Evolution News & Views
Editor’s Note: If you’re anywhere that’s remotely convenient to the Seattle area on July 17, you really must join Discovery Institute for our theatrical premiere of Flight: The Genius of Birds. Go here for more information.
The vast array of feather patterns seen across the avian world is truly astonishing. From black ravens to snowy egrets, from monochromatic sparrows to vivid rainbow-colored birds of paradise, from robins to pheasants, birds create a tapestry of feather patterns and colors unequalled in the animal kingdom. How do they do it?
The new documentary Flight: The Genius of Birds from Illustra Media takes viewers into the close-up details of a single feather, showing its complex organization of barbs, barbules, and hooks. The feather shown, though, is of a single color. What about multi-colored feathers with stripes, spots and other patterns? As a feather grows from its follicle, what turns the colors on and off at the right time to produce the result?
Scientists from Taiwan and the U.S. recently figured out part of the solution. Publishing their results in Science, they found a remarkable suite of regulators in the follicle working together, able to produce any pattern — somewhat like an ink-jet printer. A lay-level discussion of the paper was published on PhysOrg.
At the base of the follicle, a ring of melanocyte stem cells (MsSC) called the proximal follicle produces melanocytes containing the dark-colored protein melanin. The stem cells can switch between creating melanocytes with or without melanin as the feather grows. The precise firing of melanin-containing cells allows the feather to emerge with stripes, spots, vibrant colors and a multitude of patterns. Another protein in the follicle named agouti modulates the production of melanocytes, providing another level of regulation. From the Abstract:
Melanocyte progenitors are distributed as a horizontal ring in the proximal follicle, sending melanocytes vertically up into the epithelial cylinder which gradually emerges as feathers grow. Different pigment patterns form by modulating the presence, arrangement, or differentiation of melanocytes. A layer of peripheral pulp further regulates pigmentation via patterned agouti expression. Lifetime feather cyclic regeneration resets pigment patterns for physiological needs.
When the feather is complete, the stem cells go quiescent, until needed again if the feather has been damaged or plucked and a new one is required, or if it’s time to molt. Think of a male peacock feather and you begin to realize the remarkable spatial and temporal control needed to generate an “eye” pattern at the ends of the tail feathers.
The authors identified a dozen other signaling molecules that participate in the regulation of melanocytes. These players use feedback mechanisms to monitor and control the output as the feather grows. From a single melanocyte cell type, after all the players do their work, a rainbow of colors and a dizzying array of patterns can be produced. Simultaneously, other mechanisms must control the geometry of a feather, generating the barbs, barbules, hooks and the “herringbone pattern” illustrated in the film.
The researchers transplanted a pigment-producing quail follicle into a white chicken and found the chicken produced colored feathers. This led them to believe that the patterning on the bird’s coat is not controlled directly by the DNA code, but rather by the spatial organization of MsSC’s in the follicle and the regulatory mechanisms. Something in the genome must control these factors, though, for each member of a species, like a black-capped chickadee or bald eagle, to produce identical feather patterns.
The paper is titled, “Topology of Feather Melanocyte Progenitor Niche Allows Complex Pigment Patterns to Emerge.” The topology may allow the patterns to emerge, but what makes them emerge? That’s like saying “The topology of an ink-jet printer allows a printed photograph or page of text to emerge.” It doesn’t mean it will emerge, unless something higher is controlling the output. The production of patterns on feathers is actually more like the new 3-D inkjet printers, allowing production of different colors and patterns on the front, back, tips and edges of the feathers.
Surely something even more marvelous than described in the paper is going on in those feather follicles. Studying individual follicles is instructive on one level, just as studying individual ink jets in a printer would be, to see how they work. But the finished product, whether it be a peacock, chicken, or iridescent hummingbird as shown in Flight: the Genius of Birds, is more than the sum of the parts. Consider that each follicle is part of an overall 3-D program that can produce the overall look of a quail, ring-necked pheasant, or parrot. Moreover, the program continues over the lifetime of the bird, allowing feathers to regenerate the pattern if needed. The program adapts over the bird’s lifetime, furthermore, leading to those sleek, black-and-white-and gold mature emperor penguins from the gray downy chicks that hatch out of the egg.
Another question not addressed by the paper is how some birds exhibit optical nano-structure in their feathers. The brilliant iridescent colors on some hummingbirds and tropical birds is produced not by pigment, but by geometrical patterns at the scale of wavelengths of light, creating optical interference that intensifies some colors and cancels out others. That’s why some feathers switch from drab to brilliant depending on the angle of view. This ability, apart from melanin production, must be guided by additional precision regulatory mechanisms as the feather grows. (Similar optical tricks are found in the wing scales of some butterflies.)
Darwinian evolutionists are prone to see sexual selection as “the” explanation for such elaborate patterns. To what extent it operates to modify patterns is debatable, but sexual selection presupposes the existence of the exquisite mechanisms in the feather follicles before it can come into play. Why would stem cells, hidden in the dark crevices of a follicle, “know” they need to switch melanin on and off at particular times, in relation to neighboring follicles and those on distant parts of the bird? At best, sexual selection is like using Photoshop to tweak an already-existing picture.
The sexual selection angle is also a vacuous argument, as it accounts for opposite outcomes in terms of sexual dimorphism — males and females that look very different, as in mallard ducks, as well as sexes that are patterned nearly identically, as in crows. It’s not a standalone theory, therefore, but one that must call on auxiliary hypotheses for support, compromising its explanatory power. Additionally, it doesn’t explain the high degree of artistry that makes bird watching such a passion for humans. Many bird patterns appear over-designed for simple mate choice. Worst of all, sexual selection doesn’t explain the origin of the mechanisms responsible for patterning in the first place.
Sexual selection theory must not have been important for the authors. They mention it only briefly in passing: “Complex pigment patterns have coevolved with feather shapes to generate spectacular plumage, as seen in male peacocks, which inspired Darwin to propose sexual selection.” That’s it; moving right along, they focus their attention on molecular mechanisms that create the patterns. “Feather colors are complex, and the mechanisms for pigment patterning are poorly understood,” they write. “Here, we explore the cellular and molecular basis of pigment pattern formation.”
Mammals use equally complex mechanisms to produce fur patterns (think of a ringtail, zebra, leopard, or your pet cat). What is the evolutionary explanation for that? Why, “convergent evolution,” of course, since no one believes that mammals evolved from birds. The best explanation would not force observations into a predetermined belief, but would find the true cause able to account for the phenomenon. Whenever we find complex patterns produced by interacting parts in our experience, the best explanation is intelligence.
As Timothy Standish says at the end of the Illustra film, “They’re engineering marvels. They’re works of art. We know where engineered things come from. We know where works of art come from. Why would we attribute a bird to anything but intelligence or mind?”