In recent decades biologists studying speciation have come to consider that the process does not necessarily require the presence of a geographical barrier. Rather, it now seems to be possible for reproductive barriers to evolve within what was hitherto a single ‘‘species.’’ The intertidal snail Littorina saxatilis has been the focus of a considerable amount of work in this context, and it is now thought of as a good case study of ‘‘ecological speciation.’’ We review some of this work and briefly consider prospects for future developments. 1. Introduction In recent decades, there has been a considerable shift in our view of speciation—ecology has come in [1]. Or rather, it has come back in because the role of ecological processes in diversification dates back to Darwin, although some biologists of the 20th century gave a prominent role in speciation to geographical isolation (allopatry) [2, 3]. This shift of view has been reviewed by Mallet [4]. It is our intention here to give an account of work on Littorina saxatilis (Olivi) over the last three decades, highlighting its contribution and promise to the study of speciation. This marine snail is a species in a small and young genus and is probably the most derived member of the genus Littorina. It is thought to have originated in the eastern North Atlantic about 0.65？Ma？bp [5], rapidly colonizing both sides of the Atlantic. Phylogeographic patterns make it likely that more northern populations have undergone repeated subdivision and recontact as shorelines have been subject to glacial action and concomitant sea level changes. Populations on the northwestern coasts of the Iberian Peninsula appear to be genetically distinct from those elsewhere, suggestive of relatively long isolation [5–7]. L. saxatilis is strictly intertidal, though within the intertidal it has a fairly wide vertical distribution, and is found on rocky shores and in estuaries and salt marshes [8]. There are few published data on longevity. Hughes [9] gives a maximum of about four years. Littorina saxatilis is also highly polymorphic, and this has given rise to a great deal of taxonomic confusion and synonymy. It is part of a species complex, the group of rough periwinkles, with its sister species Littorina compressa (Jeffreys) and Littorina arcana Hannaford Ellis [8]. These latter produce egg masses which are deposited in sheltered crevices on the shore, while L. saxatilis females carry their embryos in a brood pouch in the dorsal mantle cavity until they are released as “crawl aways,” with similar morphology to the adult snails.
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