[kelipso]

Well more typical would be something like the tribe that survives into the future has always removed women's ability to not get pregnant while the other tribe dies off.

I guess there are two types of social evolution, or evolution in general... One is where one tribe changes its customs over time, so evolves through intra tribe competition. Another is where you have multiple tribes with different customs, so evolves through inter tribe competition. Reality would be a mix of these two models.