UCLA

Speciation usually occurs in geographic isolation, with lineages accumulating differences through time without exchanging genes. Yet how have closely related taxa speciated with potential for gene flow? I address this question in Linanthus, a phenotypically diverse flowering plant genus that rapidly diversified in a small geographic space. I integrated phylogenomics, trait evolution, chemical ecology, and landscape genomics to investigate patterns of Linanthus diversification. In Chapter 1, I reconstructed the evolutionary history across all species of Linanthus, including an average of seven individuals per species with a third of samples co-occurring with congeners. I found that most species were monophyletic, despite potential gene flow. The perennial and annual night blooming clades had unresolved species relationships, suggesting rapid speciation or cryptic diversity. Perenniality evolved once, night blooming three times, and flower color polymorphisms were likely ancestral. Classic allopatric speciation was likely not the only geographic mode of speciation across the genus, with young species overlapping in range. In Chapter 2, I investigated patterns of floral scent variation across populations and species of Linanthus. Scent is a complex and cryptic dimension of diversity that flowering plants use to ensure reproductive success through pollinator attraction. I found unusually high variation in scent profiles of Linanthus species. Within clades of Linanthus, species scent profiles were differentiated, suggesting fragrance is a mechanism of reproductive isolation in closely related species with range overlap. In Chapter 3, we assembled the reference genome of Linanthus parryae, which serves as a genomic resource for Chapter 1 and 4, and for understanding how biological variation originates and is maintained across Linanthus. Lastly, in Chapter 4, I used whole genome sequencing to assess the relative role of natural selection and genetic drift maintaining genetic variation across populations of L. parryae. I found evidence that both isolation by distance and by environment structure genetic divergence in this species. Precipitation and temperature variability had the highest contribution to genetic differentiation associated with climate, reflecting previous findings that fluctuating rainfall patterns maintain phenotypic variation. Together, this work provides integrative evidence to understanding how Linanthus species and their phenotypic diversity has originated and persists in nature.