[Physicist]

You are right on this one -since I mentioned I wanted to keep "coral larvae coming again to Lord Howe" outside the scope of my answer, I should have used an extended scope for "from scratch", including also the coral that eventually arrived there.

So why isn't that sufficient to explain the coral on Lord Howe Island?

[Truth666]

coral doens't interbreed - not a good startpoint for theories on evolution during the last 10,000 years ...