Selection and evolution: describe question

A population of the warbler species Phylloscopus trochiloides inhabits a forest in Southeast Asia. Within this forest, two types of fig trees, Ficus benghalensis and Ficus religiosa, fruit at different times of the year. Some warblers have developed a preference for feeding on F. benghalensis, while others prefer F. religiosa. Over many generations, individuals feeding on different fig trees have evolved distinct courtship songs, and females preferentially mate with males that sing the song associated with their own feeding preference. Genetic analysis of mitochondrial DNA shows that the two groups are more closely related to each other than to any other warbler species, but they have several fixed differences in their alleles. No geographical barrier separates the groups. Describe how this scenario illustrates the process of sympatric speciation, including the roles of behavioural isolation and DNA sequence data in supporting evolutionary relationships.