Patterns of genetic and phenotypic variation in invasive Cakile species in Australia

Biological invasions are one of the key drivers of biodiversity changes. In spite of their environmental and economic impacts, invasive species provide a valuable system for studying evolution over contemporary timescales. However, the complexity of the habitats makes it hard to infer about the evolutionary processes, such as founder events, drift, and admixture and novel selection pressures, in the invasive populations. Invasion studies, however, can benefit from the simplicity of coastal systems where dispersal happens along a linear, one-dimensional habitat with usually less complex biotic interactions. Such a confined habitat makes eco-evolutionary changes in the population more tractable as compared with most two-dimensional terrestrial habitats. My dissertation addresses the patterns of genetic and phenotypic diversity in invasive populations of two coastal species; Cakile maritima and Cakile edentula. These two species have a unique history of invasions in Australia and in two other regions (West coast of North America and New Zealand): in all these three regions C. edentula arrived earlier than C. maritima; however, later on C. maritima replaced C. edentula in most regions. Using these two coastal invaders, the main questions of my thesis were: does variation follow geographical patterns expected from particular invasion mechanisms? If so, what mechanisms predominate? And, does hybridisation/introgression between the two Cakile spp. contribute to population variation and therefore affecting invasion outcomes? These questions were addressed through studies of microsatellite variation on a continental scale, followed up by the use of CAPS markers derived from genome sequencing. A common garden experiment examined phenotypic differences between populations. The species were hand-crossed in the glasshouse to examine the viability and mating systems of hybrid progeny, while putative hybrids from the field were examined using genetic techniques. It is concluded th