A single human, though, can NOT undergo a speciation event and create a new species. At most they would be considered sterile.
A speciation event is only such 'cos the new species can not sexually reproduce with another species.
If a single human underwent a speciation event from the rest of humanity - it would DIE OUT - as it could only produce offspring with someone of the same species!! And there wouldn't be any.
So a new species requires at least TWO people from the same species in order to survive and propagate.
Also, if you have a chain of mutations from A to B to C, and C can no longer reproduce with A and are thus different species - then surely B, who can reproduce with A
AND C, must be a member of TWO species???