Zoologists recognize two kinds of species, monotypic and polytypic.' A monotypic species contains a single pattern of genetic composition, usually because it is a single population that occupies a single, environmentally unified lebensraum in which interbreeding is easy from one end of its territory to the other. Monotypic species are in the minority. A polytypic species, on the other hand, is broken up into a number of separate populations, each occupying its own territory. Usually these territories adjoin each other but are partially separated by environmental barriers. Gene flow across the barriers is infrequent enough to permit the development of separate genetic patterns but frequent enough to prevent the different populations from becoming individual species. When these barriers become absolute, local speciation can occur. Once a new species has arisen, it is likely to expand into a number of territories, where adaptation to new conditions will be rapid. This is undoubtedly what happened to our ancestors once they had acquired the erect posture and begun to use their hands for something beside locomotion and their mouths for something other than feeding and biting.
Regional populations of a polytypic species, once it has become established and has spread, are normally allopatric, a term which means simply "occupying different territories." If they were ntft allopatric, they would compete with each other for food, and one would drive out or absorb the other. Normally the one longest in situ has the advantage over newcomers because it has adapted itself to its new environment by favorable genetic changes, unless a geographical principle is involved, as in the case of isolated populations like those that arise on islands. Because they evolved with out competition, such populations are usually vulnerable when their territories are invaded by newcomers which evolved on large continental areas where competition is keen.
Related species, however, can be sympatric, which is zoologese for saying that they can occupy a single territory without interfering with each other, just as zebras, wildebeeste, and giraffes feed together on an African plain. Sympatric occupation is the rule for animals that belong to different genera, families, orders, and even higher categories of classification, which is why we have regional faunas. It is not very common among closely related species because they usually compete for food.
Whether or not related species are sympatric or allopatric depends to a large extent on their eating habits. If a species specializes in a narrow dietary range, it can coexist with another that specializes in a different range. The Australian koala lives essentially on the leaves of a few kinds of eucalyptus, the presence of which limits its range but allows it to coexist with other species of marsupials on the ground below; the giant panda of western China subsists largely on bamboo shoots whereas the smaller red panda eats a variety of foods.
Animal species that specialize in food are called > stenophagous, the Greek term for narrow-feeding. Those that eat many kinds of food are called euryphagous, or wide-feeders. Like any other specialty, stenophagy permits a rapid expansion in a narrow milieu, but it is not the road to evolutionary success. Euryphagy involves an animal in heavy competition, but if it survives, it has a better chance of expanding over areas with differing food supplies, and of undergoing further speciation.
In the case of man, he is euryphagous and always has been. Man can eat roots, succulent leaves, fruits, berries, eggs, and flesh. Except for grass, he can eat virtually everything that other animals eat, and this puts him in competition with many other species and with other populations of his own and related species.