While most of the DNA in a cell is found in the nucleus, a small amount of DNA, called mitochondrial DNA is found in the mitochondria. The mitochondria are responsible for converting chemical energy into adenosine triphosphate. In most organisms, mitrochondrial DNA is derived from the mother, however exceptions include some species of mussels, honeybees and some flies among others (Mitrochondrial DNA n.d.).
Mitochondrial DNA provide a tool for biologists to look at the evolutionary or speciation history of individuals given its brevity and the fact that it experiences a high mutation rate. It is particularly useful when comparing individuals of close relatedness due to the high mutation rate, which allows more differences to be apparent (Wikipedia). It has been shown however that there can be discrepancies between phylogenies constructed with mtDNA and nDNA indicating that for some species a validation may need to be completed for accuracy (Shaw 2002).
Shaw, K. L. (2002). Conflict between nuclear and mitochondrial DNA phylogenies of a recent species radiation: what mtDNA reveals and conceals about modes of speciation in Hawaiian crickets. Proceedings of the National Academy of Sciences, 99(25), 16122-16127.
Mitochondrial DNA. (n.d.). In Wikipedia. Retrieved March 17th, 20015, from http://en.wikipedia.org/wiki/Mitochondrial_DNA