The undeniable existence of altruism always has been a thorn in the side of Darwinism. Natural selection, together with genetic shift, explains pretty much everything - except why a man might jump into an icy Potomac to save an airplane passenger he doesn't know, exert huge energy to help unknown victims of natural disasters, or reach into somebody's parked car to turn off its headlights.
Evolutionary biologists have come up with all sorts of mathematical reasons for altruism - from calculations of relatedness (you'd do more for a first cousin than you'd do for a second cousin), to theories about how the welfare of a particular grandchild depends upon whether the grandmother is a paternal or maternal granny.)
In the end, altruism remains a mystery. But I suspect the way to think about it has to start with a feeling common to us all - the ability we have to actually enjoy another's happiness - even at our own expense.
For example, this evening I watched a nightly newscast (ABC, on this particular day) and learned not only about efforts to respond to Chile's monster earthquake, but the joy of Canadians celebrating their country's hockey victory over the U.S. Sure, most Americans' only think about hockey if they happen to walk past a TV broadcast and glance at a hockey puck flying toward a goal, a puck about the size of the period at the end of this sentence. Sure, Americans like to win. Sure, we care about our team. But still ... to experience, however vicariously, those Canada folks' glee was a joy that was seriously real. Watching the news, seeing the wide grins of little Canadian boys and girls, my smile was nothing but sincere.
I think that kind of empathy is, in the end, from which altruism flows. It's not about genetic math. It's about what humans, and a few other higher animals, can't help. And it is what makes evolution importantly cool - even in the face of the other, far more grim, facts of life. Which this evening I don't care to think about.