Having shown to his satisfaction, if to no one else's, that there's almost certainly no Deity, Professor Dawkins next assays to consider where the whole business of religion came from anyway. He concludes in chapter 5 that religion is an evolutionary misfiring, or by-product, of something else. By way of explanation he invites us to consider the self-destructive behavior of moths which spiral into a flame. Why do they do this? Well, over the eons they have evolved light sensors that enable them to navigate by the moon and the stars. These luminous objects are very far away and seem to the moths like stationary beacons in the night sky, but when artificial light was introduced into the moths' environment the lights were so close that they appear to shift as the moth moves, requiring any moth that's fixing on them to also deviate from a straight-line path to keep the light at a fixed point. The result was that the confused moth takes a spiral path toward the light, or something like that.
Professor Dawkins doesn't trouble himself to explain why moths need to navigate by celestial objects in the first place since they don't migrate and spend much of their adult lives confined to a localized area. When they do travel it's along chemical trails of pheromones produced by females. So why would they have evolved these light sensors? But this is a digression. His point is that the spiraling behavior of moths is really a by-product of something else and that likewise religious behavior in humans is a by-product of some other behavior which evolved because it conferred a selective advantage.
Dawkins avoids the simpler explanation that religion itself confers a selective advantage and thus humans evolved it. This is an unacceptable explanation, even if it has the merit of being less cumbersome, because if it were the case Dawkins would have to admit that atheism is a maladaptive mutation, and he certainly doesn't want to have to make the case that atheists are genetic mutants.
So what is religion a by-product of? It turns out that all we have are guesses, but one guess is that natural selection produced in children the tendency to believe whatever their parents and other elders tell them, a bit of news that'll surprise most parents. This aids the children in survival. Parents tell kids about God so kids grow up believing in God.
It's not clear whether children lose this gullibility as adults, but if they do why do they retain belief in God when they don't retain other childhood beliefs like belief in Santa Claus or the tooth fairy? Why, too, do so many people come to belief as adults? Why isn't Dawkins himself religious since he received a lot of exposure to it when he was a child? If one's belief about God is a result of psychological misfires in the brain then isn't atheism also a result of such a misfire and thus can't we conclude that atheists don't believe in God because of evidence but because of some psychological quirk? Professor Dawkins doesn't help us with these questions. He's in too much of a hurry to rush on to his next grievance against the religious - their irritating tendency to be dualists - which he also sees as a holdover from childhood.
There are other guesses as to what religion is a by-product of, of course, - love, projection, wishful thinking - but the general idea Dawkins wants to advance is that it's a by-product of something advantageous for survival.
Another reason why religion has survived and diversified has to do with memes. A meme is like a mental gene. It embodies an idea or set of ideas (called a memeplex) that spreads through a culture. For example, the belief in human rights is a meme, as is any belief. Natural selection acts to weed out unsatisafactory memes in the same way it culls unfit genes. Religious beliefs are also memes which have spread, not because they are true, but because they afforded those who held them some survival advantage.
Dawkins is obviously pleased with this explanation for the widespread occurrence of religion even though the theory is completely speculative and even self-defeating. After all, if all our beliefs are merely memes then so is atheism a meme, so is Darwinian evolution a meme, and, indeed, so is belief in memes a meme.
He closes the chapter with a rambling discussion of cargo cults, religions that spring up among primitive people when they encounter for the first time the "magic" of modern technological society. He notes that ignorant people often regard the radios and machines of visiting Europeans as being supernaturally produced because they never see them being made or repaired. None of this, like much else in the remainder of the book, has anything at all to do with whether God exists.
Dawkins is apparently convinced that the existence of God and the manner in which some people express their homage to that God are one and the same thing. He seems to think that if he can discredit religious beliefs then he can discredit belief in God. Perhaps this is the strangest "God delusion" of all.
Critiques of previous chapters in The God Delusion can be found here:
RLC
