Creationism in Crisis - Scientists Find New Clues to How Hummingbird Diversity Evolved

Creationism in Crisis

Scientists Find New Clues to How Hummingbirds Evolved Coloured Feathers

Pink-throated Brilliant, Heliodoxa gularis

© Carlos Calle Quispe
16 April 2017

Pink + pink = gold: hybrid hummingbird’s feathers don’t match its parents | Field Museum

The gold-throated hybrid, center, with its parent species H. branickii (left) and H. gularis (right).

© Kate Golembiewski, Field Museum

When scientists from the Field Museum of Natural History, Chicago, Illinois, USA, discovered a hummingbird with an iridescent gold throat, they thought they had discovered a new species, but DNA evidence revealed that it was a hybrid between two closely related species - the Pink-throated Brilliant, Heliodoxa gularis and the Rufous-webbed Brilliant, H. branickii.

The problem was that the two parent species both had pink throats, so you might expect a hybrid to have a pink throat also, not glittering gold.

Colour can be achieved in bird plumage in two ways - pigmentations and, depending on small-scale structures in the plumage, by diffraction of light to produce structural colour that changes a little, depending on the angle the light hits it and the angle from which you look at it, hence the shimmering iridescence. The precise range produced by structural colour depends on tiny structural differences, and it is these that hold the clue to the gold-throated hybrid. The same colour can also be produced by slightly different micro-structures, as in the two pink-throated species, each of which has a unique way to produce the same result.

DNA evidence also showed that the gold-throated specimen was not a first-generation hybrid, but the descendant of one that had then interbreed with the Rufous-webbed Brilliant for several generations. The gold iridescence was produced by the right mixture of the two slightly different micro-structures in the plumage of the two ancestral species, as confirmed by differences in their DNA.

The interesting thing about this discovery from an evolutionary biology point of view - and so something Creationists will want to ignore - is that it sheds new light on what had been something of a mystery in hummingbird evolution. How was plumage diversity achieved so quickly when the mechanism of iridescence requires some major changes to the genetics behind the micro-structure of the feathers. This find suggests that hybridization could have provided a short-cut to what would normally be expected take 6-10 million years.
The discovery is explained in the Field Museum’s news release:

The Pink-throated Brilliant hummingbird, Heliodoxa gularis, has, unsurprisingly, a brilliant pink throat. So does its cousin, the Rufous-webbed Brilliant hummingbird, Heliodoxa branickii. When scientists found a Heliodoxa hummingbird with a glittering gold throat, they thought they might have found a new species. DNA revealed a different story: the gold-throated bird was a never-before-documented hybrid of the two pink-throated species.

I looked at the bird and said to myself, ‘This thing doesn’t look like anything else.’ My first thought was, it was a new species.

We thought it would be genetically distinct, but it matched Heliodoxa branickii in some markers, one of the pink-throated hummingbirds from that general area of Peru.

John Bates, co-author
Curator of birds
Negaunee Integrative Research Center
Field Museum of natural History, Chicago, IL, USA

John Bates, the senior author of a new study in the journal Royal Society Open Science reporting on the hybrid, first encountered the unusual bird while doing fieldwork in Peru’s Cordillera Azul National Park, which protects an outer ridge on the eastern slopes of Andes mountains. Since the area is isolated, it would make sense for a genetically distinct population to emerge there.

When Bates and colleagues gathered more data about the specimen in the Field Museum’s Pritzker DNA Lab, however, the results surprised everyone … If it was H. branickii,it didn’t make sense for the bird to have gold throat feathers; in the hummingbird family, it’s rare for members of the same species to have dramatically different throat colors.

The initial run of DNA sequencing looked at mitochondrial DNA, a type of genetic material that only gets passed down through the mother. That mitochondrial DNA gave a clear result matching H. branickii; the researchers then analyzed the bird’s nuclear DNA, which includes contributions from both parents. This time, the DNA showed similarities to both H. branickii and its cousin, H. gularis. It wasn’t half branickii and half gularis, though-- one of its ancestors must have been half-and-half, and then later generations mated with more branickii birds.

The question remained how two pink-throated bird species could produce a non-pink-throated hybrid. The study’s first author, Field Museum senior research scientist Chad Eliason, says the answer lies in the complex ways in which iridescent feather colors are determined.

It’s a little like cooking: if you mix salt and water, you kind of know what you're gonna get, but mixing two complex recipes together might give more unpredictable results. This hybrid is a mix of two complex recipes for a feather from its two parent species.

Based on the speed of color evolution seen in hummingbirds, we calculated it would take 6-10 million years for this drastic pink-gold color shift to evolve in a single species.

There's more than one way to make magenta with iridescence. The parent species each have their own way of making magenta, which is, I think, why you can have this nonlinear or surprising outcome when you mix together those two recipes for producing a feather color.

Chad M. Eliason, lead author
Negaunee Integrative Research Center
Field Museum of Natural History
Chicago, IL, USA

Feathers get their base color from pigment, like melanin (black) and carotidnoids (red and yellow). But the structure of feathers’ cells and the way light bounces off them can also produce something called structural color. Color-shifting iridescence is a result of structural color.

The researchers used an electron microscope to examine the throat feather structure on a subcellular level, and an analytical technique called spectroscopy to measure how light bounces off the feathers to produce different colors. They found subtle differences in the origin of the parents’ colors, which explain why their hybrid offspring produced a totally different color.

While this study helps explain the strange coloration of one unusual bird, the researchers say that it opens the door to more questions about hybridization.

[T]his study gives us clues about the nanostructural basis of evolutionary shifts in color.

Mark E. Hauber, co-author
Department of Evolution, Ecology, and Behaviour
School at Integrative Biology
University of Illinois, Urbana-Champaign, IL, USA.

Separate species are generally defined as lineages that are genetically distinct and don’t interbreed with each other; hybrids break that rule. Sometimes hybrids are weird one-offs or are sterile, like mules; in other cases, hybrids can form new species. It’s not clear how common hummingbird hybrids like the one in this study are, but the researchers speculate that hybrids like this one might contribute to the diversity of structural colors found across the hummingbird family tree.

Figure 1.
Gorget colour and feather nanostructural differences in Heliodoxa hummingbirds. Images show iridescent gorget coloration in the two pure species, Heliodoxa gularis (a) and H. branickii (c), as well as the putative hybrid individual FMNH 511084 (b). Reflectance spectra (middle panels) were taken at the angle of maximum reflectance (see electronic supplementary material, table S1 for list of specimens) and show a distinct lack of peak at the 450 nm wavelength for the hybrid. Lower panels are transmission electron microscopy (TEM) images of nanostructures responsible for iridescent colour production (scale bars are 500 nm). Note thinner cortex and thicker top platelets in the hybrid (see labels in (e)).

Photo credits: Chad M. Eliason.
Source: Chad M. Eliason, Jacob C. Cooper, Shannon J. Hackett, et al (2023)

Copyright: © 2023 The authors.
Published by The Royal Society. Open access. (CC BY 4.0)

Abstract Hybridization is a known source of morphological, functional and communicative signal novelty in many organisms. Although diverse mechanisms of established novel ornamentation have been identified in natural populations, we lack an understanding of hybridization effects across levels of biological scales and upon phylogenies. Hummingbirds display diverse structural colours resulting from coherent light scattering by feather nanostructures. Given the complex relationship between feather nanostructures and the colours they produce, intermediate coloration does not necessarily imply intermediate nanostructures. Here, we characterize nanostructural, ecological and genetic inputs in a distinctive Heliodoxa hummingbird from the foothills of eastern Peru. Genetically, this individual is closely allied with Heliodoxa branickii and Heliodoxa gularis, but it is not identical to either when nuclear data are assessed. Elevated interspecific heterozygosity further suggests it is a hybrid backcross to H. branickii. Electron microscopy and spectrophotometry of this unique individual reveal key nanostructural differences underlying its distinct gorget colour, confirmed by optical modelling. Phylogenetic comparative analysis suggests that the observed gorget coloration divergence from both parentals to this individual would take 6.6–10 My to evolve at the current rate within a single hummingbird lineage. These results emphasize the mosaic nature of hybridization and suggest that hybridization may contribute to the structural colour diversity found across hummingbirds.

Chad M. Eliason, Jacob C. Cooper, Shannon J. Hackett, Erica Zahnle, Tatiana Z. Pequeño Saco, Joseph Dylan Maddox, Taylor Hains, Mark E. Hauber, John M. Bates.
Interspecific hybridization explains rapid gorget colour divergence in Heliodoxa hummingbirds (Aves: Trochilidae).
Royal Society Open Science, 2023; 10 (3) DOI: 10.1098/rsos.221603

Copyright: © 2023 The authors.
Published by The Royal Society. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

Normally, hybridization serves to blur the boundaries between genetic populations and, especially in birds, pre-zygotic barriers to hybridization in the form of plumage differences, arise after diversity begins, so reducing the undesirable effects of hybrid generality in specialized species. Here we have an example of where hybridization itself could have led to plumage diversity which then provided pre-zygotic barriers to genetically isolated populations with different iridescent plumage.

Of course, another little point, incidental to the main point of the research, that Creationists might like to ignore is the finding that pink-throats in two related hummingbirds were achieved by different genetic mechanisms. In other words, creationism's putative intelligent [sic] designer designed two different ways to achieve the same result, metaphorically reinventing the wheel.

Hardly the work of a supreme intelligence!

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