UCLA

Climate regimes are changing rapidly at a global scale and the rate of change often outpaces the ability of many species to evolve in response to changing selective pressures. It is possible that other evolutionary processes, such as introgressive hybridization, allow species to exchange alleles across species boundaries at rates that exceed the creation of novel adaptive alleles through mutation alone. By analyzing genomic sequence data for evidence of ancient adaptive introgression, we can characterize the role that this process played in the evolutionary history of these species and its potential under predicted future climate regimes. Here, we test for evidence of adaptive introgression in two hybridizing white oaks (Quercus sect. Quercus): the drought tolerant California shrub oak (Quercus berberidifolia) and the comparatively drought intolerant tree Engelmann oak (Quercus engelmannii). In Chapter 1, we use single nucleotide polymorphisms (SNPs) generated by reduced-representation sequencing to test the likelihood of various ancient demographic models to identify the presence, timing, and direction of ancient introgression in this system. We discovered evidence of asymmetric introgression from Q. berberidifolia during the advent of a Mediterranean-type climate, indicating a potential association between ancient gene flow between these species and large-scale climatic change. In Chapter 2 we used whole genome sequence data to characterize landscape-scale distribution of genetic diversity in both Q. engelmannii and Q. berberidifolia. We also identified and compared candidate SNPs significantly associated with climatic gradients to neutral genetic diversity under current and future predicted climate models. Evidence showed that Q. engelmannii is likely well-adapted to current local climate conditions throughout its range and is more likely to become maladapted under future climate scenarios. In Chapter 3 we test for evidence of introgression in both species and identified any functional genes found in physical regions of introgression. We discovered significant genome-wide evidence of introgression of functional genes associated with stress response in Q. engelmannii but did not find any significant evidence of introgression in Q. berberidifolia. This combined evidence suggests that ancient adaptive introgression with Q. berberidifolia may have allowed Q. engelmannii to adapt to past large-scale climate change.