Evolution — practice question

The greenish warbler, Phylloscopus trochiloides, is a small bird species that first evolved in northern India, along the southern edge of the Himalayan mountain range. Thousands of years ago, greenish warbler populations spread around the western and eastern sides of the Himalayan mountain range and became established in north-eastern Europe and Siberia. As time passed, their characteristics changed gradually, producing different forms of the greenish warbler. One gradual change is in the male warbler’s song, which is highly distinctive and is part of mating behaviour. When greenish warblers from north-eastern Europe meet those from Siberia, mating does not occur. The greenish warblers from north-eastern Europe and Siberia are now regarded as two separate species. Fig. 1.1 depicts a greenish warbler. Fig. 1.2 illustrates the spread of the greenish warbler.