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Genetic conservation in the Anthropocene: The case for assisted gene flow in forest trees
S.N. Aitken and M.C. Whitlock
Assisted migration is being evaluated and in some cases already being used as a tool for maintaining resource production or conserving species as climates change. However, there is a lack of scientific consensus on this subject. Much of the debate arises from a lack of a common definition for assisted migration. To some, it primarily refers to the human movement of seed or individuals within existing species ranges, which we define here as assisted gene flow (AGF). To others, assisted migration primarily refers to species introductions outside of their historical range. In order to evaluate the risks and benefits of assisted migration, it is necessary to consider AGF and assisted migration outside of native ranges separately. AGF has greater genetic implications for existing native populations than assisted migration outside of native ranges as existing recipient populations will be altered, while assisted migration into novel areas has greater ecological implications than AGF as species will be introduced to ecosystems. Here we focus on the genetic effects of AGF.
To provide a more nuanced perspective on the potential for AGF to facilitate adaptation to climate change, we have evaluated genetic risks and potential benefits through a review and synthesis of the theoretical and empirical literature in combination with simulation modeling of populations. AGF can increase average fitness in a population through introducing or increasing the frequency of pre-adapted alleles or genotypes. However, AGF may result in outbreeding depression due to the disruption of co-adapted gene complexes, called intrinsic outbreeding depression. AGF may also disrupt local adaptation to non-climatic factors such as soil properties, photoperiod, or the presence of other organisms such as competitors, pathogens or mutualists through hybridization of residents and immigrants, termed extrinsic outbreeding depression. AGF may result in lineage swamping and the loss of local populations; however, it may also maintain genetic lineages from rear-edge populations that are likely to be extirpated if left in situ.
Many tree populations have large effective population sizes, experience relatively high levels of gene flow among populations, and are locally adapted to climate, thus are ideal candidates for AGF. Intrinsic outbreeding depression is unlikely to occur unless populations have been long isolated, and evolution should resolve weak outbreeding depression due to epistasis in a few generations. Similarly, natural selection should resolve mild extrinsic outbreeding depression due to adaptive differences in large populations. To weigh the risks of maladaptation to climate change against the risks of AGF for a given species requires information on the extent of local adaptation to climate versus other environmental factors, and the patterns of gene flow. We conclude that AGF should be a powerful tool for the management and conservation of many widespread tree species that show historical adaptation to local climatic conditions.
Aitken, S.N. and M.C. Whitlock. 2013. Assisted gene flow to facilitate local adaptation to climate change. Annual Review of Ecology, Evolution and Systematics DOI: 10.1146/annurev-ecolsys-110512-135747