"resident", and "offshore" – demonstrate partial sympatry, crossing paths with relative frequency. The results of recent genetic analyses using mtDNA indicate that this is due to secondary contact, in which the three types encountered one another following the bidirectional migration of "offshore" and "resident" whales between the North Atlantic and North Pacific. Partial sympatry in these whales is, therefore, not the result of speciation. Furthermore, killer whale populations that consist of all three types have been documented in the Atlantic, evidencing that interbreeding occurs among them. Thus, secondary contact does not always result in total reproductive isolation, as has often been predicted. Great spotted cuckoo and magpie: brood parasitism The parasitic great spotted cuckoo (Clamator glandarius) and its magpie host, both native to Southern Europe, are completely sympatric species. However, the duration of their sympatry varies with location. For example, great spotted cuckoos and their magpie hosts in Hoya de Gaudix, southern Spain, have lived in sympatry since the early 1960s, while species in other locations have more recently become sympatric. Great spotted cuckoos, when in South Africa, are sympatric with at least 8 species of starling and 2 crows, pied crow and Cape crow. The great spotted cuckoo exhibits brood parasitism by laying a mimicked version of the magpie egg in the magpie's nest. Since cuckoo eggs hatch before magpie eggs, magpie hatchlings must compete with cuckoo hatchlings for resources provided by the magpie mother. This relationship between the cuckoo and the magpie in various locations can be characterized as either recently sympatric or anciently sympatric. The results of an experiment by Soler and Moller (1990) showed that in areas of ancient sympatry (species in cohabitation for many generations), magpies were more likely to reject most of the cuckoo eggs, as these magpies had developed counter-adaptations that aid in identification of egg type. In areas of recent sympatry, magpies rejected comparatively fewer cuckoo eggs. Thus, sympatry can cause coevolution, by which both species undergo genetic changes due to the selective pressures that one species exerts on the other. Acromyrmex ant: isolation of fungal gardens Leafcutter ants protect and nourish various species of fungus as a source of food in a system known as ant-fungus mutualism. Leafcutter ants belonging to the genus Acromyrmex are known for their mutualistic relationship with Basidiomycete fungi. Ant colonies are closely associated with their fungus colonies, and may have co-evolved with a consistent vertical lineage