A population is all the organisms of the same group or species who live in the same geographical area and are capable of interbreeding. In ecology the population of a certain species in a certain area is estimated using the Lincoln Index. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals from other areas. Normally breeding is substantially more common within the area than across the border. In sociology, population refers to a collection of human beings. Demography is a social science which entails the statistical study of human populations.
Population growth was not always linear. Famine, war, or disease often decimated local or regional cultures. In fact, as population grew, another pattern of human history emerged - natural and human-induced disasters that killed large numbers of people. Historically, human numbers were greatly limited by disease. This was especially true as growing populations became more concentrated in cities, where people were more easily exposed to infectious agents. (It takes a minimum population for diseases to sustain themselves. Measles, for example, requires about 7,000 susceptible individuals to assure its survival. A regional population of from 300,000 to 400,000, with regular contact, is probably the minimum necessary to sustain that disease.) Plague devastated Athens in 429 B.C.E., and large parts of China 200 years later. It ravaged the Roman Empire from 160 to 184 C.E., killed a large percentage of the population of Constantinople in 542 C.E., and reached Britain by 547. By the end of that cycle in 594, the population of Europe had been halved. Plague returned periodically, peaking in the fourteenth century, when it killed an estimated one third of the population of Europe. Other diseases were equally devastating, if more localized. When Spanish...