How can normalizing selection (stabilizing selection) be involved in speciation?

In Mayr’s book What Evolution Is, he discusses about normalizing selection in rapidly evolving lineages.

"However, normalizing selection is equally active in rapidly evolving
lineages."

What is meant by "rapidly evolving lineages" and what examples can be found about either the rate of evolution in population where normalizing selection is the dominant evolutionary process or the fact that normalizing selection can lead to speciation. How important is normalizing selection compared to disruptive and directional selection?