Submitted to: Journal of Arid Land Studies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2013
Publication Date: 6/1/2013
Publication URL: http://handle.nal.usda.gov/10113/56816
Citation: Espeland, E.K. 2013. Predicting the dynamics of local adaptation in invasive species. Journal of Arid Land. 5(3): 268–274.
Interpretive Summary: Plant invasions are sometimes restricted to a single habitat type, but after some time, some species are able to continue to expand their populations and invade other, secondary habitats. These secondary habitats are invaded later because they may provide more stressful environments for plant growth. If evolution is required in order to invade secondary habitat, we can predict what conditions make that evolutionary process faster. In order for local adaptation to occur, populations must harbor genetic diversity. Genetic diversity is generally low in invasive plants, but can be increased through repeated introductions, through movement of plants, pollen and/or seeds, through mutation, and through hybridization. An index of the ability of a plant population to maintain genetic diversity is effective population size (Ne). This describes the vulnerability of a population to genetic drift, or the loss of genetic diversity through random events rather than culling by natural selection. The environment can increase Ne by decreasing the amount of variation in reproduction among plants. For example, when competition is for light, larger plants shade smaller ones, and at the end of the growing season there is a high degree of variation in plant sizes. When competition is below-ground, there is less variation among individuals as larger plants cannot take resources away from smaller ones. In annual plants, size is highly correlated with seed set. This means that when soil nutrients are contested, the population will have low variation in seed set per plant. This population will then have lower Ne and less ability to maintain genetic variation. The effects of the environment on opportunity for selection and response to selection are parallel: when there is low variation in seed output among plants, there is less opportunity for natural selection to sort genotypes. This, again, allows a population to continue to maintain genetic diversity. If the primary invasion habitat is one that reduces variation among individuals, this habitat will allow genetic diversity to be maintained. If the invasion then spreads to a secondary habitat that increases variation among individuals, there is opportunity for selection to act and the population to locally adapt. If the secondary invasion habitat also reduces variation, then genetic diversity will continue to be maintained, but the process of adaptation will be slower because of reduced opportunity for selection and reduced response to selection. If, on the other hand, the primary habitat is one that increases variation, the population will lose genetic variation to random drift processes and may not house the genetic diversity required for local adaptation to either the primary or secondary habitat.
Technical Abstract: An invasive plant species may restrict its spread to only one habitat, or, after some time, may continue to spread into a different, 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, adaptive 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 environment induces a plastic response of little variation in reproductive output among individuals, such as environments where below-ground resources are contested, Ne is maximized and genetic variation is maintained. The same environments that maintain high Ne also reduce the opportunity for selection and minimize the response to selection, 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 secondary habitat, particularly if the secondary habitat has high opportunity for selection. When the primary invasion habitat supports low Ne, there may not be enough genetic diversity for local adaptation to occur.