As I see it, there are two primary variables which control the way humans evolve: climate and agriculture. One has purely natural causes, one is purely social because humans are social/political animals. Even Heartiste is now picking up on the idea that agricultural society causes dysgenia:
There’s a theory floating around alt-blogs that human IQ in the developed world has been steadily decreasing since about the dawn of agriculture. The working hypothesis is that agriculture enabled dense urban life to develop, and cities are known population sinks (lack of space/high cost/disease vectors all contribute to lower fertility rates in cities).
The thinking goes that cities attract smarter people, who upon settling into urban mimosavilles promptly forget the Darwinian Prime Directive and fail to reproduce themselves in sufficient numbers. 1.5 sprog per hipster village yenta is a recipe for extinction. (Which is not necessarily a bad thing.)
This is actually not my logical explanation for this. Agricultural society, unlike gender, is a sociopolitical construct.
Agriculture causes dysgenic breeding, not r-selection, because through economic specialization and overproduction of group resources it promotes the growth of parasitical humans who merely survive on the excess, having neither social nor natural fitness.
A more cynical person than myself might suggest that the breeding of these useless malcontents is actually encouraged by Top. Men. as a blunt weapon used against the productive class in the great alliance of the high and low to enslave and exploit the middle. Fortunately, I’m not, like, this negative person, or whatever.
Warm weather, contrarily, supports a very large population with a great deal of species diversity. Think Amazon jungle. An extraordinary number of little bug species, some slightly larger things, a few pretty big things. And everything’s poisonous because everybody’s trying to eat everybody else (r-selection).
(Actually, I have a recurring nightmare where I’m in the Amazon and I try to take a step, but there’s a snake where I want to step, so I try to put my foot back down, but there’s another snake, and then there’s a giant tarantula on my shirt, and I get all anxious so I try to climb a tree to get away from all this wildlife, but there are also poisonous bugs on all the tree limbs. If you want a mental picture for the modern neanderthal hybrid, imagine a saber-toothed tiger trying to survive in the Amazon, eating bugs and constantly getting stung by little semi-poisonous bugs and snakes.)
Compare this to the two temperature extremes: hot and cold. A hot world is just a desert, with little biodiversity, and no animals much larger than a skinny rabbit, fox, or scorpion. Nothing large and warm-blooded can survive because their surface areas aren’t large enough to expel heat from their volumes. A cold world also has little biodiversity, but in the form of macrofauna. This causes k-selection.
Whoops, late for work.