Their interaction - and relative weight - impact so many debates today. But one of the most fascinating digressions into the subject that I've read recently is Gregory Clark's "Farewell To Alms." Benjamin Friedman has an excellent review of the book in the NYT today, that captures both its merits and its obvious weaknesses. Money quote:

Darwinian evolution is usually seen as a process that works over very long periods of time, with consequences for humans that we can observe only by looking far into the past. By contrast, Clark’s explanation for the Industrial Revolution is a change in “our very nature our desires, our aspirations, our interactions” that occurred within recorded history, indeed within the last half-dozen centuries. His idea also stands in contrast to the entire orientation of Enlightenment thinking, including Adam Smith’s, toward accepting human nature as it is and asking what social institutions would allow humankind with that nature to flourish (as Rousseau put it, “men as they are and laws as they should be”).

The book doesn't finally persuade entirely; but it sure provokes.