I'm by no means an expert, but like most people I have my opinions. (Hey! Can I help it if I'm usually right? )
What we term "conscience" is, I believe, primarily a learned response. Primarily social, though religion (which is also social) can play a part. But I also think there is an inherent, evolved sense of right and wrong. We see such concepts in the animal kingdom frequently: a mother protecting her cubs, a herd banding together for protection. There have even been documented cases of groups of animals working together against predators, protecting and even saving some of their members. Altruism, sharing, even self-sacrifice are not uncommon in the animal world. So at least some of our "conscience" comes from our genes.

I think people tend to share whether they have plenty or little. Those who have little seem to share a greater percentage of what they have than those who have plenty, sometimes. But I put those disparities down to the vagaries of human nature. Some people are selfish, some are not. In a post-apocalyptic world, those who are selfish will become social outcasts, I think, while those who can form functioning social groups will share what little they can accumulate within that group! Outsiders will be shunned, for the most part. There just won't be enough to share outside of the group. And as resources grow smaller, so too the sizes of the groups.

As civilizations developed, social groups went from small family groups, through tribes and clans, city-states and eventually countries. As civilizations decline, I think the trend will reverse, social groups will become smaller, and the sharing will be more restrictive. Just my opinion, for what it's worth.