A lot of oversimplification. It assumes that what drives an organism is only its rewarding itself.
For most the nearby pack is as important as themselves. They are your family. Thus your reward is having a stronger family.
Assume the classical prisoner dilemma:
If A and B decided not to talk, then A and B gets 1 year of other charges.
If A and B decided not to talk, then A and B get 10 years.
If A decides to talk, and B does not A gets free, B gets 20
If B decides to talk, and A does not B gets free, A gets 20
But what if your organization is important to you? Or what if deciding to talk means that you will get wacked by the other members of the organization? In real life this happens.
If your son is going to go to jail and you can prevent it by taking the blame? Many people will choose to help their son. The survival of the fittest model is flawed, as it does not take into account that not all that drives a living being is their own survivability. Sometimes the survivability of those around you (aka your family) means more to you.