1. Answer;
They evolved separately as they moved north and became two separate species.
Explanation;
The greenish warblers weren't geographically isolated, but they migrated around the Tibetan Plateau, evolving separately until they were no longer able to interbreed when both species (East and West) contacted in Siberia.
Greenish warblers were thought to have evolved from a single ancestral population that gradually diverged into two new species as it expanded northwards around the Tibetan plateau.
2. Answer;
- Their plumage patterns.
- Genetics and history
Explanation;
West Siberian greenish warblers (P. t. viridanus) and east Siberian greenish warblers (P. t. plumbeitarsus) differ subtly in their plumage patterns, most notably in their wing bars, which are used in communication. While viridanus has a single wing bar, plumbeitarsus has two. Around the southern side of the ring, plumage patterns change gradually.
Additionally, the two northern forms viridanus and plumbeitarsus are highly distinct genetically, but there is a gradient in genetic characteristics through the southern ring of populations. The greenish warblers were once confined to the southern portion of their range and then expanded northward along two pathways, evolving differences as they moved north. When the two expanding fronts met in central Siberia, they were different enough that they do not interbreed.
