Selection and evolution: explain question

A species of finch (Fringilla montana) is found in a large, continuous forest. A mountain range formed, splitting the forest into two separate regions: a northern region with cooler, wetter conditions and a southern region with warmer, drier conditions. The finch populations in the two regions became geographically isolated. DNA sequencing of a mitochondrial gene was carried out on individuals from each region 10 000 years after the split. The results showed that the northern and southern populations had accumulated several fixed differences in their DNA sequences. Today, the two populations are morphologically distinct: northern finches have thicker beaks and darker plumage, while southern finches have thinner beaks and paler plumage. Explain how the changes in the gene pools of these two populations could lead to the formation of two new species.