The movement of people from place to place is a maj or determinant of the biological structure of human populations. Such movement injects new genetic, physiological, and morphological variability into the recipient populations. It may also deplete these sources of biological variation from the non-migrating donor population. Rural-to-urban migration is one of the most prevalent types of human movement. The extent of this type of migration in today's world is enormous. In 1800, there were about 25 million people living in urban areas. In 1980, there were about 1.8 billion. By the year 2000, it is estimated that this number will rise to 3.2 billion, a 128-fold increase in two centuries (Rogers & Williamson, 1982). In contrast, the natural increase in total world population will only be 6.4-fold in the same two centuries (one billion people in 1800 to 6.4 billion in the year 2000).

This chapter reviews the biological consequences of rural-to-urban migration for human populations. The study of the process of rural-tourban migration and its biological effects is important for four reasons. First, it entails movement into a habitat and an ecological niche – the city – that is evolutionally novel for our species. Secondly, it is the most common type of migration that has occurred in all periods of recorded history (Smith, 1984). Thirdly, it is occurring more rapidly today than ever before, especially in the least developed nations of the world (United Nations, 1980).