It takes so long to learn instincts by mutation and selection, it's intuitively obvious that learning should come first and then be replaced by instinct over time. But Lorenz didn't propose a mechanism for that to happen and I don't know a mechanism either.

The mechanism seems straightforward: evolution by mutation and natural selection. If a species is in a stable (enough) environment, and the learning process is costly in energy, time, or both (and there will necessarily be some cost), then those who don't have to learn as much will be selected for.