Time and again, my sociobiological colleagues have upbraided...

Time and again, my sociobiological colleagues have upbraided me as a turncoat, because I will not agree with them that the ultimate criterion for the success of a meme must be its contribution to Darwinian "fitness". At bottom, they insist, a "good meme" spreads because brains are receptive to it, and the receptiveness of brains is ultimately shaped by (genetic) natural selection.