Résumé
After publication of the Larus phylogeny in Crochet et al. (2002), the taxonomic status of the “Larus thayeri“ and the “L. occidentalis“ specimens that were used in that study came into question. For each of those species, we sequenced the same mitochondrial DNA regions in new specimens of known identity. In addition, specimens of L. glaucescens were included. Results from using those specimens confirm that L. occidentalis was the first to diverge from the large white-headed gulls. Larus glaucescens, on the contrary, is part of the Arctic species clade, which also includes L. hyperboreus, L. glaucoides, L. thayeri, and L. schistisagus. The three new L. thayeri specimens differ substantially in mitochondrial DNA from the previously used sample of L. thayeri and share the same haplotype with L. glaucescens. The significance of this finding is unclear, because relationships within the “Arctic species“ clade are still unresolved; that is attributable to the unusually high incidence of lineage sharing and extremely low divergence of haplotypes in the group.