Near speciation in about one century.
Below see comments on Ernst Mayr a major evolutionary biologist. One of his main interests was in speciation that is one species evolving into two separate species. The division of (sexually reproducing) living things into species is not arbitrary. Within a species genes are mixed by interbreeding (this is called gene flow). In principle, one can test if a gene in organism A can end up in a long stream of descendents of organism B (takes a long time). Thus the claim that A and B are part of different species can, in principle, be falsified. Generally the standard definition that only members of the same species can produce fertile offspring works well enough. It implies that horses and donkeys are different species because mules are sterile. Dogs however are one species even though a great dane and a chihuaha can't produce children, because they can be great to the n grandparents of the same dog through a chain of mates of different sizes through generations.
Mayr argues that speciation is a nail in Plato's coffin, because it shows that there are not permanent ideal types.
An accidental experiment suggests that speciation can occur very very quickly. For about a century Drosophila melanogaster (fruit flies) have been bred in laboratories. From now on I will drop the species name, melanogaster , which I really shouldn't do since I am writing about speciation.
The distant cousins of the lab bred Drosophila are flying around making pests of themselves.
Interestingly, if a wild male mates with a lab line female, their offspring have all sorts of developmental abnormalities which turn out to be genetic mutations (this is called hybrid disgenisis). It turns out that they are caused by an extremely benign virus called a p-element which mainly sits dormant in the chromasomes of wild Drosophila but not lab strain Drosophila. The p-element is kept dormant by a protein which it makes which tells it to lie quiet. The protein is not packed into sperm. Thus when wild type sperm fertilizes a lab line egg, the p-element activates. It then makes numerous copies of itself in the offsprings chromasomes. They cause mutations which are generally damaging (if the p-element pops into the middle of a gene it destroys it).
Apparently p-elements spread throughout the wild Drosophila population after the lab stocks of Drosophila were started and while the lab stocks were isolation from the outside world.
Hybrid disgenesis implies that there is already a partial barrier to interbreeding (gene flow9 among Drosophila which wasn't there a mere hundred or so years ago. One might imagine that, if biologists keep breeding lines of Drosophila, in another few hundred years there will be a new species "lab line Drosophila melanogaster" because accumulated stuff like p-elements will make wild-type X lab-line hybrids sterile or dead. If this occurs it will be an amazing event in experimental evolutionary biology (a field founded by Leo Szilard co-inventor of the Atomic bomb by the way).
Finally to pat myself on the head in a very juvenile way, I wrote a paper alleging that sympatric speciation occurs when in high school. This after reading papers Mayr discussed and dismissed in his text book. When I read about his current course a few years later, I found that he had been convinced by further research. The question is whether for one species to evolve into two (a process called speciation) the population has to be physically divided and in different regions (allopatric) or if it can occur with overlapping populations (synpatric). It appears that allopatry is not strictly necessary. For example, there are two species crickets in the same range in the USA that differ mainly because one lays eggs in the Spring and the other lays eggs in the Fall. For another there is a species of Drosophila in North America which eats only apples - a fruit introduced to the new world after 1492. Johnny appleseed seems to have zapped both Plato and early Mayr (I learned of the examples from Mayr's dismissive presentation in a textbook).