Recent hybridization has been reported between the Herring Gull (Larus argentatus) and Glaucous Gull (Larus hyperboreus; the larger of the species) in Iceland, after the arrival of Herring Gulls around 1925. We used samples of both species from various locations in Iceland and from three periods spanning >40 years. To analyze the nature of hybridization and the extent of introgression between the species, we compared morphological variation using genetic information based on microsatellite and mtDNA data. Both comparisons of single traits and multivariate analysis pointed to hybridization and introgression. Variation in genetic markers and morphological traits revealed two distinct groups, connected by a number of intermediates. A discrepancy in classification based on genetic or morphological information revealed introgression. Variation within each species reflected the effect of hybridization; for example, individuals with less pigmentation (i.e., more hyperboreus-like) tend to be larger. The differences we observed among samples and periods reflect changes in the overlap of the populations' distributions.
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Vol. 126 • No. 2