avocationist
Posts: 173
Joined: Feb. 2006
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Henry,
| Quote | | A question here - in what way would 7% of a wing differ from simply an arm? |
Soren Lovtrup, professional biologist in Sweden, said "...the reasons for rejecting Darwin's proposal were many, but first of all that many innovations cannot possibly come into existence through accumulation of many small steps, and even if they can, natural selection cannot accomplish it, because incipient and intermediate stages are not advantageous."2 Well known evolutionist vertebrate paleontologist Robert Carroll asked if the gradual processes of microevolution can evolve complex structures:
"Can changes in individual characters, such as the relative frequency of genes for light and dark wing color in moths adapting to industrial pollution, simply be multiplied over time to account for the origin of moths and butterflies within insects, the origin of insects from primitive arthropods, or the origin of arthropods from among primitive multicellular organisms? How can we explain the gradual evolution of entirely new structures, like the wings of bats, birds, and butterflies, when the function of a partially evolved wing is almost impossible to conceive?"10
Feuccia and Martin believe birds evolved from reptiles but not dinosaurs (I didn't know there was a difference):
It's biophysically impossible to evolve flight from such large bipeds with foreshortened forelimbs and heavy, balancing tails," exactly the wrong anatomy for flight. (15)
There is also very strong evidence from the forelimb structures that dinosaurs could not have been the ancestors of birds. A team led by Feduccia studied bird embryos under a microscope, and published their study in the journal Science." (16) Their findings were reported as follows:
New research shows that birds lack the embryonic thumb that dinosaurs had, suggesting that it is "almost impossible" for the species to be closely related. (17)
Most of the rest from chapter 9 of Denton's Crisis
According to Denton, who critiques Gerhard Heilman's book The Origin of Birds, his 'scheme is highly speculative. He attempts no rigorous mathematical aerodynamic approach, which would give estimates of wing area, body weight, and lift at the various stages to show that his frayed scaled aerofoil would work and that the transition to gliding, and from gliding to powered flight was at least feasible...there are serious doubts about the feasibility of the transition from gliding to powered flight. ...the physical adaptations for powered flying are in opposition to those of gliding flight. The aerofoil of a glider is usually a membrane attached to the body...which extends out to the fore and hind limbs. In the case of a powered flying, lift and thrust are usually generated by surfaces such as the wings and tail, which are some distance from the main mass of the animal."
The arboreal (trees down) theory is also considered implausible by Ostrom on the grounds that all birds, including Archae, exhibit anatomical features which seem to preclude them from having descended from arboreal climbing ancestors:
'The critical point is that in order to fly, the animal first had to be able to climb. However, according to the design of modern birds, (including archae) that skill may not have been part of the repertoire of primitive birds or their ancestors.'
Problems in getting airborne from running include loss of thrust when the hind feet get off the ground, not overcome by the primitive enlarged scale "wings" and therefore would not be selected.
Remarking that the running, bipedal insectivore leaping after prey scenario is somewhat plausible, Denton nonetheless says that "no known animal regularly catches flying insects by leaping after them...nearly all insectivorous vertebrate species take their prey on the ground. Only the most skilled flyers, the bats and a few birds, are able to capture insects in the air."
The Mexican roadrunner is fast, never leaps, and can barely fly, so Denton thinks the niche envisaged for proto-avis is not very attractive.
"Altho many variants of both the arboreal and cursorial theories have been proposed over the past century, to date no overall scheme has ever been developed which has not seemed impausible to some degree to a significant number of authorities...and there are a host of more specific problems, such as...the difficulty of explaining the origin of the feather.
"The central difficulty with all gradual schemes for the evolution of the feather is that any aerofoil will only work if the feathers are strong, capable of resisting deformation and capable of forming an impervious vane. Moreover, there has to be a sufficient surface area to achieve the requisite degree of lift. "
Per Heilman, the original impervious vane which supported these pre-avian species as they glided was a set of 'longish scales developing along the posterior edge of the forearms and the side edes of the flattened tail' and then,
'By the friction of the air the outer edges became frayed, the fraying gradually chaning into still longer horny processes which in the course of time became more feather like.'
But Denton says, "it is difficult to understand what the adaptive value of frayed scales would be to a gliding organism, when any degree of fraying would make the saales pervious to air, thereby decreasing their surface area and lift capacity...all gliding organisms present an unbroken surface to the air...it would seem reasonable that selection for gliding would always tend to... decrease the tendency to fray.
Apparently Ostram envisages the forelimnbs evolving into an insect-catching net, but Denton points out that it is difficult to envisage a net catcher turning into an impervious aerofoil, because a net must be pervious to air.
And,
"It is not easy to see how an impervious reptiles scale could be converted gradually into an impervious feather without passing thru a frayed scale intermediate which would be weak, easily deforemed, and still quite permeable to air. It is true a feather is basically a frayed scale - a mass of keratin filaments - but...the filaments are ordered in an amazingly complex way to achieve the tightly intertwined structure of the feather. Take away the exquistite coaadaptation of the components,..hooks and barbules, the precisely parallel arrangement of the barbs on the shaft and all that is left is a soft pliable structure utterly unsuitable (to flight). ...it seems impossible that any transitional feather-like structure could possess even to a slight degree the crucial peroperties. In the words of Barbara Stahl, in Vertebrate History: Problems in Evolution, "how they arose initially, presumbly from reptires scales, defies analysis."
Jeanno-
Thanks for the cute pic. As mentioned above and below, gliders are in a different track from winged fliers:
A gliding stage is not intermediate between a land animal and a flier. Gliders either have even longer wings than fliers (compare a glider's wingspan with an airplane's, or the wingspan of birds like the albatross which spend much time gliding), or have a wide membrane which is quite different from a wing (note the shape of a hang-glider or a flying squirrel). Flapping flight also requires highly controlled muscle movements to achieve flight, which in turn requires that the brain has the program for these movements. Ultimately, this requires new genetic information that a non-flying creature lacks
And last but not least: (how can I resist?)
Do they not see the birds suspended in mid-air up in the sky? Nothing holds them there except God. There are certainly Signs in that for people who believe. (Qur’an, 16:79)
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