Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Stephen J. Novak, Ph.D.


Richard N. Mack, Ph.D.


James F. Smith, Ph.D.


High propagule pressure is correlated with invasion success, and has important implications for the genetic diversity and evolutionary potential of a species in its introduced range. Here, I examine an invasive annual grass Bromus tectorum and document the population genetic consequences that resulted from multiple introductions of genotypes native to different Eurasian regions into the North American Midwest. Herbarium collections showed that B. tectorum was first recorded near-contemporaneously throughout the Midwest in the late 1800s. Allozyme diversity data from 60 populations were used to assess the origin and frequency of introductions into the Midwest. Genetic variation and structure was compared to similar measures for the Eurasian native range and other North American regions to infer the introduction pathways of B. tectorum across the continent.

A minimum of four to five introduction events contributed to genetic diversity in the Midwest. A genetic bottleneck associated with introduction was detected across populations, yet more Midwestern populations were polymorphic (53%), and populations were more diverse (HT = 0.187) and less structured (GST = 0.582) than native range populations, indicating high propagule pressure was associated with the invasion of B. tectorum into the Midwest. Within the Midwest, genetic diversity and structure values were compared for individual polymorphic loci to infer propagule pressure for individual genotypes, and scenarios for their likely invasion pathways were drawn based on historical collections and the history of the region. The intermixing of previously allopatric genotypes within populations in the Midwest may have allowed the formation, via outcrossing, of a novel multilocus genotype found only in this region of North America.

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Evolution Commons