I have read some of the results of the mitochondria DNA studies that show all humans descended from the same male female couple. This only makes sense to me since all race can mate and produce offspring that we are all the same species and descended from the same parents. I wonder what your opinion would be if this research had been extended to other primates. If this study had included other primates such as gorillas, baboon, and such. Do you think it would show the same thing, but further back, that all primates had a common parent? Or do you think the branch would have been too far back for mitochondrial DNA to show this consistency? In evolution there is a point when something become a new species over time. Would the mitochondrial DNA of the females be changed at that point or not?
Yes, what you are describing (about species descending from single genetic sources) is accurate. Species usually develop very slowly over time, but it is always one couple at a time, forming a single lineage. Entire populations do not evolve into entire new species, but rather, source parents have off spring, which mate with others, and have their own offspring, and after many generations form into new species. But it always comes down to one male and one female at a time.
Mitochondrial DNA, in case you weren't aware, exists because mitochondria used to be independent single-celled lifeforms, call "prokaryotes", which formed into a symbiotic relationship with other single-celled lifeforms. That's why they have their own DNA. Even today, mitochondria reproduce independently of the host cells, and maintain some metabolic functions. Cool, eh?
I doubt they could be used to track back much past a couple million years, however (ex: from humans to, say, non-primate mammals) because there just isn't enough source DNA to work with. Eventually tracing the mutations would become impossible. However, we can use regular DNA to trace back much further, by measuring the accumulated differences between two DNA strands. For example, we know that humans diverged from chimpanzees about 3 million years ago, based on the measured rate of change between the two DNA samples. The exact same technique has been used countless times to measure the point at which lots of other species diverged as well, from dolphins & whales to even types of body lice.
To answer your other question, a new species is defined as when one individual can no longer mate with some other individual to create viable offspring. So, if two groups of birds were separated from each other, but could still mate and create viable offspring, then they would be members of the same species. If they *couldnt* create viable children, then they would be different species.
For example, look at a relatively recent split: tigers and lions. They can and do mate, and produce offsprings (called "Ligers"), but ligers cannot reproduce further, so they are not viable. Therefor, tigers and lions are different species.
The reason that two groups could no longer mate is simply because their DNA has changed so much that it is no longer compatible, though they may still actually look the same. Consider poodles and great danes. They are still members of the same species, though they look nothing alike. Meanwhile a sparrow and a robin look very similar indeed, but are of different species, as they cannot reproduce.