How do we know that homologous structures really do support evolution?

The problem here is that these assessments are dependent on how independent something must be to be 'completely independent', and managing to agree on a specific level.Both bat and bird wings developed from the tetrapod forelimb so there is an arguable basis for this being an example of parallel rather than convergent evolution.But even if we look at insect limbs we see that the mechanisms of limb development are still to a large part conserved suggesting the existence of deep homologies at at least the molecular level in the esatblishment of these structures. Is this independent enough or not?So in one respect parrallel and convergent evolution are often the same if we allow the development of shared homologous structures into novel structures with similar functions to represent a loss of 'complete independence'. In this way the question of parallelism or convergence is mostly a matter of perspectve.Parallel evolution has been defined as "the independent occurrence of similar changes in groups with a common ancestry and because they had a common ancestry" while "convergence is the development of similar characteristics separately in two or more lineages without a common ancestry pertinent to the similarity but involving adaptation to similar ecological status" (Simpson, G. G., 1961. Principles of Animal Taxonomy. New York, Columbia University Press). Clearly discriminating between these two has a subjective element in terms of deciding exactly what counts as a 'pertinent' element of common ancestry.For my part I like to take parallel a little more strictly and only really consider things examples of parallel evolution if the traits are arrived at by the modification of the same genetic loci, though not necessarily the same mutation, this makes the assessment much more objective. A good introduction to parallel genotypic evolution is in a review by Wood, et al. (2004).TTFN,WKThis message has been edited by Wounded King, 07-14-2005 12:07 PM

Interesting Wounded King, I've never heard of this division, I'll have to check out Wood's review. It has always bothered me that convergent evolution and parallel evolution have such overlapping meanings.That being said, this technique is fundamentally useless when it comes to extinct lineages (i.e. 99.9% of life), so the million dollar question is the cross-application of this method to paleontology.Perhaps this process of mapping out the divergence of convergent and parallel evolution genetically... and I have to add that I have a headache following that sentence myself... should start with Homeobox genes as arguing for convergent structures, with subsequent control genes reflecting inner-clade parallel evolution.Homeobox genes are fairly universal, so it widens the definition of convergent evolution, but it certainly is a place to start. If nothing else, the presence of LINE's and SINE's and other such repeats in microsattelite DNA could trace the difference in related linneages, and then through reverse-engineering we can come up with a satisfactory threshold to be applied throughout all taxa.Oh, and before I jump out, the comment that belittles the contribution of selective pressure. Ancestral bats were almost certainly in competition with ancestral birds, perhaps selection favors imitation over innovation? Could we look at this issue from a more classical Darwinian approach?Auf weidersehen,
Theus

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