This passerine family occurs across Eurasia and contains two genera, Phylloscopus and Seicercus. Hybridization has only been documented in the former genus.
The Chiffchaff (P. collybita) complex is composed of several subspecies of which some are in secondary contact (Helbig et al., 1996, Rakovic et al., 2019). In Sweden, P. c. collybita is expanding north and engaging in hybridization with P. c. abientinus (Hansson et al., 2000) and in the Southern Urals, P. c. tristis and P. c. abientinus are interbreeding (Marova et al., 2009; Marova et al., 2017; Shipilina et al., 2017). A genomic analysis of these species revealed a handful of candidate regions that might be important for trait variation between the two species (Talla et al. 2017).
The most studied hybrid zone in this complex involves P. c. collybita and P. c. brehmii in the Pyrenees. It was first described based on morphological and vocal characteristics (Salomon, 1989; Salomon & Hemim, 1992; Salomon et al., 1997). Genetic analyses indicated extensive hybridization (Bensch et al., 2002b). However, gene flow was mostly restricted to nuclear alleles, possibly by means of hybrid males, while mitochondrial gene flow was inhibited by sterile hybrid females (Helbig et al., 2001). Both species also differ in flight-related morphology due to different migration patterns. Hybrids might be maladapted in terms of migration strategies (Perez-Tris et al., 2003).
Subspecies of the Willow Warbler (P. trochilus) form a migratory divide in Sweden, confirmed by isotope analyses (Chamberlain et al., 2000) and experiments (Ilieva et al., 2012). The subspecies are clearly separated morphologically and behaviourally, but there is little genetic differentiation (Bensch et al., 1999; Bensch et al., 2002a). However, three genomic regions – on chromosomes 1, 3 and 5 – are highly differentiated and house putative ‘migration genes’ (Lundberg, et al. 2017). Possibly there is strong selection on hybrid migration strategies, because premating isolation is weak (Liedvogel et al., 2014). In addition, the circular distribution of this species around the Baltic Sea might make it an example of a ring (sub)species (Bensch et al., 2009).
Another likely example of a ring species is the Greenish Warbler (P. trochiloides), which extends around the Tibetan Plateau. The two terminal populations of this ring (viridamus and plumbeitarsus) meet, but do not interbreed (Irwin et al., 2001a; Irwin et al., 2001b). These populations sing different song and do not recognize each other’s song (Irwin, 2000; Irwin et al., 2001a). Song divergence is mainly driven by male competition. In the south, population density is higher, leading to more interactions between the males and consequently strong selection for shorter songs. In the north, competition between males is weaker and there is a stronger selection by females for longer, complex songs (Scordato, 2018). Genetic analysis using AFLPs demonstrated gene flow between the connected populations (Irwin et al., 2005). But recent studies are questioning the validity of the Greenish Warbler as a classical ring species. Morphology and song seem to be converging in sympatry (Kovylov et al., 2012), and a genomic analyses showed that there have been occasional geographic breaks during the development of the ring (Alcaide et al., 2014).
Hybridization between Western Bonelli’s Warbler (P. bonelli) and Wood Warbler (P. sibilatrix) has been confirmed genetically (Dietzen et al., 2007).
Description of the Pallas’ Warbler (P. proregulus) complex indicates the existence of contact zones and possible hybridization between certain races (Martens et al., 2004).
Alstrom et al. (2010) describe a new species, Limestone Leaf Warbler (P. calciatilis) in Vietnam, that might be interbreeding with Sulphur-breasted Warbler (P. richetti).
The Tickell’s Leaf Warbler species complex contains three species: the nominal Tickell’s Leaf Warbler (Phylloscopus affinis), the Alpine Leaf-warbler (P. occisinensis) and the Sulphur-bellied Warbler (P. griseolus). The Alpine Leaf-warbler was only recently recognized as a distinct species, mainly based on divergence in the mitochondrial DNA. Analyses of this circular genome revealed that the Alpine Leaf-warbler diverged from the Tickell’s Leaf Warbler about four million years ago (Martens et al., 2008). However, the nuclear genome of these species is very similar, pointing to a speciation event only 600,000 years ago. This pattern – called deep mitochondrial divergence (DMD) – can be explained by ghost introgression from an extinct species (Zhang et al., 2019).
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