competitive environment? Can the patterns of genotype performance among seasons be responsible for preserving genetic diversity within a population? Demography of a natural population of Taraxacum officinale in Morgantown, WV, USA, examined with Lefkovitch transition matrices, was season-specific. Population growth was sensitive to perturbation of growth and fertility of small individuals from October through July. Large individuals contributed disproportionately to population growth during late summer. Detecting these dynamics is important for understanding long-term population growth and viability. Lefkovitch matrices calculated for five biotypes (leaf morphs) within the population indicated that differential biotypic demography within the year contributes to preserving genetic diversity. A common environment experiment confirmed that biotype differences were genetically-based. Replicates of five genotypes (representing each of the biotypes) planted into the field in each of four seasons confirmed the importance to diversity of differential fitness to seasonal environments. Several genotypes performed better in warm months. A second group performed better in cool seasons. Despite seasonal differences in fitness components, genotypic performances across seasons were equivalent. Genotypes (representing three of the previous biotypes) planted with Plantago major, Poa pratensis and Trifolium pratense suggest that differential performance to seasons is influenced by biotic as well as abiotic components in the environment. Differential genotype responses to seasonal environments represent the temporal variation in fitness necessary for maintenance of genetic variation. Therefore, temporal environmental fluctuations are expected to contribute to the persistence of the genotypes present in this natural population. This effect of temporal environmental heterogeneity, in combination with spatial heterogeneity, may be important to many species which exist in cyclically fluctuating environments as even subtle differences in the environment may result in genetic adaptation within populations.

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