The thistle Cirsium palustre exists over most of lowland Britain as a predominantly purple flowered species. However the populations of seacliffs and mountains exhibit a flower colour polymorphism, occurring as homozygous white morphs, homozygous and heterozygous intermediate morphs, and homozygous and heterozygous purple morphs. The degree of polymorphism of the seacliff populations of the Gower Peninsula is correlated with population size, the larger populations being less polymorphic. This might be interpreted as indicating chance fixation of the white alleles. Such an explanation would gain support from the very small size of the more polymorphic populations and from the drastic fluctuations in population size which have been demonstrated as having occurred in these populations over a period of four consecutive seasons. However other explanations based on selective effects are possible. On seacliffs the frequency of the white morphs is inversely related to exposure, and decreases in population size are accompanied by differential survival of the purple morphs. On mountains the distribution of the polymorphism is markedly correlated with altitude. For the mountains of southern Mid Wales, populations below 1000 ft. are strongly monomorphic purple. Above 1000 ft. the degree of polymorphism increases abruptly, with white frequencies reaching over 80%. Morph frequencies among North Wales populations bear a similar relation to altitude but the increase in polymorphism occurs at about 1200 ft. In each case the general trend is that purple frequency declines with increase in white frequency, and that the frequency of intermediates shows a unimodal distribution with a quite precise peak. For both sets of populations this peak occurs at an altitude about 250 ft. higher than that at which the increase in white frequency occurs. It is possible that the occurrence of the polymorphism on seacliffs and mountains may be related to a limitation of cross pollination consequent upon the exposure of seacliffs and the combined climatic characteristics of mountains, which include increase in exposure, mist and rain and decrease in temperature. Evidence on the levels of outbreeding in these populations was inconclusive but evidence in other species suggested that pollination might indeed be limited in these conditions. An increase in homozygosity consequent upon inbreeding would promote the frequency of the white morphs. Moreover the white morphs were subject to preferential pollination and both this and certain forms of heterogeneity in morph distribution were likely to promote the frequency of inbreeding among white morphs. However in conditions of limited pollination the degree of general outbreeding of the white morphs will be increased by preferential pollination and this may be assumed to be a fitness advantage which may be of particular importance in the maintenance of the polymorphism. In addition it is likely that the presence of white morphs within a population may result in the attraction of higher numbers of pollinators or encourage foraging for longer periods in which case the polymorphism may be said to be adaptive in the sense of Fisher (1930). Other selective effects are also apparent. The occasional presence of highly polymorphic populations in valley bases and the regular occurrence of predominantly purple populations in mountain forests may both to some extent provide evidence for an effect of temperature other than upon pollination. Some evidence suggests that both exposure and moisture may also be of individual importance. Selection was apparent even by the arrangement of morph types within a population subject to no obvious environmental heterogeneity. It is possible that the polymorphism is maintained by a physiological heterozygous advantage and that this may be responsible for the maintenance of white and intermediate morphs in low frequency in the predominantly purple populations of inland lowland regions. The maintenance of the polymorphism imposed a significant selective mortality upon the species. This was indicated by the above instance of selection within a uniform community and also in several instances in which intrapopulational selection occurred between segments of population subject to differing exposure. However the growth in cultivation of seeds set in natural populations revealed that the complexity of the genetic system was sufficient to allow widely different morph frequencies to be maintained in different populations without the necessity of high selection in each generation.

Thesis (Ph. D.)--University of Chicago, Dept. of Ecology and Evolution, June 1999. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Washington, 1995. / Vita. Includes bibliographical references (leaves [85]-97).

Cranberry (Vaccinium macrocarpon), a commercially grown evergreen dwarf shrub, is a dominant taxon in temperate bogs in North America. It spreads clonally by runners, and reproduces sexually predominantly by self-fertilization on upright stems. The objective of this project was to investigate genetic and clonal variation and phenotypic plasticity of V. macrocarpon. Specifically, I wanted to test whether there exists an inverse relationship between population genetic variation and the amount of overall phenotypic plasticity of vegetative characteristics. As background information I assessed the vegetation and edaphic factors of marginal cranberry bogs found in the mid-to-southern Appalachians. A gradient of nutrient availabilities was found among bogs that was positively associated strongly with the dominance of the more generalist Rubus hispidus and negatively with ericaeous bog shrubs such as V. macrocarpon. Eutrophication may lead to the replacement of endemic bog species with generalist plastic species. Theoretically, it would be plausible for environmental heterogeneity or stress to allow selection for more phenotypically plastic clones within a species. A single adaptively plastic clone for growth strategy could sweep a site, excluding intraspecific competitors. That is, selection could favor clones with high plasticity that could subsequently lead to a loss of genetic variation within a population. The environmental and genetic conditions favoring this would more likely exist in distributionally marginal sites because of spatial and temporal heterogeneity and island-like biogeography. Field and common garden experiments in which nutrients were manipulated were performed to test for an inverse relationship between phenotypic plasticity and genetic heterogeneity. Random amplified polymorphic DNA (RAPD) profiling was coupled with ecological measurements of plant growth on the experimental clones and other clones from the experimental populations to estimate genetic heterogeneity. Genetic heterogeneity was found to be significantly lower in marginal populations than in central populations. Phenotypic plasticity was somewhat higher in a more marginal population in the field sites, but direct statistical comparisons could not be made. The common garden study was inconclusive, possibly due to transplant shock, but a trend among natural populations was towards higher plasticity among marginal clones. Additional research on other species is needed to clarify the possible inverse relationship between phenotypic plasticity and genetic variation within populations. / Ph. D.