CONCLUSION
Our theoretical review highlights that short- and longer-term impacts of natural immigration on local population demography and evolutionary dynamics will depend on the relative fitness and frequencies of existing natives, F1s, F2s and various potential backcrosses that emerge across generations. In turn, these fitnesses will depend on underlying genetic effects, their interaction with the environment, as well as the relative contribution of adaptive and non-adaptive mechanisms to the evolutionary history of population differentiation. Quantifying the relative fitness of parents and filial generations in principle allows estimation of multiple components of dominance and epistasis impacting fitness, with the potential to reveal underlying genetic architectures of fitness and of population differentiation, or even complement population genetic and local adaptation studies aiming to understand the role of adaptive and non-adaptive divergence. By corollary, quantifying such multi-generational effects, and hence resulting time-courses and manifestations of realized migration rates and effective gene flow, is necessary to predict population and micro-evolutionary dynamics in spatially structured populations. Our literature review indicates this resource remains almost entirely untapped within the context of natural dispersal and resulting immigration in wild populations and yet, spatial structure has the potential to generate a multitude of hybrid fitness effects across demes in a metapopulation. Therefore, studies of hybrid offspring at the metapopulation level may not only prove some of the most interesting and rich fields of research, but will be of fundamental importance to understanding eco-evolutionary dynamics in a world where fragmentation of populations is an ever-growing threat to biodiversity.