Wouldn`t natural selection - select "Against" bigger brain humans?

Question

Since the human brain is so big - sometimes causing female pelvics to break while giving birth - wouldn`t natural selection work against humans evolving to have bigger brains?

Or are my facts all wrong? Let me know, this question comes from a gap in my understanding.

Answer 1

You can have selective pressures acting independently in a variety of ways, even in 'opposite' directions. Yes, one possible response to the problem of getting the head through the birth canal is to have smaller brains - which is one reason why human babies are born so relatively undeveloped. So the larger brain increases the risk to the mother and the infant; still, it's big now and has increased significantly over the time span of human development, so we would guess that there are stronger pressures favoring larger brains. A crude estimate of the benefit of larger brains is to look around - how many larger-brained humans are there, compared to smaller-brained humanoids, protohumans and other anthropoid primates?

Answer 2

Well natural selection is what it sounds like: traits are naturally selected, because some traits will kill their owners and some will give their owners an advantage. However, remove the "selection" itself, and there is no more natural selection.

In this example, human brains might well be selected against, if mothers died whilst giving birth. However, if the babies head is too big- it is not a problem. A Caesarian will allow the baby and mother to live. Therefore selection is eliminated, so bigger brain humans will not be selected against.

However, our heads are nowhere near too big to worry about big brains being selected against, even if Caesarians did not exist. One integral part of reproducing is the ability to interact socially so that one may meet a mate! This involves complex thought, and so I believe there is much more selection against "smaller" brains.

Answer 3

I've never heard of a woman pelvis breaking while giving birth.

a. For evolution to select against bigger brain sizes, there would have to be equal or greater selection for smaller pelvises in women. If there is a net gain in one or the other, that is what you will wind up having (e.g. bigger brain sizes and bigger pelvises)
b. You do know what a cesarian section is? Well, there is no limit to brain sizes in the modern world. And science has determined that humans still continue to evolve (although whoever said that humans stopped evolving was an idiot to begin with).

Answer 4

As everyone else has said - no - we would suggest it would lead to wider set hips.
Unfortunately society and culture is something natural animal populations don't really have to concern themselves with. So - "being thin is in" - isn't exactly a problem faced by elephants or hippos.
This is where modern medicine kicks in.
Though - it's interesting to point out, that in human sexuality and behavior studies it's been noted time and time again that on average, men inseminate more sperm per ejaculation into women with wider set hips and bigger breasts than the alternatively smaller set women.

Answer 5

What is the selection pressure?
Humans unlike other animals have enormous capacity to reason and manipulate the environment, and themselves, which interferes with natural selection. Perfect example is how we have facilities to care for a person who has been diagnosed with mental retardation. In a normal wild environment, this person will have to fight the elements on his/her own but our humanity intervenes. See, you don't have to eg. hunt for food anymore like our forefathers, which would work against a less intelligent person. I am almost sure that the mentally retarded individual probably has family or social welfare facilities that provides food, housing and other basic needs.

Answer 6

Wouldnt you think that it would just lead to women with wider hips to push all those big brained kids out easier?

Answer 7

according to your reasoning, it would work against women with small pelvics

Answer 8

...or could cause women to evolve with wider hips

Answer 9

In such a case they would do a cesarian. So the answer is no.