What Prevents Macroevolution?

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What prevents macroevolution? This says it best

Loci that are obviously variable within natural populations do not seem to lie at the basis of many major adaptive changes, while those loci that seemingly do constitute the foundation of many if not most major adaptive changes are not variable.- John McDonald, “The Molecular Basis of Adaptation: A Critical Review of Relevant Ideas and Observation”, Annual Review of Ecology and Systematics: 14, 1983, p77-102

IOW the mutations responsible for microevolution are not the same genes that can possibly produce macroevolutionary change. And the genes responsible for microevolution are variable while the genes  that can possibly produce macroevolutionary are are not.