Paulsamuel,
Glad to hear I'm an appreciated contributor. I must say that although evolution is mentioned frequently in classes and seminars I attend, I am amazed at the lack of explicit treatment and discussion of evolutionary theory in my program (maybe it's presupposed - but I was a bioengineer undergrad). Seeing as I've never actually taken a class on evolution I would be interested if you could refer me to a good text for overview and introduction to the topic appropriate to one that has a fair degree of tangential knowledge and general familiarity.
It's good to see a discussion with some interesting meat, in addition to being busy (PhD program) I had stopped visiting so much because the clutter seemed to often threaten to overwhelm most topics.
I'm a big fan of integrative efforts of various biological disciplines as well as insights from other scientific backgrounds (my point being I think an evolutionary approach is important to understanding the way developmental systems are organized). It seems unfortunate that the excitement of the potential to apply biological knowledge to medicine has de-emphasized questions of evolution in much of the molecular biology research done (although insight into the evolutionary process would aid in portability of knowledge between organisms). Though it was also typically very one molecule oriented, which removes the necessary context required to actually consider the evolutionary potential of a gene.
Anyways, enough personal clutter ... morphological blockage of reproduction - I certainly didn't mean to suggest it wouldn't happen. I think biological systems are such that any mechanism that would do something would occur at some frequency, but there are biases given the details. The chirality swap is an interesting one - I was thinking too symmetrically - my presupposition of morphological mechanisms being uncommon was borne on the idea that most morphological changes that would disallow the physical act of mating would typically not improve fitness and would be blocked for that reason.
In the spider example not knowing what exactly those structures are makes any hand waving thoughts on their genetic/developmental basis difficult. I guess though, that in any creature that has complicated specialized mating structures the potential of morphological blockage is also greatly potentiated. I was thinking more of the typical rod and hole arrangement ...
As to saltation and graduation - I'm not clear on the exact definitions, but would imagine graduation is a gradual change and would therefore guess saltation is a radical jump. So here I think it really depends on the trait to be discussed and the molecular mechanisms involved. Something like the length of a limb could be changed by the increased diffusion of a morphogen (diffusing signalling molecule controlling cell differentiation and programming) leading to an increased limb length. When it comes to generating shapes the situation is obviously more complicated and one would suppose that there can be more radical shifts. I guess my point would be the division may be an arbitrary one differentiating two possibilities given the characteristics of molecular mechanisms shaping the morphology.
The hox genes are also relevant here. In Drosophila one can convert the haltere (which is a derivative wing like structure used in balance during flight) to a wing by knocking out Ubx a hox gene (which are all transcription factors). This leads into Carroll's main argument regarding morphogenetic evolution and hox genes - apparently there is a tendency for repeated body structures like centipedes to give rise to organisms with fewer segments that are more differentiated. It is interesting that the event comes from the differentiation of duplicated structures similar to the genetic situation of duplicated genes become differentiated in function.
Indeed there seems to be some type of scale invariance. It happens with organisms (speciation), segments, probably cell types, genes, and sets of genes (Hox clusters are actually duplicated - there are 2 in Drosophila and more in human and mouse though I don't remember how many). All of evolution seems to hinge on copying with mistakes (edit: I should probably say changes as evolution certainly takes advantage of this so it's encorporated and the religious might attack the word usage). It's interesting to note that without fluctuations/temperature/"randomness" there would be no evolution.
Anyways enough pontificating - some hox references (yanked from the suggested readings in Carroll's book):
Duboule D, Dolle P.
The structural and functional organization of the murine HOX gene family resembles that of Drosophila homeotic genes.
EMBO J. 1989 May;8(5):1497-505.
Krumlauf R.
Hox genes in vertebrate development.
Cell. 1994 Jul 29;78(2):191-201.
Graham A, Papalopulu N, Krumlauf R.
The murine and Drosophila homeobox gene complexes have common features of organization and expression.
Cell. 1989 May 5;57(3):367-78.
enjoy ... I haven't read them myself, but they looked like the best to start with (despite the need to dig up the hard copy).