Dispersal affects evolutionary processes by changing population sizes and their genetic composition, influencing the viability and persistence of populations. Investigating which mechanisms underlie variation in dispersal phenotypes and whether populations harbor adaptive potential for dispersal is crucial to understanding the eco-evolutionary dynamics of this important trait. Here, we investigate the genetic architecture of dispersal in an insular metapopulation of house sparrows. We use an extensive long-term individual-based ecological dataset and high-density single nucleotide polymorphism (SNP) genotypes for over 2500 individuals. We conducted a genome-wide association study (GWAS), finding a relationship between dispersal probability and a SNP located near genes known to regulate circadian rhythmic, glycogenesis and exercise performance, among other functions. However, this SNP only explained 3.8% of variance, suggesting that dispersal is a polygenic trait. We then used an animal model to estimate heritable genetic variation (Va), which composes 10% of the total overall variation in dispersal probability. Finally, we investigated differences in Va across populations occupying ecologically relevant habitat types (farm vs. non-farm) using a genetic-groups animal model. We found higher mean breeding value, Va, and heritability for the farm habitat, suggesting different adaptive potentials across habitats. Moreover, dispersal phenotypes may depend on genotype-by-environment interactions. Our results suggest a complex genetic architecture of dispersal, and demonstrate that adaptive potential may be environment-dependent in key eco-evolutionary traits. The eco-evolutionary implications of such environment-dependence and consequent spatial variation are likely to be ever more important with the increased fragmentation and loss of suitable habitats for many natural populations.

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.

Early-life telomere length (TL) is associated with fitness in a range of organisms. Little is known about the genetic basis of variation in TL in wild animal populations, but to understand the evolutionary and ecological significance of TL it is important to quantify the relative importance of genetic and environmental variation in TL. In this study, we measured TL in 2746 house sparrow nestlings sampled across 20 years and used an animal model to show that there is a small heritable component of early-life TL (h2=0.04), but with a strong component of maternal inheritance. Variation in TL among individuals was mainly driven by environmental (year) variance, but also brood and parental effects. We did not find evidence for a negative genetic correlation underlying the observed negative phenotypic correlation between TL and structural body size. Thus, TL may evolve independently of body size and the negative phenotypic correlation is likely to be caused by non-genetic environmental effects. We further used genome‐wide association analysis to identify genomic regions associated with TL variation. We identified several putative genes underlying TL variation; these have been inferred to be involved in oxidative stress, cellular growth, skeletal development, cell differentiation and tumorigenesis in other species. Together, our results show that TL is a lowly heritable, polygenic trait which is strongly affected by environmental conditions in a free-living bird.

Generation time determines the pace of key demographic and evolutionary processes. Quantified as the weighted mean age at reproduction, it can be studied as a trait that varies within and among populations and may evolve in response to ecological conditions. We combined quantitative genetic analyses with age- and density-dependent models to study generation time variation in a bird metapopulation. Generation time was heritable, and males had longer generation times compared with females. Individuals with longer generation times had a higher lifetime reproductive success but not a higher expected population growth rate. Density regulation acted on recruit production, suggesting that longer generation times should be favored when populations are closer to carrying capacity. Furthermore, generation times were shorter when populations were growing, and longer when populations were closer to equilibrium or declining. These results support classic theory predicting that density regulation is an important driver of the pace of life-history strategies.

Changes in telomere dynamics could underlie life-history trade-offs among growth, size and longevity, but our ability to quantify such mechanistic processes in natural, unmanipulated populations is limited. We investigated how 4 years of artificial selection for either larger or smaller body size affected early-life telomere length in two insular populations of wild house sparrows. A negative correlation between telomere length and structural size was evident under both selection regimes. The study also revealed that male sparrows had longer telomeres than females, after controlling for size, and there was a significant negative effect of harsh weather conditions on telomere length. The long-term fitness consequences of these changes in early-life telomere length induced by the artificial size selection were explored over a period of 11 years. These analyses indicated disruptive selection on telomere length because both short and long early-life telomere length tended to be associated with the lowest mortality rates and highest life expectancy. There was also weak evidence for a negative association between telomere length and annual reproductive success, but only in the population where body size was increased experimentally. Our results suggest that natural selection for optimal body size in wild animals will affect early-life telomere length during growth, which is known to be linked to longevity in birds, but also that the importance of telomeres for long-term somatic maintenance and fitness is complex in a wild bird species.

Generation time determines the pace of key demographic and evolutionary processes. Quantified as the weighted mean age at reproduction, it can be studied as a trait that may evolve and change in response to ecological conditions. We combined quantitative genetic analyses of individual projection matrices with age- and density-dependent models to study generation time variation in a bird metapopulation. We found that males have longer generation times than females and that it is a heritable trait. Individuals with longer generation times contributed to population growth later in life, lived longer, produced fewer recruits per year, had greater lifetime reproductive success, but not necessarily a higher expected individual growth rate. As predicted by density-dependence theory, generation times were shorter when populations were growing, and longer when populations were closer to equilibrium or declining. These results support classic theory predicting that competitive regimes are key determinants of the pace of life-history strategies.