But how do we "demonstrate" either of these points? Justin has made this point before: sure, we can create a just-so group selection story to explain how altruism persists, but that does not meet the threshold of scientific evidence in even "soft" sciences like psychology. For instance, if I want to study the effects of cognitive behavioral therapy on anxiety disorders, I can set up a rigorous experimental study to assess the claim that cognitive behavioral therapy reduces anxiety symptoms.
^^^^^^
CB: However, I believe it is true that Darwin did not demonstrate directly that any trait of any species came about through natural selection. He didn't prove that the tail of the giraffe evolved to swat flies. How could he from fossils ? He speculated that. Or he didn't have any direct proof on the selection process that resulted in the eye. He speculated. So inability to directly show that "altruism" came about through natural selection in humans is no different than the whole of Darwinism.
About the only thing directly proven might be modern viruses or bacteria evolving immunities to medicines. Or there is the study of different coloring on modern moths as camouflage as the colors of factories in England darkened from soot. Then Anthropologist Frank Livingstone demonstrated that heterzygosity for sickle cell anemia was selected for in humans in West Africa when malaria became epidemic due to standing bodies of water from newly introduced agricultural techniques. But these are all modern. Proof of ancient selection for traits is all "just so" stories and relatively soft.