I've read about positive assortment as an explanation for the evolution of cooperative mechanisms, but what about the evolution of positive assortment itself?

I get that it's possible positive assortment need not necessarily have begun as a hereditary trait. It might have resulted from the structure of the local environment of replicators or from the structure or movement patterns of the population of replicators, but, at some point, intuitively, it seems likely positive assortment has become a heritable behaviour, both genetically and culturally. So how, why, when?

I have a vague feeling maybe it's analogous to Eigen's Paradox? (I got this idea while reading E Szathmary, but I'm not sure he exactly said this, or not quite in this context.) Eigen's Paradox, as far as I understand so far (this is not my field, I just googled it last night!), is that the intrinsic error rate of RNA or DNA replication without correction or repair mechanisms is too high for ordered information to evolve beyond about 100 bits, but the genetic code for correction or repair mechanisms is itself longer than about 100 bits, so it's a chicken and the egg paradox. Szathmary's suggestions to resolve this paradox, as far as I understand, were about effects of the structure of the local environment or structure of the population external to the individual replicator first, the mechanism modifies the niche and changes the selection pressures, then pre-adaptations that promote the mechanism evolve.