Medicine & HealthRise 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.
Comments
Paul Ewald is the man who talks about this. In conditions where it can spread easily the strain of a disease will incapacitate or even kill quite quickly, but the same disease in conditions where it finds it difficult to spread will evolve into a very mild form that allows infected person to be up and about as a disase vector. Disease spreads easiest in cities. A disease that speads easily will be not be mild.
I couldn't track down a proper reference but a poster on a fitness site said that being overweight protects against certain infections, mainly TB
But I think I read somewhere that the 1918 flu which found it easy to spread amoung soldiers killed mainly young poeple partly because they had strong imune systems. Is this right?
Posted by: Tod | October 3, 2008 11:33 AM
The idea that diseases spread faster in major urban centers is an epidemiological truism, I think. You could add that long-distance trade networks between cities spread diseases more widely. In Africa truck drivers are supposedly the vectors (a girl in every port). An airline steward was apparently the one who brought AIDS to the US.
Of course, even though the theory behind this is conventional, applying it to the specific African context is still interesting.
Posted by: John Emerson | October 3, 2008 2:03 PM
The Atlantic piece with Ewald & Cochran now seems to be freely available.
Posted by: TGGP | October 3, 2008 6:19 PM
I might be set straight on this, but I don't see that easy spreading necessarily implies virulence - this assumes a trade-off that may or may not exist if transmission can be achieved without increasing virulence (common cold might be an example?)
Posted by: Ben | October 3, 2008 6:54 PM
I think decreased contagion implies a cost for virulence which wouldn't necessarily exist for more contagious diseases. Also, diseases don't exist to be virulent but contagious (in a Darwinian sense), the virulence is just their way of ensuring contagion.
Posted by: TGGP | October 5, 2008 2:41 AM
I think an important part of Paul Ewald's argument is that natural selection works several orders of magnitude faster in micro-organisms. The effects can be astounding as Robert Sapolsky's case in point Toxoplasma gondi shows.
The following is absolutely true. Our family cat is a smaller than average neutered male, he is 12 years old and overweight whith a visibly protruding belly. A rival tomcat hase been scent marking our garden, even I can smell it. Last week he killed a rat in the garden for the second time, he didn't eat the rat though.
Posted by: Tod | October 5, 2008 1:35 PM
Japan was theorised to be a unfavourable enviroment for rapid spread of HIV and hence favourable to a the natural selection of a less virulent strain, Science Links Japan article has the calculations ("A simulation sheds a light on the present HIV epidemic"). However "Japans HIV infections hit record high in 2007" (china.org.cn)suggests this will not turn out to be the case.
The theory might still be a good one if the Japanese have more sexual partners than was thought, they do have a very surprisingly low 2d4d ratio (with possible implications for immunity). Theorising about the Japanese may not be as straightforward as about HIV.
Posted by: Tod | October 7, 2008 11:25 AM