in interactions between unicellular organisms and viruses due to large population sizes, short generation times, often haploid genomes and horizontal gene transfer, which increase the probability of beneficial mutations arising and spreading through populations. Theories Geographic mosaic theory of coevolution John N. Thompson's geographic mosaic theory of coevolution hypothesizes spatially divergent coevolutionary selection, producing genetic differentiation across populations. The model assumes three elements that jointly fuel coevolution: 1) a selection mosaic among populations Natural selection on interactions between species differs among populations. Thus genotype-by-genotype-by-environment (G x G x E) interactions affect fitness of the antagonists. In other words, the specific environmental conditions determine how any genotype of one species influences the fitness of another species. 2) coevolutionary hotspots Coevolutionary hotspots are communities in which selection on the interaction is truly reciprocal. These hotspots are intermixed with so-called coldspots in which only one or neither species adapts to the antagonist. 3) geographic mixing of traits Between the communities/regions there is a continuous "mixing" of traits by gene flow, random genetic drift, population extinction, or mutation. This remixing determines the exact dynamics of the geographic mosaic by shifting the spatial distributions of potentially coevolving alleles and traits. These processes have been intensively studied in a plant, Plantago lanceolata, and its parasite the powdery mildew Podosphaera plantaginis in Åland in south-western Finland. The mildew obtains nutrients from its host, a perennial herb, by sending feeding roots into the plant. There are more than 3000 host populations in this region, where both populations can evolve freely, in the absence of human-imposed selection, in a heterogeneous landscape. Both partners can reproduce asexually and sexually. The mildew tends to become locally extinct in winter, and causes local epidemics in summer. The mildew's success at overwintering, and the intensity of host-pathogen encounters in summer, strongly vary geographically. The system has spatially divergent coevolutionary dynamics across two metapopulations as predicted by the mosaic theory. Red Queen hypothesis The Red Queen hypothesis states that both host and parasite have to change continuously to keep up with each other's adaptations, like the description in Lewis Carroll's fiction. The mathematical evolutionary biologist W. D. Hamilton further proposed that since sexual reproduction continually creates new combinations of genes, some of which may be advantageous, parasitism favours sexual reproduction in the host. The New Zealand freshwater snail Potamopyrgus antipodarum and its different trematode parasites represent a rather special model system. Populations of P.