Evolutionary change requires natural selection in the presence of heritable variation for the trait(s) under selection. Since heritabilities and selection pressures are known to vary with environmental conditions, it is crucial to know how much genetic variation is expressed under which conditions. This study addresses the question of how the expression of genetic variation for fledgling body size of Great Tits varies with the environment.Different environmental conditions were created experimentally by manipulating brood sizes. The treatment affected body size, measured as either fledging weight or tarsus length, and interacted with natural temporal variation in food availability. Both measurements show stabilizing selection.A cross-fostering design was carried out to separate genetic and environmental causes of variation. Heritabilities as measured from offspring-midparent regressions and from full-sib analyses were substantial for both traits, except that no heritability was found for weight under poor conditions. Instead, fledging weights were significantly correlated with the weights of their unrelated guardians' ( = fosterparents') weight under poor conditions. We propose that under poor conditions, when selection on fledging weights is expected to be directional and strong, only little genetic variance is expressed. Any evolutionary response to this selection on fledging weight might therefore be slow, if the increase in selection pressure is not greater than the decrease in heritability.

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21 There is increasing evidence of a pivotal role of the gut microbiota (GUT-M) in key physiological 22 functions in vertebrates. Many studies discuss functional implications of the GUT-M not only on 23 immunity, growth, metabolism, but also on brain development and behavior. However, while the 24 influence of the microbiota-gut-brain axis (MGBA) on behavior is documented in rodents and 25 humans, data on farm animals are scarce. This review will first report the well-known influence of the 26 MGBA on behavior in rodent and human and then describe its influence on emotion, memory, social 27 and feeding behaviors in farm animals. This corpus of experiments suggests that a better 28 understanding of the effects of the MGBA on behavior could have large implications in various fields 29 of animal production. Specifically, animal welfare and health could be improved by selection, 30 nutrition and management processes that take into account the role of the GUT-M in behavior.