HIV & cities

Rise of colonial African cities kick-started AIDS pandemic: scientists:

“As there must have been many opportunities for such transmission over past millennia, why did the AIDS pandemic not occur until the 20th century?
“The answer may be that, for an AIDS epidemic to get kick-started, HIV-1 needs to be seeded in a large population centre.”

There are many ways that selection can operate on populations. There can be intraspecific competition, interspecific competition, as well as variation in fitness driven by changes in environmental factors. All of these are often in flux in a complex dynamic system characterized by feedback loops (recall that the local environmental context is often strongly shaped by biological processes). Last year John Hawks and Greg Cochran made their case for human adaptive acceleration by focusing on the rapid increase in population size due to agriculture. This parameter alone can in theory result in a greater yield of beneficial mutations upon which selection can operate. But in their paper the authors did allude to rapid changes in ecological pressures due to the protean shifts in human culture starting with the shift toward agriculture. If culture can be thought of as environment, then many would make the case that rate of environmental change has been increasing as a function of time over the past 10,000 years, and presumably the buffeting of human populations by selective pressures.
I think most people would agree that disease is a big part of this. Agriculture results in the ability of many more humans living at high densities on a given unit of land. Cities take this to their natural conclusion. Only in the past few years have the majority of humans alive been resident in cities, but as our century progresses the proportion will keep increasing. And with it, we must be ever vigilant against the specter of infectious disease.