But the new gene didn’t create the new trait. It just made it easier to keep a trait that a change in the environment made valuable. The gene didn’t drive the train; it merely hopped aboard. Had the gene showed up earlier (either through mutation or mating with an outsider), back when you lived in the forest and speed didn’t mean anything, it would have given no advantage. Instead of being selected for and spreading, the gene would have disappeared or remained in just a few animals. But because the gene was now of value, the population took it in, accommodated it, and spread it wide.
I don't see how this is incompatible with the Selfish Gene. I thought selfish gene theory described why organisms seek to reproduce, and why altruism and selfishness both act together to further the organism and it's genes along. It described why life does what it does, and that genetics was the driving influence of all that, not individual genes.