that remained in a boreal coniferous forest. Although these two populations spend a period of the year in the same place, and though there was evidence of gene flow between them, the difference in prey–habitat specialization has been sufficient to maintain genetic and even coloration divergence. A different study has identified the remains of a population of extinct Pleistocene Beringian wolves with unique mitochondrial signatures. The skull shape, tooth wear, and isotopic signatures suggested these remains were derived from a population of specialist megafauna hunters and scavengers that became extinct while less specialized wolf ecotypes survived. Analogous to the modern wolf ecotype that has evolved to track and prey upon caribou, a Pleistocene wolf population could have begun following mobile hunter-gatherers, thus slowly acquiring genetic and phenotypic differences that would have allowed them to more successfully adapt to the human habitat. Aspergillus and Staphylococcus Numerous genera of bacteria and fungi live on and in the human body as part of its natural flora. The fungal genus Aspergillus is capable of living under considerable environmental stress, and thus is capable of colonising the upper gastrointestinal tract where relatively few examples of the body's gut flora can survive due to highly acidic or alkaline conditions produced by gastric acid and digestive juices. While Aspergillus normally produces no symptoms, in individuals who are immunocompromised or suffering from existing conditions such as tuberculosis, a condition called aspergillosis can occur, in which populations of Aspergillus grow out of control. Staphylococcus aureus, a common bacterial species, is known best for its numerous pathogenic strains that can cause numerous illnesses and conditions. However, many strains of S. aureus are metabiotic commensals, and are present on roughly 20 to 30% of the human population as part of the skin flora. S. aureus also benefits from the variable ambient conditions created by the body's mucous membranes, and as such can be found in the oral and nasal cavities, as well as inside the ear canal. Other Staphylococcus species including S. warneri, S. lugdunensis and S. epidermidis, will also engage in commensalism for similar purposes. Nitrosomonas spp and Nitrobacter spp Commensalistic relationships between microorganisms include situations in which the waste product of one microorganism is a substrate for another species. One good example is nitrification-the oxidation of ammonium ion to nitrate. Nitrification occurs in two steps: first, bacteria such as Nitrosomonas spp. and certain crenarchaeotes oxidize ammonium to nitrite; and