Some might call me a "social Darwinist". Let's face it, evolution is all about adaptability and our monkey brains evolved to meet the demands of a time 50K years prior to our own. Someone wonders why we're all stressed out and on meds? It's no secret to me ... we're not evolved to meet the conditions of this time - and it's only changing faster.
So I'm pretty comfortable with those who can't hack it not being able to produce as many offspring as those who can. That's how the species improves as a whole. But before you going thinking I'm some kind of dick, just because I feel this way doesn't mean I don't want to help. Helping is in my nature. It's just that I prefer giving a hand up to a hand out.
I see it like Shantideva, the 8th-Century Buddhist monk, himself godless, saw it. He said:
May I be a guard for all those who are protector-less,
A guide for those who journey on the road,
For those who wish to go across the water,
May I be a boat, a raft, a bridge.
For all those ailing in the world,
Until their every sickness has been healed,
May I myself become for them
The doctor, nurse, the medicine itself.
I understand that what benefits others benefits me as well. There's essentially nothing "altruistic" about altruism. We're all interconnected, and once we realize that, we understand that helping others helps us to. We may disagree about the means with which we help others (libertarian charity vs government socialism), but humans are so successful partly because we help each other and form social networks.
So, dichotomous individual that I am, I believe altruism is perfectly compatible with atheism and, in some respect, even more virtuous as I'm not being altruistic because I'm afraid of a god who will punish me because I'm not.
I do it because it's just good to do.