DNA, morphology and fossils: phylogeny and evolutionary rates of the gastropod genusLittorina

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Abstract
Using data from the direct sequencing of fragments of three mitochondrial genes (12S and 16S ribosomal RNA, and cytochrome-b); total length 1469 b.p.) we have reconstructed a gene phylogeny for all 19 living species of the gastropod genusLittorina. Members of the closely related generaNodilittorina,LittorariaandMainwaringiahave been used as outgroups, and it appears thatLittorinais monophyletic. An earlier morphological phylogeny has been revised, and its topology found to be almost entirely consistent with that from the molecular data. The fossil record is sparse, but likewise consistent. A consensus tree is presented, showing clear resolution of basal and terminal branches, and a central unresolved polychotomy. We have used fossil evidence and geological events to estimate the ages of some clades, and thus to calculate average rates of molecular evolution, which in turn provide approximate dates for all branches of the molecular phylogeny. The central polychotomy may be explained by a burst of rapid speciation in the northwestern Pacific during the Middle Miocene, perhaps driven by climatic fluctuation. Our results support the hypothesis that the two clades ofLittorinain the northern Atlantic originated from Pacific ancestors which took part in the Pliocene trans-Arctic migration of marine organisms.