Where Did These Wings Come From?
Flightless Birds, Vestigial Structures and the Theory of Evolution
If you ever happen to find yourself walking around the Galapagos Islands you will find a most fascinating creature – a Flightless Cormorant. What makes this fine-feathered-friend interesting is that he is the only Cormorant in the world that can't fly.
Now, you shouldn't think that the Flightless Cormorant can't fly because it doesn't have wings – that simply isn't true. It does have wings – and it does a number of things that other Cormorant's do with their wings – such as dry them in the sun after going for a swim. No, the Flightless Cormorant can't fly because his wings are not built for flight – they lack the size and other requisite properties to propel their owner into the air.
This, of course, is a curious feature – a wing that doesn't fly. It looks like a wing, has feathers like a wing, but it doesn't fly like a wing. Why not?
The answer, it is claimed (and in the case of the Flightless Cormorant I think it's a reasonable claim) is that there is a sort of de-evolution going on here. Using more technical language – this flightless wing is known as a vestigial structure and it is one of the pieces of evidence used to argue that evolution is a fact – i.e, that we know that macro-evolution takes place.
Sounds good – evidence, that is exactly what I want. So let's take a moment and look at vestigial structures and see what they can (and cannot) tell us about evolution.
A Vestigial What?
First, though, let's get a better grasp on this most unfriendly of terms – vestigial structure.
To start with, it should be noted that vestigial structures are not limited to wings – there are examples given at the molecular level, in embryos and internal organs as well as other limbs besides wings. I can't cover all of these examples in one post, so for now we'll focus on wings (and hopefully get to some of the other examples in the future).
More important than where we find vestigial structures is how we define them – for how we define a vestigial structure will be important in determining whether or not they provide any evidence for some sort of evolutionary process. As such, I will start with the definition of Professor Douglas Theobald from his famous (or infamous, depending on who you ask) article on 29+ Evidences for Macroevolution.
A vestigial character is reduced and rudimentary compared to the same complex structure in other organisms. Vestigial characters, if functional, perform relatively simple, minor, or inessential functions using structures that were clearly designed for other complex purposes. The most extreme test for vestigiality is to remove the character and observe the organism's viability and reproductive success. If these remain unchanged, the character is definitively vestigial. However, vestigial characters can certainly have functions; non-functionality is not a requirement.
This is quite an interesting definition, one that we need to take some time to analyze. As I mentioned above, definitions here are crucial for they will determine whether or not we have any evidence in the first place (independent of what that evidence means or doesn't mean).
Professor Theobald notes that the definition of functional vestigial structures depends on comparing them to their non-vestigial counter-parts. This seems reasonable enough – the question, though, is how to make the comparison. Let's see how Professor Theobald does it. He notes two key differences:
- Differences in Structure
"…reduced and rudimentary compared to the same complex structure in other organisms"
- Differences in Functionality
"…perform relatively simple, minor, or inessential functions using structures that were clearly designed for other complex purposes"
Perhaps an example will help clarify this point. Assuming that all flightless wings are vestigial then the physical structure of a flightless wing will be a simpler structure in comparison to a regular wing (i.e., a wing which enables a bird to fly). In addition, the function of a flightless wing will be simpler or more minor in comparison to the complex purposes of a regular wing. Alternatively, it will be non-essential instead of being essential.
It is this second point that I find curious and am not sure that it is a good definition. To start with, the word relative seems interesting. Relative in which way? It sounds like he means relative to the primary design which we associate this structure with (such as flying in the case of a bird).
But what if a function is not simple, minor or inessential, but is radically different? That is to say, the bird can't fly with the wings, but can do some other complex, significant or essential function with their wings. Is the wing still vestigial? I would think not. I would think that the word relative, understood this way, would not be appropriate.
I find it hard to see why a well-integrated, sophisticated re-purposing or co-opting of a structure in a new and unique manner should be considered vestigial. A vestigial organ is a relic, a shadow of its functional past. I would need a pretty convincing argument to consider a well-designed, but different, use vestigial.
Let's keep this point in mind since we'll have cause to bring it up again shortly.
Meeting the Flightless Cormorant
Alright, let's return now to our fine-feathered (but flightless) friend – the Flightless Cormorant. Let's get to know him a bit better. To make the introduction we'll turn to none other than famed evolutionary biologist Richard Dawkins. Take a look:
I must admit, that despite Richard Dawkins impressive rhetorical skills, I think this is perhaps the worst piece of 'evidence' that I have ever seen offered for the theory of evolution. Not that I disagree with the assessment that the Flightless Cormorant is a descendant of the regular Cormorant. The fact is that I think that is a perfectly reasonable explanation for this interesting creature – and that is why I find this evidence so irrelevant to the theory of evolution.
You see, this intriguing creature started out a Cormorant and ended up a Cormorant. There is no claim that he came from an earlier, but related species or that he has an ancient ancestor which he shares with the flying Cormorants. No, this flightless fellow seems to simply be a stranded version of the regular Cormorant and his loss of flight not due to any grand evolutionary scheme, but simply to lack of use or need of his wings (probably due to the fact that he doesn't need to flee any predators). And as a result, his wings have atrophied.
I wonder what Dawkins thought he was proving? Perhaps he just wanted to set up a straw-man attack on 'creationists' by suggesting that they think that the Flightless Cormorant was created in the Galapagos Islands. Or maybe he wanted to suggest that this is representative of other vestigial structures.
Either way, both claims are rhetorical, rather than scientific or factual. You can't prove the theory of evolution by knocking down creationism (the straw-man version or the real thing) and you can't create an evolutionary line from a crippled version of the Cormorant. The Flightless Cormorant is obviously not the beginning or end of a grand evolutionary line. His common ancestor was his fully functioning self. As such, we can't learn from him anything about how birds or wings originated or developed or how one creature can evolve into another.
But we should not think that we can learn nothing of interest from our flightless friend. For his atrophied wings do provide a nice contrast or benchmark by which to judge other flightless creatures – some of whom we will meet right now.
The Big Bald, Flightless Bird
Let's return back to Professor Theobald as he will provide us with a more fitting candidate for a vestigial wing. Once again, we'll let him do the talking:
…wings are very complex anatomical structures specifically adapted for powered flight, yet ostriches have flightless wings. The vestigial wings of ostriches may be used for relatively simple functions, such as balance during running and courtship displays—a situation akin to hammering tacks with a computer keyboard. The specific complexity of the ostrich wing indicates a function which it does not perform, and it performs functions incommensurate with its complexity. Ostrich wings are not vestigial because they are useless structures per se, nor are they vestigial simply because they have different functions compared to wings in other birds. Rather, what defines ostrich wings as vestigial is that they are rudimentary wings which are useless as wings.
Let's note some of the key points of this statement and draw an obvious conclusion from each point:
- "The vestigial wings of ostriches may be used for relatively simple functions…"
Therefore, if we could show that the wings actually perform complex functions then they would not be considered vestigial.
- "The specific complexity of the ostrich wing indicates a function which it does not perform…it performs functions incommensurate with its complexity."
Therefore, if we can show the specific complexity of the ostrich wing is directed towards a function that it does perform and which are commensurate with its complexity then they would not be considered vestigial.
- "…what defines ostrich wings as vestigal is that they are rudimentary wings which are useless as wings…"
Therefore, if we can we can show that the wings are not rudimentary wings and that they are very useful as wings then they would not be considered vestigial.
So here we have a litmus test – we need to show that the ostrich wings perform complex functions which are in sync with the specific complexity of the ostrich wings and which demonstrate that these wings are useful as wings. Or, in other words, the Ostrich's wings are not comparable to hammering tacks with a computer keyboard, but with using that keyboard to write a beautiful sonnet. Let's see if we can do that.
Born to Run
I'm going to go out on a limb here (pun intended) and suggest that Ostriches are built to run (they are also built to dance). Something about these 9 foot tall, 16-feed striding, two-legged, two-toed birds seems to suggest that running is essential to their design. They are the worlds fastest two-legged creatures able to sprint up to 43 MPH and cruise for long distance at 31 MPH. In short, the ostrich's long, powerful legs and uniquely adapted feet are clearly designed for running – come see what I mean:
So what about their flightless wings, might they be built to help the Ostrich run. Might they have a complex structure which is specifically directed towards a function which the Ostrich does perform - namely running? In short, might the Ostrich's wings work just fine as wings - but perhaps wings have other sophisticated uses besides flying?
Well, don't ask me – ask Nina Schaller, a biologist at the University of Antwerp in Belgium and Senckenberg Research Institute in Frankfurt, Germany. Shehand-raised a number of ostriches in a large outdoor German pastures and (after they had grown) "video-recorded them as they raced down nearly 1,000-foot (300-meter) stretches outdoors".
And what did she discover – that "Ostriches used wings as sophisticated air-rudders for rapid braking, turning and zigzag maneuvers". In other words, according to Nina Schaller "you have to stop thinking about their wings as flight organs and as stabilizers instead." Or, in more technical jargon, "these flightless birds can efficiently channel aerodynamic forces."
Let's get a glimpse of what she is talking about:
In other words, Ostriches do have a sophisticated use for their wings, it is of essential use to helping them do what they do best – run. I imagine that it is not so simple to turn or stop a 9 foot tall, two legged animal running at 30 – 45 MPH. It seems that a sophisticated rudder and brakes system is right in sync with the nature of this fine-feathered beast. And, as such, I don't see any reason to consider them vestigial structures.
A Different Definition
I think that part of the problem here is with the definition. It seems to me that in the context of trying to demonstrate that some sort of macro-evolutionary process is taking place, a more specific or relevant definition is in order. I'm not going to provide a full definition – but I will identify two characteristics.
The first factor is that the structure in question cannot be intricately integrated into the essential nature or manner of functioning of the animal in question. The second factor is that the structure in question cannot be an atrophied version of its representative population.
With these two characteristics in mind, we can note that other examples touted as evidence for macro-evolution are not evidence at all. For instance, Penguins who use their feathers to help them swim or the Steamer Duck who uses its wings to paddle in the water (more on that in a later post).
Differences in the nature or manner of function of the animal can also explain the flightless-wings of the Australian Emu who does not zigzag like the Ostrich, but rather "flee predators with short bursts and thus would not need the advanced maneuverability seen in ostriches." In short, "there seems to be a very direct link between wing-size and function."
Add into the mix micro-evolutionary factors such as change in climate and you can also explain the flight-less wings of the Cassowary. As Professor Schaller notes:
The ostrich lives in the Savannah, where the open spaces mean the investment in keeping wings makes absolute sense. The cassowary, on the other hand, lives in the rain forests of Australia and New Guinea, where having long wings wouldn't be very beneficial."
I imagine that studying other flightless birds (or insects) could reveal similar nuances based on either their location or particular manner of function. I for one would assume that the nature of the wings of flightless birds would directly correspond to the nature of the animal and that this could serve as a useful benchmark for some interesting experiments. In other words, rather than assume that these wings are primarily useless and relics, assume that they are primarily useful and needed and then work on discovering what they use and need are.
So, where does this leave us all – waiting. We are still waiting for an example of a vestigial structure that can actually serve as evidence of a macro-evolutionary process. Atrophied versions of ones healthier self and cleverly re-purposed versions of existing structures are out. Of course, there are plenty of other examples cited. We'll take a look at them and see how they stack up another time.
Photo used under Creative Commons from Riverside MC
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