Answer
All human populations can freely interbreed, producing healthy, fertile offspring. Any isolating mechanism could in theory lead to speciation in future-- say, if technology disappears and populations are isolated from one another, genetic drift might eventually lead to speciation.
Work Step by Step
Review the biological species concept and the first part of the chapter to see how humans qualify as a single species. Then review isolating mechanisms to try to find one which might eventually occur for humans.