By this, I mean extant species or taxa whose origins are the most temporally proximate to the present day, or which have required the shortest amount of time to evolve into contemporary species. I assume most of these would belong to the broad categories of bacteria or viruses, given their extremely short generation times and immense mutational potential. But what about macroscopic species? Excluding cases of artificial breeding, are there any documented natural examples? (The influence of urbanization can be included in this context.)
The plant Tragopogon mirus formed in eastern Washington, western Idaho in the early 1900’s. It originated from hybrids of T. dubius (yellow flowers) and T. porrifolius (purple flowers). These diploid species can cross to form sterile hybrids because the chromosome from T. dubius don’t line up well with those of T. porrifolius. In some hybrids, the chromosomes doubled, producing 4x (tetraploid) cells. This allowed both the T. dubius chromosomes and the T. porrifolius chromosomes to match up with corresponding chromosomes and therefore these tetraploids were fertile but cannot produce fertile hybrids with either parental types. The tetraploids are called T. mirus. Their flowers have purple outer petals and yellow inner petals. Interestingly, T. mirus tetraploids have formed this way at least a dozen times.
The marbled crayfish is a good contender. It originated in the aquarium trade in germany in 1988
Date of discovery is not a perfect proxy for date of origin but Mimulus peregrinus (Erythranthe peregrina) may well be younger than Tragopogon mirus.
I have no doubt that there are other species younger than Tragopogon mirus, but I’m working on a T. mirus project right now, as can be seen based on the large number of T. mirus observations I have posted lately, and I couldn’t resist writing about it.
The origin of this species has to be more recent than introduction of both species in the Pacific Northwest and probably since the completion of the railroad across North America (1869). Some weeds were introduced earlier, but there was a big explosion of introductions in the 1870’s, including what we now consider horrible weeds, e.g. Cheatgrass (Bromus tectorum), offered by seed catalogues.
Date of discovery certainly isn’t likely to be date of origin. However, it can be in captive or garden situation (e.g. Primula kewensis, a plant, and the crayfish mentioned above). 1988 is quite a recent date of origin!
In the 1980s a single Española cactus finch made it to Daphne Major and bred with a Medium Ground Finch. The hybrid offspring preferentially breed with each other, and seem reproductively isolated to a degree as the local medium ground finches do not recognize their songs.