Over at Philosophically Made, Stephen Cooper asks:
Is it in human nature to be rational? If one takes an evolutionary stance, then one thinks that evolution thins out irrational points of view.
But then, we get all sorts of behaviour that really isn't conducive to survival.
If I sit on the couch watching TV, how is that good for survival? Am I contributing to human extinction this way? Some might say humans need to rest, to recouperate and give more the next day. But then, wouldn't evolution provide us with a human that doesn't need to rest?
The simple answer is that it's just not energetically efficient to do so (or rather, wasn't in the evolutionary situation in which the strategy of sleeping evolved, way back in the depths of the animal phyla). While there are fairly obvious opportunity costs to sleeping (including the risk of being eaten), they fairly obviously do not outweight the extra expense of being active all the time.
But there's also a great deal of confusion in Stephen's post. Evolution doesn't act on "points of view". It acts on the genes, through their vehicles, individual organisms. Rationality is not necessarily desirable in an evolutionary sense; all an organism needs is the ability to solve common problems in its environment, and as digger wasps show, it need not do this by thinking. And if an environment is sufficiently unchallenging, you can get by without thinking at all - look at trees. In such circumstances, having a brain is simply a waste.
There's also a trace of the biggest confusion surrounding evolution: conflating its values with those of people. Evolution may "judge" behaviour by whether it is "good for survival" (or rather, reproductive success), but we can - and should - have other standards. Rape, murder and cannibalism may all be "good for survival" (if you can get away with them), but it does not follow that we should act that way.