Author ORCID Identifier

Defense Date


Document Type


Degree Name

Doctor of Philosophy


Integrative Life Sciences

First Advisor

Andrew J Eckert

Second Advisor

Amy V Whipple

Third Advisor

Brian C Verrelli

Fourth Advisor

Maria C Rivera

Fifth Advisor

Rodney J Dyer


Species range margins are often characterised by high degrees of habitat fragmentation resulting in low genetic diversity and higher gene flow from populations at the core of the species range. Interspecific gene flow from a closely related species with abutting range margins can increase standing genetic diversity and generate novel allelic combinations thereby alleviating limits to adaptive evolution in range margin populations. Hybridization driven interspecific gene flow has played a key role in the demographic history of several conifer due to their life history characteristics such as weak crossability barriers and long generation times. Nevertheless, demonstrating whether introgression is adaptive and whether it helps overcome perils associated with high degrees of landscape fragmentation remains challenging in conifers due to limited among species differentiation and the lack of well developed genomic resources.

My dissertation addresses this challenge by first investigating the divergence history and the maintenance of species boundaries between two North American species of white pines: Pinus strobiformis and P. flexilis. By combining demographic modeling with ecological niche modeling and genomic cline analyses, I illustrate a divergence history of ecological speciation with gene flow and the absence of strong genomic incompatabilities. By combining genotyping-by-sequencing datasets along with a transcriptomic dataset through a series of novel as well as established multifaceted approaches, I unravel the genetic architecture of adaptive evolution in fragmented range margin populations encompassing the P. strobiformis-P. flexilis hybrid zones.

Here, both introgressed and background genetic variants are shown to facilitate adaptive evolution along freeze and water-availability related environmental gradients, respectively. I also highlight the adaptive potential of novel allelic combinations formed by the interaction between introgressed and background genetic variants, that is unique to hybrid zone populations and will likely be crucial in responding to novel selective regimes imposed by climate change. Finally, by assaying transcriptional changes between hybrid zone population through a common garden design, I reveal strong signatures of adaptive trait differentiation and of genotype-by-environment effects that is driven by variation in hybrid ancestry among populations.

This dissertation adds to the growing body of literature demonstrating the importance of introgression in assisting species response to changing climatic conditions via range shifts and through adaptive evolution. Contrary to the notion that extant conifers will be susceptible to rapid environmental change owing to their long generation times, I posit that the mosaics of allelic variants available within conifer hybrid zones will confer upon them greater resilience to ongoing and future environmental change and can be a key resource for conservation efforts.


© Mitra Menon

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