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 potential ly an important mechanism by which additive genetic variation for quantitat ive traits is maintained and can prevent populations reaching local adaptiv e peaks. We analysed natural selection actin on three morphological traits of male and female collared flycatchers via both fecundity and survival, us ing 15 years' data from a large isolated population on Gotland, Sweden. We particularly investigated variation in the direction and magnitude of selec tion acting: (1) among years over the study period; (2) on different life s tages and (3) the consistency of observed patterns of selection with sexual size dimorphism (SSD) in this population. We found little evidence of natu ral selection on these traits over the study period. Evidence for direction al, stabilizing and disruptive selection was found for some year trait comb inations, but these patterns were inconsistent with respect to both the mag nitude and form of selection found. Consequently, our results, based on the detailed analysis of natural selection in a large wild population over a p eriod of 15 years, provide evidence for the common assumption that forces o f selection acting on quantitative traits are generally weak. They are also consistent with the suggestion that environmental stability is an importan t determinant of the degree to which organisms fit their environment. (C) 2 000 The Linnean Society of London.