The origin and maintenance of biological diversity is one of the most fundamental topics of ecological and evolutionary sciences. In the broad sense studies on this topic consider primarily the relationship between the ecological environment of organisms and phenotypic differences and changes within and across generations (Schluter 2000; West-Eberhard 2003). Of particular importance is our understanding of adaptive divergence where fi tness related phenotypic traits are sorted by disruptive or divergent natural selection resulting in discrete phenotypic varieties or morphs (Robinson and Wilson 1994; Skúlason and Smith 1995). Signifi cantly, such adaptive divergence can often involve the evolution of reproductive barriers infl uencing genetic connectedness between morphs resulting in more than one population. Thus, varying levels of genefl ow among such morphs, ranging from unlimited to none, is of major importance for subsequent evolutionary processes. In some cases, reproductive isolation can become advanced and even complete resulting in a clear population divergence and even the formation of a new species (Skúlason et al. 1999; Schluter 2000; Coyne and Orr 2004; Snorrason and Skúlason 2004; Hendry 2009). Thus, studies of adaptive divergence and studies of speciation have no clear borders but overlap and share their objectives, i.e., to understand the origin and maintenance of biological diversity.