Evolutionary psychology on compassion: More stuff you do not need to know
Robert ("Non-Zero") Wright explains the "evolution" of compassion
here. No wonder no one believes evolutionary psychology.
The reality is that some people never evolve any compassion at all, but compassion comes naturally to others - even if it hurts them. Most people can learn it, often through their own suffering.
If a gene were involved, we could predict who would have compassion, but we usually can't.
Get past that, and welcome to the world of useful human psychology. See Clive Hayden's comments here.