When I started working with DNA sequences to determine population structure and species diversity, the first things that set in my mind were to keep the DNA sequences as long as possible and also that a single base pair change is not enough to differentiate between two distinct species. This has always been my rule of thumb until noon today.
While sitting through a presentation by a Research Experience for Undergraduates (REU) student who was presenting her report on a study on the hybridization of blueback herring (Alosa aestivalis) and alewife (Alosa pseudoharengus) fish, I noticed that she had a single nucleotide polymorphism (SNP) that was used to distinguish between two different fish species. Her study was to do a DNA comparison of blueback herring and alewife from river systems in Connecticut rivers to determine if there was hybridization between the two species and she concluded that there was hybridization.
What was puzzling was that although she had sequences that were 592 base pairs long, there was a SNP at location 209 of her sequences and that was what was used to differentiate between her the blueback herring and alewife fish species. Furthermore, this analysis was made with reference to sequences that were already on GenBank and they lined up perfectly. She did say that there were other polymorphisms in her sequences too, but this one at 209 was by far the most obvious one to differentiate between the species.
So the question now is, what is the minimum number of SNP’s we need to have in a sequence before we conclude that they are different species. I am also working on some samples and apart from other polymorphisms in my sequences, I also have SNP’s at certain locations that have up to three different bases in the one location. Can I say these are three different species too?