Exchange is to cultural evolution as sex is to biological evolution.
Exchange is to technology as sex is to evolution.
These ideas are, I believe, basically incorrect and misleading. Most sex in biology involves one donor and one recipient. Exchange refers to swapping things, with both parties involved giving and receiving. That's a pretty significant difference.
In the book, Ridley defines the term "exchange" as follows:
Exchange – call it barter or trade if you like – means giving each other different things (usually) at the same time: simultaneously swapping two different objects.
This is similar to the dictionary, which defines "exchange" as meaning:
an act of giving one thing and receiving another (esp. of the same type or value) in return.
Sex is different. Sometimes it involves a mutual exchange of resources - but it is a broader concept. Typically, in biology, sex involves one-way exchange - with the male acting as donor and female as recipient.
Of course, there is an equivalent of sexual recombination in cultural evolution. However, exchange is just one type of sex - among many.
Ideas have sex mostly in people's brains - and inside computer systems. Exchange need not be involved - since culturally-transmitted ideas can get into brains in other ways - e.g. by observing the behaviour or another, or by acquiring an artifact from them. Once there, they can combine with all the existing ideas - both individually-learned and socially transmitted ones.
The alleged equivalence between exchange and cultural sex seems basically wrong to me. Sex is a significantly broader category than exchange is. Muddling these concepts together seems much more likely to lead to confusion than enlightenment.