ANTAGONISTIC NATURAL-SELECTION REVEALED BY MOLECULAR SEX IDENTIFICATION OF NESTLING COLLARED FLYCATCHERS

Natural selection may act in different directions during different lif e-history stages, or in different directions on different classes of i ndividuals. Antagonistic selection of this kind may be an important me chanism by which additive genetic variation for quantitative traits is maintained, and can prevent populations or species reaching local ada ptive peaks. This paper reports the results of a study of viability se lection on morphological traits of nestling collared flycatchers Ficed ula albicollis. Analyses performed without knowledge of the sex of nes tlings suggested that no selection was occurring on these traits. Howe ver, using molecular sex identification with the avian CHD gene, it is shown that selection acts in different directions on male and female body size from fledging to breeding, apparently favouring relatively s mall males and large females. The results suggest that differential se lection on male and female nestlings may contribute to purely phenotyp ic sexual size dimorphism in this species. These findings highlight th e potential of newly developed molecular sexing techniques to reveal t he consequences of an individual's gender for many aspects of its life history in taxa where gender cannot be determined on the basis of ext ernal appearance.