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The study of speciation in recent populations is essentially a study of the evolution of reproductive isolation mechanisms between sub‐groups of a species. Prezygotic isolation can be of central importance to models of speciation, either being a consequence of reinforcement of assortative mating in hybrid zones, or a pleiotropic effect of morphological or behavioral adaptation to different environments. To suggest speciation by reinforcement between incipient species one must at least know that gene flow occurs, or have recently occurred, and that assortative mating has been established in the hybrid zone.

In Galician populations of the marine snail Littorina saxatilis, two main morphs appear on the same shores, one on the upper‐shore barnacle belt and the other in the lower‐shore mussel belt. The two morphs overlap in distribution in the midshore where hybrids are found together with pure forms …