Loss of bird coloration genes points to evolution

Coloration in birds is produced by a number of mechanisms, one of which includes the ability to harness molecules called carotenoids. These are found in a number of plants, and quite often give the yellows, oranges and reds that you can see in fruits. By feeding on such plants, birds can actually sequester these carotenoids into various tissues, including their skin and plumage, to visually signal to members of their species, and in the retinas of their eyes, to enhance their color vision.

Bird carotenoids
Image showing yellow and red carotenoids in bird feathers and their chemical structures (Lopes et al. 2016, Current Biology)

However, not all colors in birds are produced solely by the plants that birds eat. Recently, scientists discovered that birds possess an enzyme that can modify yellow carotenoids into red ones [1], and this appears to explain how red carotenoids are formed in their feathers, skin and retinas. Yet, not all birds utilize red carotenoids for coloration or even for color vision. While it appears to be extremely rare to lack them altogether, evidence suggests that owls and penguins are the sole known exceptions to this rule.

I looked into this further by studying the gene encoding the red carotenoid enzyme, CYP2J19 [2, 3]. If the ability to make red carotenoids was present in the common ancestor of birds, we would except to find the gene in nearly every living bird species. However, we would further expect that if the gene was present in the common ancestor of birds, the gene would have become a nonfunctional pseudogene in owls and penguins since they don’t make red carotenoids.

Indeed, that is what I found! Nearly every one of the 80 species of bird I looked at possesses an intact CYP2J19 gene, ranging from loons, turacos, parrots and trogons. However, genomes of two penguins and two owls I examined retain CYP2J19 only as a pseudogene. This suggests that these birds formerly had the capacity to produce red carotenoids, but their adaptations to foraging in dim light (nocturnality, ocean) likely rendered the color discriminatory function useless, and the minimized coloration in the drab nocturnal owls and the countershaded penguins probably negated any need for bright red coloration.

CYP2J19 pseudogenes
CYP2J19 pseudogenes in penguins, owls and a kiwi

I also found evidence that another species, a kiwi, has lost the functionality of its red carotenoid enzyme. Kiwis also have particularly drab coloration, probably related to their nocturnal habits, the latter of which also likely diminished their need for extensive color vision.

Brown kiwi
Brown kiwi

This provides further evidence that owls, kiwis and penguins descended from ancestors more similar to most others birds: day-active and living on land, colorful and relying strongly on color vision. At some point, we assume these particular species adapted to more dim-lighted niches, such as nocturnality and foraging in the ocean. No longer needing the ability to make red carotenoids for their plumage, skin or eyes, the gene was disposed of once and for all.

Questions for Creationists
Why would the Creator create birds with a dysfunctional red coloration gene? Wouldn’t it have made more sense to create them without the gene? Did owls and kiwis descend from diurnal birds that had more detailed color vision? How could their specializations for dim light have evolved within only 6,000-10,000 years?

References

1. Lopes, R. J., Johnson, J. D., Toomey, M. B., Ferreira, M. S., Araujo, P. M., Melo-Ferreira, J., … & Carneiro, M. (2016). Genetic basis for red coloration in birds. Current Biology, 26(11), 1427-1434.

2. Emerling, C. A. (2018). Independent pseudogenization of CYP2J19 in penguins, owls and kiwis implicates gene in red carotenoid synthesis. Molecular phylogenetics and evolution, 118, 47-53.

3. Hanna, Z. R., Henderson, J. B., Wall, J. D., Emerling, C. A., Fuchs, J., Runckel, C., … & Dumbacher, J. P. (2017). Northern spotted owl (Strix occidentalis caurina) genome: divergence with the barred owl (Strix varia) and characterization of light-associated genes. Genome biology and evolution, 9(10), 2522-2545.

3 thoughts on “Loss of bird coloration genes points to evolution

  1. Ambopteryx

    The only YEC answer I can think of would be to say,
    “See this only proves that all kinds of birds were originally designed with the same coloration genes. Then loss-of-information mutations after creation and the Flood led to these pseudogenes. It also doesn’t prove birds came from anything else, they’re still birds and always have been.”

    Of course, if creationists acknowledge one pseudogene, that starts down the slippery slope to accepting others and their implications.

    1. Yes, your last comment hits the nail on the head! In addition, if these birds lost this gene, then it raises questions of other aspects of their anatomy, physiology, etc.

      For instance, were owls originally designed to be diurnal, and had all of the adaptations that typically are associated with day-active birds? If so, how did they adapt so rapidly to a nocturnal lifestyle, and did all owls do this in parallel, or did an ancestor of their kind adapt and they inherited the traits (shortening the time it would have occurred in). If not and owls were created nocturnal, why would the Creator throw in a gene that would simply become lost anyway?

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