I’ve also argued that there are Darwinian elements in human social behaviour, in “Some Basics of Cynical Sociology,” and that’s a premise of much of my cultural criticism. I call this the default form of social organization, namely the dominance hierarchy, since it’s genetically controlled and in a sense antisocial (the upshot of all conservative thinking).
But strict Darwinism, competition and survival of the fittest to pass on genes, applies to biology, not fully to the social level even in animals. Cooperation emerges in social species because those creatures aren’t puppets of their genes. There are degrees of genetic control of the host organisms, and the more freedom (autonomy) a species has, the more surprising and protomoral its potential behaviour.
You say cooperation and friendliness may enhance biological fitness. But there’s the old problem of the free-riders: altruistic members would be exploited and removed before the genes that support their empathy or playfulness would be passed along to the next generation. These prosocial behaviours emerge when there’s already a sociological or even a moral level of explanation required, in which case whether the behaviour supports the transmission of genes isn’t the only relevant consideration. I’d expect to see prosocial behaviour in animals when their genes enable the parents to teach their offspring lessons that aren’t encoded in the genes. The friendliness would be a byproduct of genetically-determined intelligence or of other traits of their body-type.
You seem to want to say empathy and cooperation are selected for directly at the genetic level, that natural selection deems prosocial behaviour to be fitness-enhancing. But I don’t see this kind of behaviour being transmitted across generations without the filter of learning. Once we’re dealing with learning and with mental representations, we’re no longer dealing with just the biological or genetic level, but with psychology, sociobiology, sociology, politics, and eventually morality.