yeah, it's not obvious from this quote, but having read the book, I know what he means. The utility function of the tree is the sum, over all individuals, of the fraction of genes that each other individual has in common with it. He constantly talks as if plants, chromosomes, insects etc. desire to maximize this number.

I think it works, because when an organism is in its environment of evolutionary adaptation, finding that a behavior makes this number bigger than alternative behaviors would explains why the organism carries out that behavior. And if the organism does not carry out the behavior, then you need some explanation for why not. Right?