Regardless of geography or reason, subpopulations that refuse to interbreed become separate species. Over time, certain adaptations create even greater differences between these species.
Even though the sources of genetic diversity are quite different between bacteria and sexual eukaryotes, the process by which adaptive diversity spreads and triggers ecological differentiation seems very similar. ~ American evolutionary biologist Jesse Shapiro
Speciation by timing is most mysterious. The closely related star coral and boulder star coral differ mainly by different timing in release of sperm and eggs, by 1.5 to 3 hours; enough to preclude cross-fertilization, given the ocean currents. How the schedules diverged is not known.
Coastal California pine speciated similarly. Monterey pine anthers (which release sperm) become active several months before Bishop pine stigmas (which hold eggs) become active, and so the 2 pines do not mate.
Pink salmon only breed in alternate years, after spending 2 years in the high seas. The salmon remain, for now, a single species, but in time may diverge by this reproductive isolation.