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Studies of clinal variation have helped greatly in the elucidation of the mechanisms of both adaptation and speciation (Endler 1977; Barton and Hewitt 1989). Clines have been explained by selection acting within a continuous population ( primary intergradation ) or, more frequently, as the result of contact between populations differentiated in allopatry (secondary intergradation) (Endler 1977; Barton and Hewitt 1989). However, a frequent problem in interpreting empirical studies of allozyme or morphological clines is that both primary and secondary intergradation may generate similar distributions of characters (Endler 1977; Barton and Hewitt 1989). Theoretical and experimental studies suggest that the interpretation of clinal variation is aided by historical considerations (see, e.g., Thorpe 1984; GonzalezVillasenor and Powers 1990; Gallant et al. 1993). Mitochondrial DNA analysis provides a historical perspective because it permits the construction of a phylogenetic tree of haplotypes (Avise 1994). If this tree has a deep phylogenetic split, which separates haplotypes occurring at high frequency on either side of the cline, secondary contact is favored, particularly if the phylogenetic split is more ancient than any present-day barrier to gene flow ( Gonzalez-Villasenor and Powers 1990; Avise 1994). Alternatively, low haplotype divergence would suggest primary intergradation and a lack of genetic isolation. The mussel Mytilus galloprovincialis provides an interesting model system for studies of mtDNA phylogeography. In Europe, its range extends from the Black Sea and Mediterranean to the British Isles (Gosling 1992a), a geographical region strongly affected by welldocumented Pleistocene glaciations and contemporary oceanographic boundaries (Pielou 1979; Tintore et al. 1988). Mussels, in common with many marine organisms, have a pelagic larval stage that is believed to have

A mitochondrial DNA discontinuity in the mussel Mytilus galloprovincialis Lmk: pleistocene vicariance biogeography and secondary intergradation.