Feb 11 2013

The Evolution of Feathers

Published by under Evolution
Comments: 8

As a follow up to my post last week on feathered dinosaurs, I received this question over e-mail:

How would a creature get feathers in the first place? I figure there would be some intermediary stages between no feathers and fully feathered, but what would these stages possibly be? No other family of species seems to have anything remotely like feathers. Also, what would be the evolutionary advantage of having feathers be specially since the dinosaurs discovered in China were flightless and (to the best of my knowledge) flying dinosaurs like the pterodactyl were already featherless. Any light you could shed would be appreciated.

The e-mail comes from someone who accepts evolution (not a denier), but is genuinely confused about the above questions. This is an excellent question, one that Darwin himself confronted. This also remains one of the common denialist tactics of the creationists, despite the fact that Darwin gave a very cogent answer in Origin of the Species.

The broader question is – how do complex features evolve when their utility would not come into effect until they were far along the path of evolving to their current form? What use is half an eye or half a wing? This question was rebranded in recent decades as the notion of “irreducible complexity,” but the essential question is the same.

The unstated major premise of the question is that a feature must have evolved directly to its current use. Feathers and wings are currently used for flying and so they evolved directly for that purpose. Evolution, however, does not see that far into the future. Features are evolved for immediate use. They must have current utility. Evolution is also opportunistic and chaotic. Features that evolve for one purpose can be adapted to another.

This process was originally called “preadaptation,” but that term fell out of favor because it suggests some anticipation of the later use, which is misleading. Gould and Vrba proposed the alternate term “exaptation” in 1982. This can refer to a trait that evolved for one purpose and then was coopted for another, or to a trait that arose through genetic drift (without a specific adaptation) and then was coopted.

Let me borrow the now famous example from Michael Behe of the bacterial flagella, which he claimed were “irreducibly complex.” It could not function (as a flagellum – which is logically implied but ignored by Behe) if it were less complex than its current form. ¬†Here is an excellent lecture by Ken Miller explaining how the various parts of the bacterial flagella are all homologous to proteins and structures that serve some other purpose. The base of the flagella is the Type III secretory system that some bacteria use to inject toxins into other cells, for example.

So what about feathers? First, let me address the statement by the e-mailer that “no other family of species seems to have anything remotely like feathers.” This is not true. Feathers are an adaptation of the integument (the skin). Terrestrial vertebrates developed a number of different integumentary adaptations. Reptiles have a variety of scales, while mammals have hair and fur. Take a look at the bristly scales on the Atheris hispida¬†snake.

Birds also have integumentary features other than feathers. The wild turkey is a good example – it has a hairy “beard” and skin outgrowths on the neck. Bird beaks and claws and integumentary adaptations, and birds typically have scales on their feet.

Non-avian dinosaurs, from what evidence we have, also had a variety of integumentary features. So – while I agree that feathers are perhaps the most dramatic such adaptation of terrestrial vertebrate integument, they are by no means unique.

The exact evolutionary origin of feathers has not yet been worked out, but scientists are making good progress. The question is – how far back in the history of dinosaurs, or perhaps even pre-dinosaurs, do feathers or their precursors go? This will probably be worked out through genetic analysis.

What purpose could early feathers have served before they were adequately adapted to flight? There are many hypotheses. Since we have no living examples of feathered dinosaurs or similar creatures it will be difficult to definitively demonstrate what purpose early feathers served. What we can do is to come up with plausible options and see that at least they are consistent with the fossil evidence.

One hypothesis is that early downy feathers were an adaption for thermoregulation. Down is very insulating, and would have been very useful for keeping warm, especially for young dinosaurs. These feathers could then have been coopted and adapted for mating displays. Broad feathers could also be used for trapping insects for food. More directly related to flight, early feathers could have been used to increase the range of predatory pounces, and even provide for some mid-air correction. They could also have been used to slow and guide the descent when dropping from a height, such as a tree limb. This could have led to gliding flight, and finally to full flapping flight.

The above examples are not a literal linear sequence, and also adaptive radiation means that feathers may have taken many different paths through various adaptations and uses, at least one (probably more) eventually leading to full flapping flight.

This at least answers the question – how could feathers have evolved before they were developed enough to be useful in flight. It turns out that half a wing could have had many uses.

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