Exploring the origin of hybrid bird species.
Whenever an interesting hybrid combination is reported, people ask the question: Will this hybrid evolve into a new species? The answer is almost always a resounding no. The origin of a new species through hybridization – hybrid speciation – requires very specific conditions. First of all, you need several hybrid individuals that can breed with each other. In some cases, hybridization is too rare to sustain a population of hybrids. And the hybrids should be reproductively isolated from their parental species, allowing the hybrids to carve out their own independent evolutionary trajectory. But despite these restrictions, hybrid speciation does occur. Hybrid life finds a way.
How to recognize a hybrid species? In 2014, Molly Schumer and her colleagues put forward three criteria to show that a species is of hybrid origin, namely (1) reproductive isolation of hybrid lineages from the parental species, (2) evidence of hybridization in the genome, and (3) evidence that this reproductive isolation is a consequence of hybridization. A survey of the literature revealed that only four species (three sunflowers and one butterfly) fulfilled all three criteria. The main bottleneck was the third criterion. The first two criteria make intuitive sense, but the third one does seem too strict. Why should hybridization directly lead to reproductive isolation before we can talk about a hybrid species? Other evolutionary mechanisms could lead to reproductive isolation between the hybrids and their parental species.
Several researchers indicated that the third criterion was too strict, leading to the exclusion of potentially interesting cases of hybrid speciation. In 2018, I proposed a possible solution to this clash of criteria by discriminating between two types of hybrid speciation: type I where reproductive isolation is a direct consequence of hybridization and type II where it is the by-product of other processes. Applying this approach to birds revealed that the majority of putative hybrid species belongs to the type II group. Specifically, reproductive isolation often evolved when the hybrid population became geographically isolated from the parental species. A combination of hybrid speciation and classic allopatric speciation.
This academic debate about criteria sounds rather abstract. Let’s explore an actual case to make it more tangible: the Italian Sparrow (Passer italiae). Morphologically, this small passerine resembles a cross between House Sparrow (P. domesticus) and Spanish Sparrow (P. hispaniolensis). Indeed, Jo Hermansen and his colleagues noted that:
Male Italian sparrows have a chestnut-coloured crown and nape, and white cheeks similar to the Spanish sparrow (house sparrows have a broad, grey band on the crown and nape, and grey cheeks), but a small bib and a brown-streaked back similar to the house sparrow (Spanish sparrows have a large black bib that extends all along the body flanks and a black- and yellow-streaked back). Interestingly, male F1-hybrids between house sparrows and Spanish sparrows resemble Italian sparrows.
The morphological intermediacy of the Italian Sparrow was confirmed by genetic analyses. As expected, the genome of the Italian Sparrow is a mixture of the House Sparrow (ca. 60%) and the Spanish Sparrow (ca. 40%). The hybrid speciation event probably occurred less than 10,000 years ago when House Sparrows expanded across Europe and came into contact with the Spanish Sparrow. The resulting hybrid population became geographically isolated on the Italian peninsula and adjacent islands, eventually evolving into the Italian Sparrow. A nice example of a type II hybrid species.
A scenario of hybrid speciation with an allopatric phase is not only limited to the Italian Sparrow. Another example concerns the Audubon’s Warbler (Setophaga auduboni), a hybrid between Myrtle Warbler (S. coronata) and Black-fronted Warbler (S. nigrifrons). And there is the Golden-crowned Manakin (Lepidothrix vilasboasi), a hybrid between Opal-crowned Manakin (L. iris) and Snow-capped Manakin (L. nattereri). Does this mean that all hybrid bird species belong to the type II group? Well, there is always an exception that proves the rule.
A beautiful example of a type I hybrid bird species – where reproductive isolation is directly caused by hybridization – is the so-called “Big Bird” on the Galapagos Islands. In 1981, a large Cactus Finch (Geospiza conirostris) arrived on the island Daphne Major and mated with a female Medium Ground Finch (G. fortis). The resulting offspring only bred with each other and as such established a new hybrid lineage on the island. The hybrids have an intermediate beak morphology and produce a distinctive song. These traits contribute to reproductive isolation from at least one parental species (G. fortis) because of differences in song and beak morphology. Reproductive isolation is thus directly due to hybridization.
The hybrid species discussed above show intermediate morphology. The Italian Sparrow has plumage patterns of both parental species, and the “Big Bird” lineage sports an intermediate beak. In some cases, however, hybrids exhibit extreme phenotypes that surpass the range of the parental species. Think of the excessive size the hybrid ligers compared to their parents, lions and tigers.
Some researchers have used this phenomenon – known as transgressive segregation – to pinpoint potential hybrid species. Two intriguing examples concern the Steller’s Eider (Polysticta stelleri) and the Red-breasted Goose (Branta ruficollis). Both species have exceptional plumage patterns and show signs of past hybridization with other species. The Steller’s Eider shares genetic variation with Long-tailed Ducks (Clangula hyemalis) and several eider species. And the Red-breasted Goose might have originated through hybridization between Brent Goose (Branta bernicla) and the ancestor of the white-cheeked geese (a group which includes among others, the Canada Goose, B. canadensis).
There is, however, an alternative explanation for the genetic make-up of the Steller’s Eider and the Red-breasted Goose. These birds might have hybridized with several species at different times during their evolutionary history, picking up genetic variants in each of these hybridization events. The resulting mixture looks like a hybrid species, but developed along a different evolutionary path (see figure below). Discriminating between successive hybridization events and hybrid speciation requires more detailed genetic analyses. Clearly, we need more that a handful of criteria to identify a hybrid species.
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Featured image: Italian Sparrow (Passer italiae) © Omar Bariffi | Wikimedia Commons