Four species of finch (A, B, C, D) are found across an archipelago. Species A is found on the mainland, species B and C on adjacent islands with a narrow channel between them, and species D on a remote island far from the others. Researchers compared the DNA base sequences of a specific gene across the four species. The table shows the percentage of base differences between each pair of species.
| Pair of species | Percentage of base differences |
|---|---|
| A and B | 2.0% |
| A and C | 3.0% |
| A and D | 4.0% |
| B and C | 1.5% |
| B and D | 3.5% |
| C and D | 2.5% |
Species B and C have overlapping territories on their islands and show similar diets but different beak shapes. Species D has a highly specialised diet unique to its remote island.
Explain how these DNA sequence data can show the evolutionary relationships between the four species, and how speciation may have occurred, considering both allopatric and sympatric mechanisms.