Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: Souffle on 15/06/2009 22:04:20
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I've been pondering this one for a while but remain clueless. As I understand it, geckos can stick to things because their feet are covered in millions of tiny hairs. These hairs create a large enough surface area for the tiny attractive forces that normally just hold molecules together to add up sufficiently to stick a gecko to a wall.
This system wouldn't work except in its current extremely specialised state. However, evolution wouldn't suddenly pop out a perfectly evolved gecko foot; it builds things up incrementally. So how could sticky gecko feet have evolved?
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They evolved by random mutation. The geckos who randomly evolved caster wheels instead were consequently unable to climb walls and so missed out on food available to the hairy-footed ones. Unable to compete, and partly out of embarrassment over looking so silly, the caster-wheeled gecko ultimately died out.
More seriously, random mutation isn't limited to tiny changes, it's just that small mutations are less likely to be damaging, so more of the successful mutations will be small, but as long as the mutation doesn't kill the animal, which it does in >99.999% of cases, anything that confers an advantage is in.
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A recent find by scientists from Oregon State University and the Natural History Museum in London of a gecko' foot fossilised in amber shows that these reptiles had this ability 100m years ago. This fossil is 40m years older than the previously oldest find.
Exactly how and why these tiny hairs (setae) evolved is not known, but I suspect LeeE has the best explanation we can hope for.
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Full disclosure first. I ascribe to Intelligent Design, so I'm coming at the question from that angle.
I work in R&D for a transportation-related company and we're studying (secretly) gecko technology in order to create synthetic substances with much higher coefficients of friction.
That said, the work we've studied from UC Berkeley leads me to conclude that the gecko spatula / setae multi-scale structure is created (designed), not evolved. But, if any of you sees literature that proposes a scheme, please post it. I'm professionally and personally interested.
The problem is that Van der Waals forces develop only when surfaces are extremely close. If particle size is 10(-4) meters (0.1 mm, barely visible) there is no effect. If size is 10 (-5) meters, there is negligible effect. For 10 (-6) meters, the attraction is suddenly large. For practical use on rough surfaces, a flexible, multiscale structure is mandated. There are 4 levels of such multiscale structure on geckos having this adhesion mechanism.
Evolutionary problems seem insurmountable. One must do away with claws (or the soft spatula / setae arrangement are inoperable. But, without claws, you need nano-scale structures that have a particular design. Problems don't stop there. You don't just need the surface hardware. Geckos also "roll" their toes down into contact, as the Van der Waals forces obtain very high shear adhesion but essentially zero PEEL adhesion. They roll and un-roll with lightning speed, very much unlike any other creature. Special musculature and coordination are required. But this is just hardware. They also need the software (instinct) to tell them to do this. They didn't figure it out.
Once you have this hardware and software, the effect is astounding, however. Lab experiments show that "coefficent of friction" is an imprecise term, as it's possible to run up walls, etc. With a normal force, coefficients of friction from 2 to 10+ have been measured, with structures that are non-elastomeric and therefore energy efficient (very low or non-existent tangent delta). Imagine formula one stopping power in your Honda Accord, without ground effects!
hbart
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This Intelligent Design Creationism argument is called the Argument from Personal Incredulity. It is a logical fallacy. Evolution is smarter than you are.
Further, complexity is not evidence against evolution, it's precisely one of the things evolution is expected to produce.
Intelligent Design Creationism is not science in the slightest.
To specifically address gecko feet. There is no need to assume they originated fully-formed and able to stick to smooth inclined surfaces perfectly. Is it really that difficult to visualise that gecko feet evolved slightly better gripping properties than average over many generations? A simple scenario is that the first gecko (gecko ancestor) feet needed only crude projections, such as the scales on their toes, for gripping rough surfaces (most lizards don't have adhesive pads but get along fine without them). Further gradual modifications to those projections could allow geckos to move along increasingly smoother and more inclined surfaces, until, at present, they are effective enough to adhere to glass.
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I didn't read hbart's post. As soon as I saw the 'Intelligent design' bit, I moved on.
Your estimation, Stefan, on the previous post sounds good to me though, as does your explanation on the evolution of the Gecko's foot.
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This Intelligent Design Creationism argument is called the Argument from Personal Incredulity. It is a logical fallacy. Evolution is smarter than you are.
Further, complexity is not evidence against evolution, it's precisely one of the things evolution is expected to produce.
Intelligent Design Creationism is not science in the slightest.
To specifically address gecko feet. There is no need to assume they originated fully-formed and able to stick to smooth inclined surfaces perfectly. Is it really that difficult to visualise that gecko feet evolved slightly better gripping properties than average over many generations? A simple scenario is that the first gecko (gecko ancestor) feet needed only crude projections, such as the scales on their toes, for gripping rough surfaces (most lizards don't have adhesive pads but get along fine without them). Further gradual modifications to those projections could allow geckos to move along increasingly smoother and more inclined surfaces, until, at present, they are effective enough to adhere to glass.
Not to mention the fact that "hairs on hairs" is a common biological tendency that increases surface area - think of the lining of the gut. So the genes to create similar structures exist elsewhere. Modifying existing genes is not uncommon.
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Also consider the feather, with its multiple levels of detail.
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Also consider the feather, with its multiple levels of detail.
Good point - and feathers are just modified scales.
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Thanks Ben and Stephan... hadn't thought about the vdw forces coming into play until far down the evolutionary line. Question answered!