Predicting the dynamics of local adaptation in invasive species

An invasive plant species may restrict its spread to only one type of habitat, or, after some time, may continue to spread into a different, often stressful, secondary, habitat. The question of whether evolution is required for an invasive species to spread from one habitat to another is currently hotly debated. In order for local adaptation to occur, genetic variation must be present within invasive populations. In this paper, I focus on the effect of habitat on the maintenance of genetic variation during the lag phase, the phase of population stability prior to expansion. Genetic diversity in invasive plant populations accumulates through multiple introductions, gene flow, mutation, and hybridization, but diversity is maintained by population level processes influencing effective population size (Ne). I show that when the plastic response to the environment results in little variation in reproductive output among individuals, Ne is maximized and genetic variation is maintained. Established models of plant competition show that below-ground competition reduces the variation in reproductive output, whereas competition for light increases variation in reproductive output. The same environments that maintain high Ne also reduce the opportunity for selection and minimize the response to selection, and thus the effects of the environment are synchronized to prevent genetic purges. When the primary invasion habitat supports high Ne, conditions are ripe for local adaptation to a secondary habitat, particularly if the secondary habitat has high opportunity for selection. When the primary invasion habitat supports low Ne, genetic diversity is less likely to be sufficient for local adaptation to secondary habitat to occur.