Sponsor
Portland State University. Department of Biology
First Advisor
Mitch Cruzan
Term of Graduation
Spring 2026
Date of Publication
5-20-2026
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Biology
Department
Biology
Language
English
Subjects
allele surfing, associative overdominance, Fisher-KPP, geographical cline, Ranunculus
Physical Description
1 online resource (v, 60 pages)
Abstract
Introgressive hybridization has long been hypothesized to be an evolutionary stimulus for lineages to obtain adaptive genomic elements, but the lack of ongoing adaptive introgression events observed in nature limits our understanding of this process. Adaptive introgression is also difficult to characterize as most hybrid zones are relatively old and adaptive traits have stabilized. As a result, most hybrid zone theories emphasize selection against hybrids in regions where selection and dispersal are already in equilibrium. An exception is the Fisher-KPP traveling wave model, which describes the velocity of introgression of adaptive alleles in response to varying strengths of selection. In this study, I apply Fisher's model to understand patterns of ongoing introgression in a buttercup hybrid zone between a wider-range species, Ranunculus occidentalis, and a range-limited species, Ranunculus austro-oreganus. Using reduced-representation genomic markers and geographical cline analysis, I confirmed that this early-stage hybrid zone was a rare occurrence of active adaptive introgression. Common garden fitness measurements confirmed that many of the putatively adaptively introgressing loci were associated with intrinsic survival benefits. Fitness-associated adaptively introgressing loci with additive dominance demonstrated a positive association between selection and beneficial allele movement (cline displacement), as predicted by Fisher. However, contrary to Fisher's other prediction of stronger selection leading to narrower clines, I observed a lack of association between selection and cline width, suggesting a pattern of allele surfing in the introgressed endemic populations. This study is the first to verify the positive fitness effects of genomic regions undergoing active adaptive introgression identified through cline analysis and is among the few real-world demonstrations of Fisher-KPP in the genetics of natural populations.
Rights
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Persistent Identifier
https://archives.pdx.edu/ds/psu/44830
Recommended Citation
Xie, Ziqi, "Not an Occident: Positive Selection Predicts Patterns of Adaptive Introgression in an Early-Stage Buttercup Hybrid Zone" (2026). Dissertations and Theses. Paper 7092.
Comments
Supported in parts by the Bill Dahl Graduate Student Research Awards from the Botanical Society of America (BSA), the Field Research Grants from the Native Plant Society of Oregon (NPSO), the Forbes-Lea Research Award from Portland State University, and NSF grant No.2512368.