The Tanagers comprise one of the largest bird families. They have an American distribution. Hybridization has been recorded in several genera.
Sympatric populations of Tree Finches were genetically more similar compared to allopatric ones. This can be explained by hybridization and/or sympatric origin (Grant, Grant & Petren, 2005). Indirect evidence for hybridization was provided by an analysis of historical and recent samples of Small Tree Finch (C. parvulus), Medium Tree Finch (C. pauper) and Large Tree Finch (C. psittacula). The historical sample (1852-1906) showed three morphological clusters, while the recent samples showed two clusters and an increasing amount of hybridization (19% in 2005 and 41% in 2010) (Kleindorfer et al., 2014). Gene flow is asymmetric, from Medium into Small Tree Finch, probably driven by female choice (Peters et al. 2017). From 2005 to 2013, the foraging behaviour of the parental species changed, but that of the hybrids did not (Peters & Kleindorfer, 2015). In an acoustical survey on Floreana Island, song differed significantly between Medium Tree Finch and Small Tree Finch, but it was not possible to discriminate between the song of Small Tree Finch and hybrids (Peters & Kleindorfer, 2017).
The Mangrove Finch (C. heliobates) is critically endangered and currently restricted to one small population on Isabela Island (Galapagos Islands). A genetic study found that a number of individuals have hybridized with the closely related Woodpecker Finch (C. pallidus). Possibly, there is a breakdown of reproductive isolation between these species because the Mangrove Finches cannot find a mate due to the low population size (Lawson et al., 2016).
The Warbler Finch (C. olivacea) comprises two distinct genetic lineages that are paraphyletic. This pattern can be explained by incomplete lineage sorting and hybridization (Freeland & Boag, 1999b).
Several Darwin’s Finches interbreed (Farrington et al., 2014; Grant, 1999), but a few cases have been studied in great detail. Lamichhaney et al. (2015) give phylogenetic evidence of hybridization having occurred throughout the radiation (see also perspective by Almen et al. (2016)). The findings of this study have been placed into a greater perspective of introgressive hybridization by Palmer and Kronforst (2015).
On the small island Daphne Major, three species coexist, Medium Ground Finch (G. fortis), Common Cactus Finch (G. scandens) and Small Ground Finch (G. fuliginosa). They all interbreed at low frequencies (Freeland & Boag, 1999a; Grant, 1993). Hybridization is caused by occasional errors of imprinting on parental songs that are inherited culturally (Grant & Grant, 1996a; Grant & Grant, 1997b; Grant & Grant, 1997c). Hybrids showed high fitness under certain ecological circumstances (Grant & Grant, 1992b), they are morphologically intermediate which enables them to occupy intermediate ecological niches (Grant & Grant, 1996b; Grant & Grant, 1994). For example, the El Nino effect on the island created the opportunity for Medium Ground Finch hybrids to proliferate (Grant & Grant, 1993).
Hybridization led to the exchange of genetic material between species (Grant et al., 2005), which enhances genetic diversity of populations (Grant & Grant, 1992a; Grant & Grant, 2010), counteracts inbreeding depression (Grant et al., 2003) and creates new evolutionary tracks (Grant & Grant, 1996c). Hybridization can also lead to convergence of species (Grant et al., 2004). So, the balance between selection and hybridization will ultimately result in fission-fusion dynamics of species (Grant & Grant, 2008; Grant & Grant, 2006). These studies have uncovered the important role hybridization may fulfil in speciation (Grant, 2003; Grant & Grant, 1997a). An overview of this system has been nicely summarized (Grant & Grant, 2014), including the case of a hybrid Medium Ground Finch that reached the island and is behaving like a new lineage (Grant & Grant, 2009). Genomic analyses of this lineage indicated that it concerns the founders were a female Medium Ground Finch and male Large Cactus Finch (G. conirostris). Their offspring were reproductively isolated from the parental species and have been breeding amongst each other for several generations, possibly resulting in the origin of a hybrid species (Lamichhaney, et al. 2017, but see Hill & Zink, 2018).
On Genovesa. Large Cactus Finch (G. conirostris) hybridizes occasionally with Sharp-beaked Ground Finch (G. difficilis) and Large Ground Finch (G. magnirostris) (Grant & Grant, 1989).
On Santa Cruz, there is positive assortative mating between large and small morphs of the Medium Ground Finch (Huber et al., 2007). Genetic analyses showed that this might be an example of divergence with gene flow (de Leon et al., 2010).
Mckay and Zink (2015) provide a different perspective on the Geospiza ground finches. They argue that these populations are cycling between stages of differentiation and will never attain species status, a process referred to as Sisyphean evolution. Although an interesting idea, how can you scientifically test a never-ending process (Peter Grant, personal communication)?
On Inaccessible Island, Wilkin’s Finch (N. wilkinsi) and Inaccessible Island Finch (N. acunhae) are interbreeding (Ryan, Moloney & Hudon, 1994). Hybrids are generally intermediate in size (Ryan, 2001) and egg size (Ryan & Moloney, 2002).
In Colombia, habitat modifications have led to secondary contact between two subspecies of Flame-rumped Tanager (R. flammigerus flammigerus and R. f. icteronotus), although these subspecies have probably been in contact for at least 6000 years (Bedoya & Murillo, 2012; Morales-Rozo et al., 2017; Sibley, 1958).
A study of an adaptive radiation comprising 11 species found marked phenotypic differences despite a lack of mtDNA monophyly. This pattern can be explained by incomplete lineage sorting, introgressive hybridization and demographic expansions (Campagna et al., 2012; Campagna et al., 2013). Some wild hybrids have been observed. Captive breeding experiments show that hybrids are generally fertile, although female hybrids can be infertile (Campagna et al. 2018).
A genomic analysis on this radiation found no evidence for gene flow between five sympatric species (S. hypoxantha, S. melanogaster, S. palustris, S. pileata, S. ruficollis), likely due to recent divergence and the fact that a high number of shared ancestral polymorphisms reduces the power of tests for finding signatures of gene flow (Campagna et al., 2015). Capuchinos show differences in a small proportion of their genomes, yet selection has acted independently on the same targets in different members of this radiation. Many divergent regions contain genes involved in the melanogenesis pathway (Campagna et al. 2017).
Two studies tested whether a certain taxon was of hybrid origin (S. zelechi and a variant of S. hypoxantha), but in both cases it turned out to be a colour morph (Areta, 2008; Areta & Repenning, 2011).
Subspecies within the White-collared Seedeater (S. torqueola) complex are not closely related. They belong to distinct clades within the Sporophila radiation. There was probably some gene flow in the past (Mason et al., 2018).
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