Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Evolution, Ecology and Population Biology


English (en)

Date of Award

January 2010

Degree Type


Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Alan Templeton


Matings between biologically related individuals often produce offspring with reduced fitness, a phenomenon known as inbreeding depression. The magnitude of inbreeding depression can play an important role in determining the overall fitness, and persistence, of populations, and is often considered one of the key factors driving the evolution of mating systems and their associated traits. Recent research has shown that the strength of inbreeding depression is often highly sensitive to environmental conditions, such as the availability of abiotic resources or variation in the strengths of ecological interactions between organisms. In plant populations, drought, plant density, herbivory, and infection by pathogens, among other factors, have all been shown to influence inbreeding depression in important fitness-related traits. However, the effects of interspecific competition on inbreeding depression are much less clear, as studies have yielded mixed results, leading some authors to conclude that interspecific competition is unimportant in models of mating system evolution. Mimulus ringens is a perennial plant native to North American wetlands and grows in competition with both an invasive plant, purple loosestrife: Lythrum salicaria) and its native congener, winged loosestrife: L. alatum). Additionally, it utilizes a mixed mating system and exhibits inbreeding depression in several stages of its lifecycle. Here, common garden experiments reveal that inbreeding depression in several important fitness-related traits varies between different competitive environments. Furthermore, a greenhouse experiment shows that the magnitude of inbreeding depression is altered by the density of M. ringens individuals, and this relationship is altered by the presence of purple loosestrife as a competitor. A mating system-explicit model of population growth in M. ringens is developed, and shows that variation in competition leads to differences in inbreeding depression in important fitness components, ultimately influencing cumulative estimates of inbreeding depression across the lifecycle. The model demonstrates that competition influences how mating systems affect population growth, the sensitivity of population growth to inbreeding depression in certain fitness components, and the range of outcrossing rates over which natural selection may strongly act on inbreeding depression. Ultimately, we conclude that interspecific competition does alter inbreeding depression and should be considered in future studies of mating system evolution.


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