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).