The Evolution of Feathers
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.
Did the brain then evolve through separate mutations to use the feathers for gliding and flying?
How many mutations did that take, and how many generations? And is that consistent with mutations being random?
My brain is imagining that primitive brains evolved ways of using new features through informational mutations, so to speak, which would be far faster than genetic mutations in the brain cells. In any case it took many generations and a lot of selective pressure, no doubt.
there was an interesting repeat at discovery channel hat showed that even flying birds use their feet and the help of their wings when ascending ramps up to almost 90 degrees instead of flight.
This shows that walking is still hardwired in bird.
Mutations are random, the successful traits are selected for and non random.
I find the host/parasite interplay much more a cause for wonder regarding the evolutionary process than something as simple as feathers developing from scales.
Think about the control of the liver fluke on the behaviour of an ant, and the cycle through differnet hosts it has to complete towards adulthood.
The ways that parasites are able to control host behaviour to the benefit of the parasites procreation and survival success by chemical signalling is just amazing, as it can be considered a co-evolutionary process between host and parasite.
A recent article by Prum and Brush has shown that feathers developed through a number of stages, from simple down to various types of less complex feather that still do not have the asymmetric vane and shaft necessary for flight. Thus, feathers were a body covering (and possibly a mechanism of display and species recognition) in most dinosaurs long before they were co-opted for flight.
The study you’re thinking of was done by Ken Dial, who showed that most ground birds used their feathers not for flight, but for “wing-assisted incline running (WAIR)”. This is a plausible way for dinosaurian proto-birds to use feathers to escape predators, and eventually might launch themselves off the top of such inclines and engage in short gliding flight.
Oops, typo: “Bird beaks and claws –>areand<–.
Hey, that didn’t come out right–not at all what I typed. I was trying to correct the typo in “Bird beaks and claws and integumentary adaptations…” Should be “are integumentary adaptations…”
I can’t for the life of me find the reference, but there was a study of the advantages of half a wing in modern birds, which showed partial wings help slow & control falling. The experiment involved taking baby birds of different ages and dropping them off the side of a building.
taking baby birds of different ages and dropping them off the side of a building
ouch! That’s like the archetype of cruelty :(
I’m guessing there was an airbag or something for a soft landing, in which case it’s fun.
They had IACUC approval, so I imagine the birds were kept from splatting.
Your reply is totally unconvincing. It requires more leaps of faith than a religious zealot does. Here’s the real deal. Be prepared to open your mind for a change.
Evolution isn’t exclusively a product of random mutation, as has been assumed for decades. Scientists who are smart and working on the cutting edge of Evolutionary Biology are beginning to catch on that our traditional theory of Evolution is way too simple. We’re now realizing that:
1) cells communicate with one another,
2) genetics are dynamic (they’re not static like architect blueprints.),
3) cells/DNA receive feedback from their environment and adapt to it, and
4) all this happens within the lifetime of an organism.
All this points to a more coordinated, less random, “smarter” form of DNA adaptation and evolution. Essentially, organisms themselves somehow design (in an “engineering” sort of way) new features, and systems, presumably eventually even resulting in new species. This is all based on their environment and experiences. Not just random mutation.
So how did feathers evolve when there’s nothing like it? You never explained that very well. Reptile scales and human hair are nothing like feathers. Those analogies sucked.
Feathers evolved because “smart DNA” inside organisms solved the engineering puzzle of creating a more lightweight, efficient form of insulation from the existing biological material at hand in the organism. Just like engineers in a Toyota plant might come up with new bumpers on a Camry.
Does this sound too creepy, like a scientific form of “intelligent design”? It sure does. And this creepiness is presumably one of the big reasons biologists have steered clear of this line of thinking for decades. It concludes there is intelligence at a cellular or even genetic level. And that’s exactly what the latest research is finding. Despite the fact that nobody is ready to handle this. It completely shatters the worldview that cells and genetic material are just passive cogs in the wonderful machines piloted by our even-more-wonderful brains. There’s something deeper at play here. And it’s creepy to think that tiny cells inside us are not only designing tomorrow’s humans, but tweaking our own systems to be as efficient as possible for our environments within our lifetimes (just as plants do).
If you’re not ready to handle this, don’t worry – most major scientists today aren’t either. It’s going to take a few years before the evidence becomes irrefutable and new theories have to be formed to explain such an unsettling way of looking at organisms.
I love when people come in and claim that no evidence is given and they don’t like it, cuz it messes up their man-in-the sky view. Then they use the unviersal term such as “scientist”, without citing even one name. If this is so prevelent, who exactly are you refering to? Besides the fact that you show that you haven’t either read or understand what was written, you give even less evidence than that which you chastise the author for?
Just remember, just because you aren’t capable of understanding the science, doesn’t mean that some magic man wished everything into existence.
Half wings,feathers from scales etc… Where’s the fossil record showing the transitions? The bird fossil record is one of the most plentiful I understand.