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The term "interaction" in evolutionary biology and ecology describes the relationships among variables in two classes of causal models. In the first, "interaction" refers to the influence of a single putatively causal variable on a variable of interest. In the second class of models, the term applies when a third variable mediates the relationship between two variables in the first class of models. The development of multi-factor causal models in evolutionary biology and ecology represents a stage in the construction of theory that usually follows from complexities discovered in single-factor analyses. In this thesis, I present three cases that illustrate how results of simple single-factor models in the population genetics and community ecology of seaweeds may be affected by incorporation of a second causal factor. In Chapter II, we consider how the effect of natural selection on genetic variability in seaweeds and other plants may be mediated by life history variation. Many seaweeds have haplodiplontic life histories in which haploid and diploid stages alternate. Our theoretical analysis and review of the electrophoretic literature show that the level of genetic polymorphism in haplodiplonts is not necessarily reduced relative to that in diploids. In Chapter III, I take an experimental approach to understanding how herbivory may mediate the effect of desiccation on the upper intertidal limit of a red alga, Iridaea cornucopiae. Iridaea appears to be grazer-limited in dry, but grazer-dependent in moist environments, suggesting that a third factor may mediate the interaction of desiccation and herbivory. Finally, in Chapter IV, we consider research strategies for studying how the outcome of competitive interactions is affected by seaweed traits. Some of the problems that arise in applying simple models of competition to plants suggest the need for theory that explicitly incorporates plant traits in two- (or more) factor models of interspecific competition. In particular, we note that unique traits of seaweeds require development of new approaches to understanding competition. Single-factor causal models represent an indispensable stage in the development of evolutionary and ecological theory. Properly conceived theoretical and empirical studies focus attention on the assumptions under which such models will hold and suggest lines of inquiry that ultimately lead to the integration of additional causal factors in conceptual models of natural processes. Identifying the circumstances under which simple models will suffice remains one of the most important challenges of evolutionary and ecological scholarship. / Graduation date: 1993