In WP1 we combined genomic and ecological data to show that inbred individuals had lower fitness (i.e. lower survival and fewer offspring), which is a clear sign of inbreeding depression. Although inbreeding levels varied across populations and time, its negative effects on fitness remained consistent. Environmental factors like weather and population density influenced survival and reproduction, which in turn determine population growth rates, but did not alter the strength of inbreeding depression. We also developed a genetic method to identify dispersers, revealing that dispersal patterns varied with geography, habitat, and population size. Notably, immigrants and their hybrid offspring had higher fitness, suggesting heterosis - where new genetic variation from immigrants offsets the negative effects of loss of genetic variation due to inbreeding and genetic drift.

WP2 reviewed and synthesized theory on multi-generational fitness effects of dispersal. We found that genetic architecture of fitness and the relative roles of adaptive and non-adaptive mechanisms in population differentiation can lead to diverse outcomes, and especially in species with the spatial population structure we often see in nature. Furthermore, using our house sparrow data, we showed that inbreeding reduces genome-wide variation and is likely driven by many mildly harmful recessive alleles, rather than a few major ones - an insight with major implications for conservation.  

WP3 developed a statistical model to estimate how adaptive potential and the evolution of traits vary across space and time, especially when the traits are influenced by many genes. 

WP4 examined how genetic drift (effective population size, Ne) and dispersal (immigration rate, m) affect genetic differentiation. We found that differentiation is lowest when both Ne and m are high, but one can compensate for the other. Interestingly, observed differentiation was often lower than theory predicted, likely due to higher-than-expected gene flow. 

WP5 synthesized findings to assess how genetic processes impact population growth and viability. Populations with more inbreeding - often small and isolated - suffered greater fitness costs. Thus, our results suggest that inbreeding may have important negative effects on population growth rates and viability. However, immigrants and their offspring boosted both local and metapopulation viability, reinforcing the importance of gene flow in fragmented populations.