Joined: Oct. 2005
|Quote (Cornelius Hunter @ Jan. 28 2007,12:11)|
|I came here asking for a justification/ defense of one of the most fundamental claims of evolution. My question was, how is it that similarities such as the pentadactyl pattern are such powerful evidence for evolution, in light of equal and greater levels of similarity in distant species, such as displayed in the marsupial and placental cousin species? |
This was my question. There are many, many more examples of similarities that do not fit the common descent pattern.
Unfortunately the explanation that one type of similarity is "deep" and the other "superficial" isn't going to satisfy very many people. This explanation really just raises more questions. When I asked for details, he [Wells, i.e. me] deferred to Deadman who did give an answer:
This seems like a perfectly reasonable answer, as far as it goes. The problem is it is farily subjective. Do we really want to make one of the fundamental evidential claims for evolution contingent on an opinion about what might, and might not, be more difficult for evolution to accomplish?
Again, you are overemphasizing some superficial similarities while disregarding important differences. If you give a competent scientist either the skeletons or a tissue sample from any of the marsupial & placental analogs, he or she will be able to quickly and easily tell you which of each pair came from Australia & had a pouch.
I think I answered your question more thoroughly than you acknowledged. Theory holds (and observations support) that functionally important features are subject to considerable selective pressure, so we should expect many overall similarities. 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 funtionally important.
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.
I gave you some specific examples: bats, birds, and pterosaurs all fly and all have wings, although many of the details of wing construction are unique to each group (feathers, alulas, & fusion of fingers 2 & 3 to birds, pteroid bones to pterosaurs, highly reduced radii to bats). Furthermore, many of the differences between the fliers tie each type to their ancestral group rather than to other types of fliers: 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.
I already said that if we see the same embryological tissues contribute to two features, the same genes activated during their construction, utilization of the same developmental pathways, and the same bones ending 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.