Nestling growth in the Great Tit I. Heritability estimates under different environmental conditions

TLDR: This study addresses the question of how the expression of genetic variation for fledgling body size of Great Tits varies with the environment by manipulating brood sizes and interacting with natural temporal variation in food availability.

Abstract: 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.