Patterns of natural selection on morphology of male and female collared flycatchers (Ficedula albicollis)

Natural selection may act in different directions among years, life stages, or classes of individuals. Fluctuating selection of this kind is potentially an important mechanism by which additive genetic variation for quantitative traits is maintained, and can prevent populations reaching local adaptive peaks. We analysed natural selection acting on three morphological traits of male and female collared flycatchers via both fecundity and survival, using 15 years' data from a large isolated population on Gotland, Sweden. We particularly investigated variation in the direction and magnitude of selection acting: (1) among years over the study period; (2) on different life stages and (3) the consistency of observed patterns of selection with sexual size dimorphism (SSD) in this population. We found little evidence of natural selection on these traits over the study period. Evidence for directional, stabilizing and disruptive selection was found for some year-trait combinations, but these patterns were inconsistent with respect to both the magnitude and form of selection found. Consequently, our results, based on the detailed analysis of natural selection in a large wild population over a period of 15 years, provide evidence for the common assumption that forces of selection acting on quantitative traits are generally weak. They are also consistent with the suggestion that environmental stability is an important determinant of the degree to which organisms lit their environment. (C) 2000 The Linnean Society of London.