Begging the question, but how do we know there were tall trees?
And what was the selection pressure which led to the trees getting taller?
For that matter, what were the list of genetic changes all of which would have to occur in tandem in order for the neck to get longer successfully? (Size of vertebrae, together with proper structure to support the neck; changes in ligaments and musculature; changes in the blood vessels and hormones to signal to the blood vessels in the neck to keep blood pressure to the brain correct despite the increased hydrostatic pressure; etc. etc. ad infinitum).
Some things scale continuously; some have abrupt changes in behaviour beyond a certain threshhold.
Has anyone *done* a bioengineering study on the giraffe to see if there are any discontinuous physical characteristics which would require elemental changes to aspects of the physiology, once the neck got beyond a certain length? And the genetic changes necessary for the individual proteins coded for, the macroscopic structures, and the inbred ('instinctive') behaviours to accomodate these things?
Can you actually demonstrate this, or is it nothing more than hand-waving to be accompanied by personal attacks on the questioner?
Cheers!
1) Fossil evidence
2) Photographic evidence
The fact that we can look at pictures of giraffes in their native habitats and see tall trees strongly suggests that tall trees also existed in the past.
And what was the selection pressure which led to the trees getting taller?
One selection pressure (but not the only one) would be that short trees keep getting eaten by all the short herbivores. Have you ever noticed that in a pasture containing cows, the height above the ground of the leaves and branches of the trees in the pasture almost exactly coincides with how high the cows can reach? *That* is a selective pressure.
For that matter, what were the list of genetic changes all of which would have to occur in tandem in order for the neck to get longer successfully?
Probably not as many as you would think. A random mutation in a promoter of a gene responsible for vertebrae formation that causes the neck vertebrae to grow longer would be sufficient. The muscles, ligaments, and blood vessels would automatically grow to fit (just like they always fit people of different heights and bone structures). The only other change might be vascular changes to strengthen vessels against the higher pressures resulting from greater elevation of the head; that would not have to occur simultaneously with the vertebrae elongation. Evolution does not occur through sudden massive changes throughout the entire genome; it progresses change by change.
Has anyone *done* a bioengineering study on the giraffe to see if there are any discontinuous physical characteristics which would require elemental changes to aspects of the physiology, once the neck got beyond a certain length? And the genetic changes necessary for the individual proteins coded for, the macroscopic structures, and the inbred ('instinctive') behaviours to accomodate these things?
Yes, people research all of those issues, and if you are genuinely interested in finding the latest research on biomechanical features of giraffes, I'm sure you can find it in a zoological/veterinary research database. Giraffe-specific research is not generally found in PubMed (although studies covering all of those aspects of physiology are there, if they concern humans, pets, or laboratory animals).
Although I did find this in PubMed:
Pressure profile and morphology of the arteries along the giraffe limb.
Østergaard KH, Bertelsen MF, Brøndum ET, Aalkjaer C, Hasenkam JM, Smerup M, Wang T, Nyengaard JR, Baandrup U.
Zoophysiology, Department of Biological Sciences, Aarhus University, Aarhus, Denmark. kho@hst.aau.dk
Abstract
Giraffes are the tallest animals on earth and the effects of gravity on their cardiovascular system have puzzled physiologists for centuries. The authors measured arterial and venous pressure in the foreleg of anesthetized giraffes, suspended in upright standing position, and determined the ratio between tunica media and lumen areas along the length of the femoral/tibial arteries in the hindleg. Volume fraction of elastin, density of vasa vasorum and innervations was estimated by stereology. Immunohistological staining with S100 was used to examine the innervation. The pressure increase in the artery and vein along the foreleg was not significantly different from what was expected on basis of gravity. The area of the arterial lumen in the hindleg decreased towards the hoof from 11.2 ± 4.2 to 0.6 ± 0.5 mm(2) (n = 10, P = 0.001), but most of this narrowing occurred within 2-4 cm immediately below the knee. This abrupt narrowing was associated with a marked increase in media to lumen area ratio (from 1.2 ± 0.5 to 7.8 ± 2.5; P = 0.001), and a decrease in mean volume fraction of elastin from 38 ± 6% proximal to the narrowing to 5.8 ± 1.1% distally (P = 0.001). The narrowing had a six-fold higher innervation density than the immediate distal and proximal regions. The sudden narrowing was also observed in the hind legs of neonates, indicating that it does not develop as an adaptation to the high transmural pressure in the standing giraffe. More likely it represents a preadaptation to the high pressures experienced by adult giraffes.
Can you actually demonstrate this, or is it nothing more than hand-waving to be accompanied by personal attacks on the questioner?
I don't engage in personal attacks, and I endeavor to show the evidence, or at least have it available, to support anything I say.