Some species, not currently divided in any subspecific taxa, have a very disjunct distribution with thousands miles separating the known present areas where they occur.
Two examples are Mesembryanthemum nodiflorum and M. crystallinum, both occurring EDIT: AS NATIVE in the Mediterranean basin, the Arabian area and in southern Africa. As regards, since Southern Africa is a known center of biodiversity for mesembs one could be allowed to wonder if plants growing in the extra-African populations are native or not.
Similar examples regard plants that were known as trans-Atalntic but eventually turned out to have been introduced in Europe.
Xanthosia atkinsoniana in Australia. Two completely disjunct populations on opposite sides of the continent, thousands of kilometres apart. Evidence of a historically contiguous population or actually two cryptic species? No idea
Rhipsalis baccifera – range is Madagascar, Seychelles, Mauritius, Reunion, Sri Lanka.
All other Rhipsalis spp. – range is South America and into Costa Rica.
All other Cactaceae – range is in the Americas.
Rhipsalis baccifera is the only cactus native outside the Americas, and is endemic to islands of the Indian Ocean, which is the only ocean with no shore in the Americas.
Bromeliaceae… 3700 species known, all native in the Americas… Except for one, Pitcairnia feliciana, which is native to western Africa… Some very old relict I guess from when those two land masses were still one?
Start in central Texas (“Lost Maples”), jump to Guadalupe Mountains (about 400 miles in the car), north to Manzano Mountains near Albuquerque (another 300 miles by car), jump again to SE Utah or SE Arizona, then main population in northern Utah. Interesting relics of cooler climate in the West.
Maybe both (or like @bobmcd suggested, a recent dispersal). In one sense they are already cryptic species, since unless and until there is gene flow between the two areas, they are already on separate evolutionary trajectories. Maybe just not for long enough to have fixed any visible differences yet.
We have a few species here in the Southern Appalachian mountains that have disjunct occurrences in the coastal areas (e.g. BONAP map for Kalmia buxifolia) but not quite as dramatic in terms of distance as some of the other examples mentioned earlier in this thread. I think genetic studies are ongoing to determine what’s going on and how distinct they are on a genetic level.
Going back further into time, a lot of our Southern Appalachian plant species have Asian counterparts - a well-known and studied disjunction on genus level - they usually have evolved into sister species by now rather than still being considered the same. The explanation I’ve heard from colleagues studying biogeography is that these species were once spread throughout the entire northern temperate zone, but the ice ages pushed them up against the Alpes and to extinction in Europe while they could escape into southern refugia in Eastern North America and Eastern Asia and then recolonized those areas once the glaciers receded and diversified into our modern day sister species.