hypothetical construct

Tom DiBenedetto tdib at UMICH.EDU
Tue Dec 9 01:23:57 CST 1997

>sylvia hope wrote:

>>I wrote:
>>Homology is real. It is biological structure or form
>>which is passed from generation to generation amongst organisms
>>throught the information transmission processes of genetics and

>          Homology is a *generalization* about the *correspondence* of
>          biological structures - - - It is *not* those structures themselves.

It is more than that. The correspondance you refer to is, in fact,
identity. Homology is an assertion that two structures are the *same
thing*. At the level of general homology, this might be seen as a
mere concept of correspondance, akin to strong similarity. But the
historical perspective on homology is grounded explicitly in the
notion that there is a real physical continuity in the structures;
they are manifestations of matter and form which are physically
transmitted from organism to organism through time. There is physical
continuity between some of your genetic sequences and those found in
the first life form. The character "bump on this bone" is widely seen
in systematics as a shorthand for the novel or modified ontogenetic
pathway that resulted in that bump, i.e.the substitution of one or
more chemicals in the genetic "blueprint" or the modification of some
epigenetic pathway. The causitive factors which are responsible for
this novel ontogeny are physically transmitted in an unbroken chain
to all organisms in all species which possess it. This should
illuminate a concept of character as a lineage of information and

>          Synapomorphy refers to those homology hypotheses
>          - - - you said it yourself!

Yes, but the act of coding a character for phylogenetic analysis
entails entrance into the historical dimension of homology.
Synapomorphy is a corroborated historical homology hypothesis - it
represents acceptance of the assertion of historical, physical
continuity between the character manifestations in two species.

> Is the recognition and naming of Aves (with a proper name, no less) a
>reification of mere ideas?

>                   This seeming contradiction is resolved by
>          understanding that taxa are not the sum of their
>          synapomorphies, they are individuals connected by lineal
>          descent.

Sorry, but I dont think the contradiction is resolved at all.
Homologies are connected by lineal descent in the same manner. The
relationships of homology (for characters) and monophyly (for taxa)
are the exact same relationship. I do not see how you can assert or
deny the reality of one without admitting the same for the other.

Tom DiBenedetto                 http://www-personal.umich.edu/~tdib/
Fish Division                                   tdib at umich.edu
University of Michigan Museum of Zoology

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