Imagine that part of a population of flies is blown from the california coast to an offshore island. the island flies have no contact with the mainland flies for 10,000 years. then an earthquake rearranges the landscape and the island is rejoined to the mainland. the former island flies can now mingle freely with the mainland flies. if attempts at mating between flies from the two groups are successful and the resulting offspring grow up strong and healthy and have offspring of their own, you could conclude that

Speciation didn't occur over the past 10,000 years

Explanation:

We conclude this, since the two populations could mate (if speciation occurred, there would be no reprodution).

For example, in allopatric speciation which occurs as a result of geographic isolation, the part of population becomes physically separated from the initial main population. There is no gene flow between these two populations and as a result the two populations  reach a high level  of genetic divergence. They can no longer interbreed (reproduction between them) which means they become two different species (speciation).

New populations evolve as result of mutation, genetic drift and natural selection.

not sure if this is what you'r looking for nnew= nold