Caesarian Sections influencing Evolution

Birth weight is always the classic example of Stabilising Selection in Evolutionary Biology.  Basically, humans have difficulty giving birth; there is a trade off between large human heads and small fused pelvises required for bipedalism and human babies get stuck in the birth canal.  

Selection operates on survival of the fittest, or death of not fit.  Those babies that are too small may be born easily but are so underdeveloped they are unlikely to survive. Those babies that are too big may be well developed but they get stuck during birth, and are unlikely to survive either by being starved of oxygen or fatally haemorrhaging their mother.

So the perfect size is in the middle and selection makes sure that these individuals survive, reproduce and put their genes into the next generation.

This is an interesting article that shows that interventions in birth, ie Caesarian sections, is causing directional selection, not stabilising, and birth weight is increasing.

http://www.bbc.co.uk/news/science-environment-38210837