A common creationist argument against evolution is the existence of morality. How and why do we use the terms good, evil, right, wrong, bad?
This video has the start of an answer. I have elsewhere used a similar argument -without Dawkins' eloquence or the neat animations - and am glad to see it so clearly described.
As I see this, I think:
As a population, humans are comfortable with or capable of carrying perhaps 5% cheaters. In the example above, if one person cheats once in a while, s/he gets a great benefit but no one suffers much of a loss. Losing one fish a month isn't that bad.
This does explain how many people (I think nearly all) will cheat or steal in rare instances but few do so most of the time.
It doesn't really explain how we feel or experience good, evil or conscience. In my opinion, this is an unconscious psychological effect. Just as women are said to be attracted to different types of men depending on their menstrual cycle but not be aware of their change in judgement; just as apparently rational men and women irrationally treat tokens as less valuable than money ( see Dan Ariely on Youtube or his books. The talk I haven't had time to find was one about casinos using tokens as guests don't attach the same significance to them.) despite their equivalent value, people feel pangs of conscience as a result of worry or nervousness.
I want to explore this further but this is where I am heading.