Adaptation and colonization in Hypericum canariense: Past and present invasions of an island endemic

When individuals colonize a new location, they face challenges to successful establishment as well as opportunities to exploit unused resources. There is an increasing awareness that evolutionary changes can help founding populations overcome obstacles and capitalize on opportunities. My dissertation examines the evolution of quantitative traits during the colonization history of Hypericum canariense. This plant is native to the Canary Islands and introduced to the United States, Australia, and New Zealand. It has naturally colonized five islands in its native range, and is expanding aggressively in several recent (< 50 years old) introductions. I explored patterns of evolution during the spread of H. canariense (1) across its native range, (2) from native to introduced populations in California and Hawaii, and (3) across individual introductions. Using molecular genetic surveys and a common garden of plants from the native range, I found that two major lineages (varieties) have followed different patterns of colonization across the islands. The varieties differ in growth rate, flowering date, flower production, and floral morphology. Selection appears to be maintaining stable values of growth and flowering date for each variety across different islands, indicating that each has colonized islands to which it was pre-adapted. Using similar methods, I found that growth and flowering date have evolved in introduced populations, relative to their source in the native range. Growth rate has increased and flowering date has developed a latitudinal cline. In a field common garden, I found selection for increased size via survival and the likelihood of flowering, suggesting that these changes are adaptive. Finally, I examined the possibility that invasions evolve reduced levels of inbreeding depression, due to an increased reliance on self-fertilization as a mechanism of reproductive assurance. I used hand-pollinations to assess the levels of inbreeding depression in older (invasion core) and younger (leading edge) areas of each introduction. I found novel evidence of a decline in inbreeding depression (in seed production, germination, or growth) in all populations. Taken together, my results indicate that colonizing populations H. canariense are responding to local selective pressures, and that isolated populations are sites of surprisingly rapid evolutionary change.