The phrase demographic transition came up this week during a discussion about cultural impacts to human evolution. A good working definition came from an article by Milot and Pelletier (2013). I have copied some passages below:
"The demographic transition is one major step in the history of modern human populations that impacts evolutionary processes. It typically begins with a sharp decrease in mortality arising from technological and societal improvements in living conditions, such as hygiene, disease prevention and health care, followed by a decrease in fertility. Typically this causes population size to grow then to stabilize or even decline towards the end of the transition. While the reasons for the decrease in mortality are well understood, the decrease in fertility still puzzles demographists, social scientists and biologists."
"Ultimately, the demographic transition is just one amongst the panoply of new playgrounds offered by the modern human life-style to natural selection. Who knows what other playgrounds can emerge from features such as exposure to newsynthetic molecules, large-scale mobility, changing climate, and so forth."
"Understanding how culture and modern life-style lead to new selective environments should provide major insights into human evolution. The next challenge will be to assess whether selection and evolution can have any concrete impact on human affairs such as public health, demographic forecasts or mate choice."
Milot, E., & Pelletier, F. (2013). Human Evolution: New Playgrounds for Natural Selection. Current Biology, 23(10), R446-R448.