Natural dispersal between populations, and resulting immigration,
influences population size and genetic diversity and is therefore a key
process driving reciprocal interactions between ecological and
evolutionary dynamics. Both ecological and evolutionary consequences of
dispersal fundamentally depend on the relative fitnesses of immigrants
and their various descendants manifested in the context of natural
environments. Yet, despite this commonality, recent research advances in
predicting immigrants' legacies remain substantially disconnected across
disciplines. To bridge resulting divides, we synthesize empirical and
theoretical work examining fitness consequences of inter-breeding across
the full spectrum of genetic divergence from inbred lines to
inter-specific hybridization. We demonstrate how common underlying
processes can generate positive or negative fitness consequences of
immigration depending on interacting genetic and environmental effects.
Impacts of inter-breeding following natural dispersal among
sub-populations could consequently vary dramatically, shaping
eco-evolutionary outcomes. Yet, our systematic literature review reveals
a striking paucity of empirical studies that quantify multi-generational
fitness consequences of immigration in natural metapopulations,
precluding general inferences on outcomes. Hence, to provide new
impetus, we highlight key theoretical and empirical gaps, and outline
how cutting-edge statistical and genomic tools could be combined with
multi-generational field datasets to advance understanding of
dispersal-mediated drivers and constraints on eco-evolutionary dynamics.