New directions
Our theoretical review highlights that fitness outcomes from natural dispersal between demes can be complex. Unraveling such complexity will require natural studies reporting fitness comparisons beyond that of the parental generation of residents and immigrants, and relative frequencies of filial generations in the wild. These studies must also report proper error estimates. These conditions inevitably require methodological considerations pertaining sources of biases and inaccuracies, as discussed in the section above, but will allow for future synthesis studies.
Future studies should also aim to estimate fitness of hybrid offspring under natural conditions as well as possible environment-specific effects, which have so far seldom been considered. Environmental conditions may influence not only the genetic architecture of traits but also their fitness consequences. As the magnitude of genetic effects change across filial generations, environmental effects may influence filial generations differently. For example, even if F1hybrids consistently show positive heterosis, negative heterosis may be environment-dependent in recombinant filial generations, due to loss of locally adapted beneficial epistatic interactions. Consequently, selection against hybrid individuals, introgression rates and effective gene flow will differ across demes and pairs of demes in a metapopulation, likely influencing isolation-by-ecology patterns of population differentiation and the evolution of habitat-matching dispersal. Non-random gene flow, in turn, can have significant cascading consequences to the eco-evolutionary dynamics of natural populations (Edelaar & Bolnick 2012).
Estimates of sex-specific effects are also fundamental to the understanding of eco-evolutionary dynamics of populations. Some studies reported that the direction of the cross between populations or the sex of the immigrant individual affected the fitness outcome for the hybrid offspring, suggesting that not only the sex of the offspring needs to be considered, but also the sex of the parents with different origins. Since sexes can present different degrees of local adaptation (Svensson et al. 2018), and populations may experience different degrees of intersexual conflict (de Lisle et al. 2018), outcomes of outbreeding that depend on the sex of the immigrant may be ubiquitous, and drive the evolution of sex-biased dispersal. In fact, sex differences in the propensity and distance of dispersal are common in animals (e.g. Trochet et al. 2016) and in plants, where male gamete dispersal can occur at higher rates and for longer distances than seed dispersal (reviewed in Ellstrand 2014). Sex-specific dispersal, in turn, may further influence the dynamics of sexual conflict and sex differences in local adaptation. For instance, it can impact the evolution of uniparental inheritance via maternal or paternal effects (Revardel et al. 2010) or the evolution of parental care (Trochet et al. 2016). Sex-biased introgression due to sex-specific differences in fitness or sex-biased dispersal can further influence the evolution of uniparental gene expression (Raunsgard et al. 2018), and change dynamics of nuclear-cytoplasmatic conflict and the degree of sexual dimorphism which alter intrapopulation levels of sexual conflict (Runemark et al. 2018).