Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Arts and Sciences



First Advisor

Dr. Philip H. Crowley

Second Advisor

Dr. D. Nicholas McLetchie


Despite the two-fold advantage to asexual reproduction and its prevalence in a variety of organisms, sexual reproduction is prevalent across all taxa. The maintenance of two sexes is required to ensure genetic diversity and to prevent “evolutionary dead ends,” especially in clonal organisms. Many mechanisms have been proposed for the maintenance of two sexes, ranging from environmental variation and stochasticity, parasites and predators, and mutation rates. Spatial configuration, the size and location of populations with respect to other populations, can allow two competitors to coexist when one would normally be lost. This is especially important when the two competitors are the two sexes. In the clonal organism Marchantia inflexa, I determined that spatial configuration of populations can directly influence the maintenance of both sexes in a population and in an aggregate of populations (a metapopulation) using a combination of theoretical models and field studies. Based on field studies, population size has a significant influence on whether a subpopulation will contain both sexes, with populations smaller than 1m2 being more likely to contain only one sex while populations greater than 1m2 are more likely to contain both sexes. Based on mathematical models, the spatial arrangement of subpopulations within a metapopulation can greatly influence whether a metapopulation maintains both sexes as well as whether the metapopulation persists once one sex has been lost. Field data suggest that distance to nearest neighbor, a measurement of spatial arrangement, influences the maintenance of the sexes within subpopulations, but could affect maintenance differently depending on the metapopulation identity. In some metapopulations, both sexes are maintained when the nearest neighbor is close, while in other streams, one sex is lost when the nearest neighbor is close. When mathematical models are used to explicitly simulate natural metapopulations, the mathematical model predicts the observed sex ratios in one metapopulation, predicts the observed bias in another metapopulation, and fails to predicted observed values in two other metapopulations. Understanding spatial configuration helps us understand the maintenance and loss of sex, but other factors, such as environmental differences, may be required to accurately predict which sex will be lost.

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