Speciation is defined as the formation of new and distinct species in the course of evolution. Speciation involves the splitting of a single evolutionary lineage into two or more genetically independent ones.
here is real life example
A hurricane washes the bananas and the immature fruit flies they contain out to sea.
The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature
and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland
and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — any
fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
2) So we meet again:
When another storm reintroduces the island flies to the mainland, they will not readily mate
with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland
flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has
split now that genes cannot flow between the populations.