When forbidden love creates a new species

Choosing your mate is an important decision. If you are a species that cares for your offspring, you and your mate need to be able to coordinate your care, and you must be able to rely on your mate to pull their weight. (Watch the albatross cam to see how the mated albatrosses depend on each other—one stays back with the chick while the other flies off to get food. If the food-getter never came back, or the chick-minder wandered off, the chick would die.) More fundamentally, whether or not you care for your offspring, you and your mate must be able to have biologically healthy offspring together.

Animals do not always get this right. The recently-in-the-news stories of Thomas the goose, who bonded with a black swan and spent his days helping that swan and his mate raise their cygnets, and Nigel the gannet, who was devoted to a concrete gannet dummy, are good examples of the errors an animal heart can make. From an evolutionary perspective, these are bad decisions: they prevent the lovestruck individual from passing on their genes to future generations. (All reports suggest that Thomas and Nigel appeared happy, so from an individual perspective, the mistake may not be so bad.)

But sometimes—very, very rarely—an error in mate choice, instead of being an evolutionary dead end, is the beginning of an entirely new lineage.

In 1981 in the Galápagos islands, a young male large cactus finch traveled over 100 km from his home island to an island with no other members of his species. Researchers monitoring the finches dubbed him individual 5110 (and, unofficially, “Big Bird”), and observed as he mated and raised chicks with a medium ground finch. This hybridization is not in itself unusual: animals do occasionally mate with other species, and even raise hybrid offspring. These hybrid offspring are usually less fit than either of their parents, however: think of the mule, a hybrid between horses and donkeys, which is infertile.

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A large cactus finch, like Big Bird. Quite the bill! (Photo by Wildlife Travel on flickr, used via a Creative Commons license.)

Big Bird’s hybrid chicks weren’t less fit. They flourished, and had offspring of their own. They looked a bit like each of their parents: smaller-bodied, like their mother, but with their father’s large, strong bill. That bill allowed them to eat certain seeds better than any other finches on the island, giving them a competitive advantage.

The researchers expected that these hybrid finches would breed with their neighbors on the island, other medium ground finches like their mother. Over the course of generations, the unusual genes from Big Bird would be diluted, so that his great-great-great-grandchicks would be almost pure medium ground finches. But that didn’t happen: instead, the new hybrid finches bred almost exclusively with each other.

Galápagos finches learn to identify their own species by looking at their parents. When they grow up, they look for mates with a similar bill size and body size as their parents. The sons learn to sing from their father, and the daughters look for males who sing a song similar to the song sung by their father. The distinctive bill size of the hybrids prompted them to breed only with each other. Their song, too, was different, due to an unlikely circumstance: Big Bird mostly learned what his song should sound like on his home island, but then seems to have attempted to imitate some of the songs on the new island to which he immigrated. The result was an entirely new song, neither exactly the one from his old home or his new home. His sons learned that song, and his daughters learned a preference for that song, and no other finches on the island sung a song like it.

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A female medium ground finch, like the one that bred with Big Bird. (Photo by budgora on flickr, used via a Creative Commons license.)

Well, okay, that’s unexpected, said the researchers, but all that inbreeding is bad for the lineage, so the hybrid line will soon die out. But that didn’t happen either. The hybrids continued to be successful, and continued to breed only with each other, and looked and sounded distinctive from all the other finches on the island: the hybrid lineage became a new species.

The Big Bird hybrid lineage is very young. It is restricted to a single small island, and its population has never been larger than three dozen individuals. Perhaps this is a temporary evolutionary fluke? Perhaps hybrid speciation can result only in fragile, short-lived species?

Consider these manakins:

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Left-to-right: snow-capped manakin, golden-crowned manakin, opal-crowned manakin. Rectangles show the crown feathers for each species. From Fig. 1, Barrera-Guzmán et al. 2017.

The golden-crowned manakin is found only between the ranges of the other two species. This led some researchers to suspect that it might be a hybrid between them, but a quick look at their feathers seems to clearly disprove that: how would two white-crowned birds produce a yellow-crowned bird? The question went unaddressed for a long time, as the golden-crowned manakin was lost from the 1950s until 2002, when they were rediscovered. Detailed analysis of the three species’ genomes revealed that the golden-crowned manakin is its own species—i.e., golden-crowned manakins are not currently interbreeding with opal-crowned or snow-capped manakins. The analysis also showed that the golden-crowned manakin originated from hybridization between the two other species.

How do two white-crowned birds produce a yellow-crowned bird? Gradually. The opal-crowned manakin has an iridescent crown that can look white, blue, or purple depending on the angle; it produces this effect through tiny spherical structures in its feathers, carefully organized into layers that scatter specific wavelengths of light. The snow-capped manakin’s white crown also uses tiny structures to scatter light, but its structures are disorganized, so that they scatter all wavelengths of light rather than a specific few.

An intermediate between these two crowns is… not so pretty. Half-organized, half-disorganized structures in the feathers produces a sort of dull whitish, not nearly as impressive as either of the parental species. Having a nice crown is important: the manakins choose their mates based on their crown color. So what is a poor, dull hybrid to do?

Apparently, incorporate a different technique entirely to give their crown some flair. The yellow in the crown comes not from tiny structures in the feathers, but from a yellow pigment, a carotenoid (the same kind of molecule that turns house finches red, orange, and yellow). In a population of dull hybrids, whichever bird developed a mutation that covered its crown in yellow pigment must have been highly attractive, and so the hybrid population quickly became yellow-crowned.

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That’s cool, can I get a yellow crown? (Female white-beared manakin; photo by Nathan Rupert on flickr, used via a Creative Commons license.)

The golden-crowned manakin probably speciated within the last 260,000 years. In evolutionary terms, that is still a young species, but clearly the species is viable: hybridization can produce strong species. (That sound you hear is all the botanists in the world rolling their eyes: plants speciate through hybridization all the time, and nobody gets all worked up about it. But animals don’t do it very often, botanists!)

This Valentine’s Day, as you eat chocolate or drink wine or try to forget that the holiday exists at all, spare a thought for Thomas and Nigel, and the female medium ground finch in 1981 who thought that that new, large male looked really sexy, and the snow-capped and opal-crowned manakins who liked each others’ styles. You never know where a love story will lead.

References

Barrera-Guzmán AO, Aleixo A, Shawkey MD, Weir JT. 2017. Hybrid speciation leads to novel male secondary sexual ornamentation of an Amazonian bird. Proceedings of the National Academy of Sciences 115(2):201717319.

Lamichhaney S, Han F, Webster MT, Andersson L, Grant BR, Grant PR. 2017. Rapid hybrid speciation in Darwin’s finches. Science 359:224-228.

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