natural selection and head size
How is it that humans have continued to evolve with increasing brain/head size when having a larger head leads to more complications and death in child birth and the fact that we use such a small portion of the actual brain. It seems that natural selection would be acting in a means to evolve more efficiently used brains, not simply larger ones.
First, I'd like to clear up a misconception. Human beings (like all other animals) absolutely use 100% of their brains. It is an old but persistent myth that we only use a small portion of our brains.
As for why would humans evolve large heads when it can sometimes cause complications in child birth, is because evolution doesn't cause animals to become perfect-- rather, just good enough to avoid extinction. The fact that human beings haven't gone extinct from child birth-related deaths is proof that our heads are not too large. And those females with too small of pelvises *did* die in child birth, thereby removing their genes from the pool. That may sound harsh, but I do not mean it to be. This is simply the way evolution works. Traits which are deleterious to a species usually die off and remove themselves from the gene pool within a few generations.
Consider this: thousands of human beings choke to death every year, because we breathe out of the same hole we eat. This is a terrible design for an organism which requires a breath of oxygen every few *seconds* to stay alive! But as unfortunate as those deaths are, they do not threaten the species with extinction, nor do they affect reproduction. In other words, if a man dies from choking *after* he has had children, then natural selection wouldn't play a part.
As for natural selection favoring more efficient brains over larger ones, that is somewhat true, though intelligence is not always the most important trait. Most animals, in fact, favor defensive or offensive adaptations (like claws, good eye sight, etc) rather than intelligence. Also consider that the whale has the largest brain on Earth, not humans, but it obviously is not as sophisticated as ours.