Answer:
The mitochondrial gene tree suggests a later split because it occurred hybridization later in time, with the transference of the brown bear mitochondrial genetic information to the polar bear population. So the mitochondrial gene tree exhibits a later divergence.
Because the first divergence event of the common ancestor occurred 60,000 years ago, evidenced by nuclear DNI and mitochondrial DNI. However, 100,000 years ago, concerning the mitochondrial genetic material, the hybridization event defined a new more recent common ancestor between the two species.
Explanation:
The use of mitochondrial DNA might present a few problems that need to be considered in evolution studies. It involves only one locus, so it has some limitations when reconstructing history, especially if the locus went through any process, such as natural selection, bottleneck events, or its transference to another population due to hybridization. There is also a high possibility of an incomplete reconstruction of the population history if males and females have different dispersal patterns.
The most proper way of reconstructing history is by complementing mitochondrial information with nuclear genetic information and evaluating their concordancies.
In the exposed example, 600,000 years ago, the original species split into the polar bear and the brown bear. But 100,000 years ago, hybridization occurred between a brown bear female and a polar bear male. The offspring joined the polar bear population, carrying the female genetic information, nuclear and mitochondrial. With time, these individuals interbred with the polar bears, diluting the nuclear brown bear genetic information. However, females carrying the brown bear mitochondrial DNI seemed to be more successful reproductively than the polar bear females. The mitochondrial information, hence, has been in the population for many generations, until today. And this information came from a more recent common ancestor, compared with nuclear genetic information.
