It is important to understand, as one reads the group-selection arguments in Descent, that group-selection is not presented as an alternative to the model in Origin that has the individual as the locus of selective action. Rather, it introduces a second selective field that shapes the destinies of 'tribes'. It is also important to remember that a Darwinian 'tribe' is not just a mob of great apes moving through the landscape. Tribes are institutional structures that exist in the minds of affiliates by negotiation and common consent. The interplay between these two Darwinian systems, one ideational and the other corporeal, was sufficient, Darwin believed, to explain the emergence of pro-social altruism. For Darwin, the tribe seems to be a hybrid construct - partly a physical community of co-operating organisms and partly an abstract sense of identity, obligation and belonging.
The model Darwin presents in Descent and the later editions of Origin has a complex structure. Natural selection had become a bridging concept that not only linked dynamics on three space-time scales (event, conjuncture and deep-time) but could occur simultaneously in two separate contexts. There was one three-level system governing the fates of individuals and another governing the fates of tribes. Altruism, which can only be valorised at the event-level as patterns of co-operation between neighbours, influences the fitness of the social unit as a whole and this communal fitness (or the lack of it) feeds back to modify the selective landscape that shapes the destinies of individuals.
Darwin clearly believed that the selective field that shaped the evolution of tribes, and the selective field that underpinned descent with modification at the individual level must have reinforced each other to act as a synergetic multiplier. The upshot of this two-level interaction was the emergence of a new species - a smart, sociable, altruistic ape. However, it is equally possible, indeed likely, that the traits that would enhance the tribe's fitness would so severely compromise the fitness of carriers that the two selection pressures would cancel each other out. Stable, cohesive tribes containing self-sacrificing altruists could only have emerged in circumstances where the two dynamic systems reinforced each other. There had to be a congruence of selective interests at the levels of the tribe and the individual.
Had these two types of dynamic been locked into a double-bind, with group selection favouring tribes that contained altruists and individual-based selection punishing altruism, then the evolutionary system would have become trapped in a stable attractor that would have prevented directional change. In cybernetics, the difference is often described as that between positive and negative feedback loops. Negative feedback is more likely to act homeostatically, preventing irreversible change and maintaining stable attractors. Positive feedback is more likely to become manifest as a synergetic multiplier that drives directional change. In Descent, Darwin implies that the two selective dynamics must have reinforced each other to create a positive feedback loop.