Morphometric differentiation across House Sparrow Passer domesticus populations in Finland in comparison with the neutral expectation for divergence

To understand the biology of organisms it is important to take into account the evolutionary forces that have acted on their constituent populations. Neutral genetic variation is often assumed to reflect variation in quantitative traits under selection, though with even low neutral divergence there can be substantial differentiation in quantitative genetic variation associated with locally adapted phenotypes. To study the relative roles of natural selection and genetic drift in shaping phenotypic variation, the levels of quantitative divergence based on phenotypes (P-ST) and neutral genetic divergence (F-ST) can be compared. Such a comparison was made between 10 populations of Finnish House Sparrows (n = 238 individuals) collected in 2009 across the whole country. Phenotypic variation in tarsus-length, wing-length, bill-depth, bill-length and body mass were considered and 13 polymorphic microsatellite loci were analysed to quantify neutral genetic variation. Calculations of P-ST were based on Markov-Chain Monte Carlo Bayesian estimates of phenotypic variances across and within populations. The robustness of the conclusions of the P-ST-F-ST comparison was evaluated by varying the proportion of variation due to additive genetic effects within and across populations. Our results suggest that body mass is under directional selection, whereas the divergence in other traits does not differ from neutral expectations. These findings suggest candidate traits for considering gene-based studies of local adaptation. The recognition of locally adapted populations may be of value in the conservation of this declining species.