I recently learned something about selective breeding of chickens:
If you repeatedly select for the individual chicken that lays the most eggs, generation after generation, then egg production goes _down_. This is because the chook that lays the most eggs is the dominant chook, and selecting for dominance results in a flock of chickens that are hyper-aggressive and waste a lot of energy fighting.
Conversely, if you selectively breed chickens at the level of the flock, selecting for flocks that have overall better egg production, then you select for _cooperative_ flocks of chickens and this is the best way to optimise overall egg production*.
I think that humans have been selected-for at the group level more than the individual level, and this is why it's so critical to preserve tolerance and teamwork at the societal level. Groups of humans that can't cooperate are out-competed by groups that can.
* I'm not advocating for chicken factory farming 🙂