Date:
Title: Evolutionary history, local adaptation, and reproductive isolation in the Phlox pilosa complex
Abstract: Four evolutionary forces – mutation, natural selection, genetic drift, and gene flow – are responsible for producing the incredible diversity of life. One of these forces, gene flow, is typically understood to counteract evolutionary divergence, and therefore much of the research on how species form is devoted to understanding how barriers to gene flow between lineages evolve. However, in some cases, it is clear that gene flow can act as a creative evolutionary force by generating novel traits, transferring adaptive variation between lineages, and even giving rise to new species. Intrigued by this duality, I investigated a group of four closely related taxa in the wildflower genus Phlox that presented open questions about the role of gene flow and local adaptation in the origin, divergence, and reproductive isolation of these lineages.
In Chapter 1, I review the existing body of work on hybridization in plants, and I trace the origin and development of many influential ideas about hybridization to scientists who studied plants. In Chapter 2, I use genomic data to infer the evolutionary relationships among my focal taxa and overturn the longstanding hypothesis that two of the taxa represent hybrid species, independently evolving lineages that arose through hybridization between the other two taxa. In Chapter 3, I use a reciprocal transplant experiment including three of the taxa to show that they are locally adapted to different ecological conditions, which is likely to present a barrier to gene flow in spite of living in sympatry. I also show that local adaptation among populations within one taxon is driven by different ecological factors than those that drive local adaptation between taxa. Finally, in Chapter 4, I study a contemporary hybrid zone between two of the taxa and find that ecological divergence driven by canopy cover, but not pollinator-mediated assortative mating, likely contributes to reproductive isolation between them.
Committee: Robin Hopkins (Advisor & Chair), Dan Hartl, Missy Holbrook, Jim Mallet