1) Demographic fluctuations that may have resulted in divergent lineages within species as a result of past climatic and geomorphologic disturbances.
Climatic fluctuations during the Pleistocene had a major impact on species assemblages and population structure. Contemporary populations reflect the new climatic conditions that were established ~11,000 years ago. We investigate the effects of past climatic conditions on divergence among populations of Mediterranean woodland species including the red oaks of California, tanoak and others, on redwood and giant Sequoia and on populations of mangrove species.
2) The role of hybrid events in colonization particularly under past and future climate change and the response of hybrids to pests and diseases.
Oaks are well known for their propensity to hybridize and have been used as an example of the failings of the biological species concept. We have shown that hybrids occur naturally among all combinations of species of the red oaks in California. In heavy-seeded species such as oaks, dispersal is likely to be quite slow and dependent on the vagaries of long-distance dispersal events. Our hybrid studies show that pollen, also, can serve as a vector of colonization, providing a hybrid-compatible species is present in, or close to the new habitat. We are currently looking at the effects of climate change on hybrid frequency as a possible model for rapid species’ movements.
One of the consequences of hybridization is combined gene pools that may influence resistance to pests and diseases. In California Sudden Oak Death and the gold spotted oak borer are causing severe mortality to coast live oak, but interior live oak appears to be a little risk. We are investigating the susceptibility of hybrids between these two species to these threats.