Rapid speciation after the flood

So lets say all life of earth arose by normal processes in a few hundred years when approximtely would you expect life to have reached the diversity that we observe today (or observed pre human intervention). Surely this period of low biodiversity would leave traces like reduced diversity pollen. I believe there are deposits that contain pollen that extend well into pre flood times so they sould also cover this post flood time.

I am no expert on flood geology but surely there would have been large amounts of exposed sediment lying around after the flood and with no plant cover this would have made ideal conditions for rapid erosion and deposition after the flood. Also were there even plants on the arc? as far as i am aware there is no mention of them being on the arc and they were just left to suvive as they could in which case certain plants (those with hard shelled seeds such as coconuts and plants where the seed is usually waterborne) would have dominated the post flood world. regarding rapid speciation apparently it would be possible to get from a type of grass to modern wheat in about 25 years if you knew what you were doing (heard it on the radio and can't find ref annoyingly).