The biology of closely related and recently separated species is of special interest in the study of speciation. Recognition of the status of such populations presents problems – it is not always easy to discriminate which populations are fully reproductively isolated from those where some hybridisation between ‘species’ occurs. In our chosen study organism, Littorina saxatilis in the prosobranch subgenus Neritrema, there is still considerable controversy over which populations are in a state of gene flow with which others. Recent opinion holds that there are three differentiated sibling species: L. saxatilis, L. arcana and L. nigrolineata, with the status of the small barnacle-dwelling L. neglecta still controversial. In this chapter, we question the genetic unity of L. saxatilis sensu stricto, and discuss the relative importance of study of shell morphology, enzyme polymorphisms and DNA polymorphisms. We conclude that in this organism, enzyme polymorphisms are a weak tool for the study of sibling species.
A ‘species’ is often held to comprise a group of populations that freely exchange genes among themselves, but not with other groups. The idea of species is, for some workers, fundamental in biology and studies of biodiversity (e.g. Futuyma, 1987), and indeed inasmuch as species are genetically circumscribed units, this has force. Our opening sentence is an expression of the Biological Species Concept (BSC) especially associated with Mayr (e.g. Mayr, 1963) – there are other species concepts.