The immortal remains of Henrietta Lacks.
When human body cells are removed and put into a cell culture, they weaken and die quickly, usually within about 50 divisions. Without the rest of the support structure—a heart, blood circulating, a digestive system and so-on—body cells can't survive. Body cells also age, so even if you were to simulate the body's environment in a test tube or petri dish, the cells would eventually perish anyway. The basic mortality of the cells reflect the basic mortality of the organism they comprise, which is why there's no fountain of youth or medicinal procedure that'll give you biological immortality.
There is, however, one human being who is biologically immortal on a technicality, and her name is Henrietta Lacks. In 1951 she showed up at John Hopkins Hospital
...another sample from the tumor was sent to George Gey, who was the head of tissue culture research at Hopkins. Gey discovered that the cells from Henrietta's tumor would not only survive and multiply outside of her body, but they didn't age either. These cells were basically immortal.
And they're still alive, even though Henrietta herself died of the cancer on October 4th, 1951...
But say you're a scientist looking at HeLa cells under a microscope. They live independently of the body they came from. They reproduce (faster even than other cancerous cells). They consume, excrete, and do everything an independent living organism usually does. A thousand years from now there will still be HeLa cells multiplying and living, even some of the original cells sampled from Mrs. Lacks, even though Henrietta Lacks herself has long since passed away. Is this a new species?
In 1991 the scientific community decided it was, and blessed HeLa cells with its own genus and species: Helacyton gartleri, named by Van Valen & Maiorana.
...Since this event occurred in nature when the papillomavirus transformed Henrietta's cells, and not in the laboratory, it's a strong piece of evidence supporting Evolution (although not one that suggests you could go from an ameoba to a chordate, which would probably take more than one mutation).