Evolution — practice question

The greenish warbler, Phylloscopus trochiloides, is a small bird species that first evolved in northern India, along the southern rim of the Himalayan mountain range. Many thousands of years ago, greenish warbler populations spread round the western and eastern sides of the Himalayan mountain range and became established in north-eastern Europe and Siberia. Over time, these populations showed gradual changes in characteristics, which produced different forms of the greenish warbler. One gradual change is seen in the male warbler’s song, which is highly distinctive and plays a part in mating behaviour. When greenish warblers from north-eastern Europe meet those from Siberia, no mating occurs. The greenish warblers from north-eastern Europe and Siberia are now regarded as two separate species.