Pattern of Phenotypic Vaviation in SILENE LATIFOLIA, an Invasion Plant Species

Owing to the fact that invasive species have a strong impact on native communities, much effort has been made to understand why they become problematic following their introduction to novel habitats. My thesis examined Silene latifolia, an invasive weed in North America that was introduced from Europe about 200 years ago. During the introduction process this plant species escaped its natural enemies and evolved a weedier phenotype in North America. However, adaptive evolution in an invasive species may seem counterintuitive. A bottleneck occurred in the colonization of Silene in North America involving only a small subset of the variation present in native Europe, and this founder effect could have theoretically reduced the amount of genetic (and therefore phenotypic) variation present in the introduced range. My thesis explored if reduced genotypic variation in North American Silene has resulted in a decrease in the expression of phenotypic variation. Furthermore, I examined whether the magnitude of variation was sensitive to plant density. To test this, a common garden experiment was conducted comparing the variation (coefficient of variation) in life history and morphological traits of native and introduced populations (25 each from Europe and North America) in different densities (1, 3, and 9 plants per pot). There were two major results. First, there was significant among-population variation in most life history and morphological traits on each continent despite the bottleneck event. Second, there was a pattern of decreased phenotypic variation among traits with increasing density and the amount of variation was greater in Europe than in North America; however, these differences were not statistically significant. The results of my research demonstrate that despite undergoing a founder event during its colonization, significant levels of genetic variation exist in important life history and morphological traits in introduced populations of Silene latifolia