This thesis examines what happens in species with populations that are genetically different and meet at a contact zone, a boundary area between the two populations. The thesis investigates reproductive barriers that exist between two different forms of Littorina saxatilis, an intertidal marine snail, and I have analyzed data on reproductive barriers found in several marine species around the mouth of the Baltic Sea.
In a review of reproductive barriers in 23 different species, including cod, herring and plaice, it was found large genetic differences between the Baltic Sea populations and the North Sea populations. These differences are maintained partly because the populations survive differently in different salinities and partly because their reproduction is separated in time or space, or both.
For the Littorina saxatilis snail, which is common in the Atlantic along the coasts of both Europe and North America, two different populations or ecotypes have formed under ongoing genetic exchange, according to previous research. This thesis shows that the size of intertidal marine snails is important not only for survival but also for mating. Mating is more common between snails of similar sizes and that small males have more matings. Both of these factors help to counteract gene exchange between the large Crab snails and the smaller Wave snails when they meet both inside and outside the contact zones.