Joined: Oct. 2005
[quote=Cornelius Hunter,Feb. 05 2007,01:31][/quote]
|(CH, addressing various people):|
Please be careful. My point was merely that you'll have a difficult time persuading people that the evidence powerfully supports your theory, if you are interpreting observations accoring to the theory.
The reasoning here is circular because you are interpreting the evidence according to the theory of evolution, and then claiming it powerfully supports evolution. It makes little sense to explain that homologies such as the pentadactyl pattern are powerful evidence for evolution because, after all, such similarities are "deeper and are the result of common inheritance," whereas those other similarities "are superficial and are appear not to have resulted from shared inheritance." What you need to do is explain why some similarities are "deeper" and others are "superficial."
We've already compared the pentadactyl pattern with the skin stretched between the fore and hindfeet of the phalanger and flying squirrel. Your answer was that skin is easier to evolve than bones (wasn't that you?). That was sufficient for me. I do not need to multiply examples, for this one is subjective, circular, and clearly shows the weakness of the claim.
I’m going to repeat some of what I said earlier, because I think I already answered your points. Theory holds and observations confirm that functionally important features are subject to considerable selective pressure, so we expect many overall similarities between unrelated organisms that are doing similar things. However, when similarities are arrived at through separate evolutionary histories, then we should also see significant differences within or underlying the grossly similar features, and most of those differences that are not brand new innovations should be consistent with features of the ancestral group rather than with features of the morphologically analogous group. Differences should be especially obvious in parts of the feature that aren't functionally important or which resolve a functional problem in substantially different ways.
The various forms of the vertebrate forelimbs are therefore important evidence for evolution because 1) they share fundamental similarities, 2) at the same time they fall into subsets that are characterized by shared differences (differences between the subgroups but shared within them), and 3) in both cases, the similarities and the differences are better explained by evolutionary history than by design.
A good specific example of this is bats, birds, and pterosaurs. They all fly and all have wings. In each of them, the wing is made of a scapula, one upper-arm bone (the humerus), and two lower-arm bones (the radius and the ulna). There is no particular reason to make a wing with those particular bones: insect wings work fine with no bones whatsoever, for example. If you were making wings for hovering (humingbirds), dynamic soaring (albatrosses, large pterosaurs), strong pumping flight (geese, sparrows), swimming (penguins), and waving around to impress potential mates (ostriches), it is unlikely that you would want to make wings out of the same basic components. (Since when are helicopters, submarines, sailplanes, and 747s constrained to utilize the same basic construction?)
However, underneath that unnecessary basic similarity, each group has a significantly different type of wing construction, with features that are unique to each group. Birds have feathers, alulas, & fusion of fingers 2 & 3; pterosaurs have pteroid bones and the tip of their wing is supported by an extremely long “little finger”, and bats have highly reduced ulnas, with four fingers stretching out to the tips of the wing. And again, many of the differences between the groups of fliers tie each type to their ancestral group rather than to other types of fliers (i.e., other functional groups): early birds had reptilian tails and teeth, and even modern birds lay reptilian eggs with chorion, allantois, and amnion membranes. Bats have fur and give milk. We see this pattern extending down into genetic and biochemical character traits, and it is also widespread (albeit with some notably complex and confusing exceptions) in the fossil records of the various groups.
The best evidence for homology is found when we see the same embryological tissues contribute to a feature in two different organisms, when the same genes are activated during the feature's construction, when the same developmental pathways are utilized, and when the same bones end up in much the same places in the same basic relationships to adjacent bones, nerves, blood vessels, and so forth. Then we can make a reasonably secure claim of homology. If we additionally have a fossil record that shows similar structures or a gradation of change in probable intermediates then the claim is that much stronger.