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Abstract

The constant race to outcompete other organisms, either from the same or another species drives the continuous emergence of new and, better-adapted individuals. As these phenotypes become increasingly specialized to the specific environmental conditions they are exposed too, they grow progressively divergent from both their ancestors as well as their conspecifics in other environmental contexts. In freshwater three-spined sticklebacks (Gasterosteus aculeatus), this has resulted in a number of specific phenotypes, which have adapted different reproductive strategies or have evolved habitat specific growth and parasite resistance. Each of which is characterized by specific adaptations to the environmental context they are exposed to and maintained through environmental pressures and sexual selection. A key difference between rivers and lakes are the different parasite communities they harbor. Parasites impose strong selection pressures on their stickleback host by diverting resources, restricting reproduction and promoting their host’s death. Hence, it is crucial for sticklebacks to minimize their parasite load, resulting in a co-evolutionary arms race between parasite and host. As the parasite community varies between habitat types they demand different adaptations, resulting in increasing divergence between sticklebacks in their respective habitats. The globally reoccurring distinction of river and lake habitats implies an equally reoccurring habitat dependent differentiation into specific river respectively lake ecotypes. Specializing to their environment bestows an ecological advantage onto the resident individuals. This is reinforced by sexual selection which privileges well-adapted phenotypes by actively selecting for such individuals with low parasite burdens and optimal immune gene composition. Incidentally, this selects against potential migrants from other habitats, thus generating genetic isolation, a prerequisite for speciation.