Skip this thread if Biogeography bores you - especially those recurrent discussions about Croizat
SDMANNING at ASUB.EDU
Tue Feb 5 15:38:50 CST 2002
At 11:28 AM 2/4/02 +0100, P.Hovenkamp wrote:
>Hi all those not deterred by mentioning the name of Croizat,
>Readers without access to the writings of Croizat may from the following
>easily gain the impression that Croizat was seeing things (even if not
>writing) clearly and was recognizing the importance of hybridization:
>>>importantly, it is becoming increasingly obvious that reticulate evolution
>>>is important in many, if not most, sexual groups.
>>Actually I would suggest that it has been as 'obvious' all along - just not
>>traditional evolutionary models (I would call them 'Darwinian' models -
>>not). Croizat referred to reticulate evolution as character recombination.
>But this last statement is not exactly true. At least, if Croizat referred
>to reticulate evolution as character recombination, he did not do so in
>writing. I checked the quite extensive index on "Things and concepts" in
>Croizat's "Space, time form" (1964) - unfortunately, his later works are
>not so fully indexed.
>There are c. 27 entries on character recombination, and c. 20 on
>hybridization and interbreeding (as placeholder for reticulation, which is
>not a word Croizat indexed on). Only on 2 pages the two concepts are
>treated together, and looking up those pages it seems clear that the two
>are seen as completely different processes:
>p. 273, footnote: "Adding to this the effect of (...some processes...)
>hybridizations and recombinations (...more distinct processes...) form
>making may reach in time a hardly credible density"
>p. 520: "(...) hybridization took place here and there during spells of
>mobilism followed in immobilism (C.means during subsequent spells of
>immobilism, note PH) by re-combination of characters of permanent nature,
>whatever the technical process in detail."
>To me, it is clear that hybridization and character recombination are to
>Croizat two completely different processes, which need not coincide in time.
>This is of course rather surprising, as hybridization offers such a good
>expanation of observed character recombination. Yet Croizat seems not to
>have thought of that. But then, what he called character recombination is
>not what we casually call character recombination.
>His concept of character recombination refers to parallel evolution of
>similar combinations of character states. To Croizat, organisms were a bit
>like banks of dip-switches. Ancestral forms had all the switches in the
>same position, and in descendant forms different combinations of switches
>would be "up" or "down". Recombination of characters refers to the
>possibility that the same combination of "up" and "down" could occur in
>"unrelated" forms. Throughout evolution, new "banks" of switches could not
>be added: "in evolution old characters are being combined and recombined
>without cease rather than new characters constantly called into being" (p.
>309). Thus, it is not necessary to invoke hybridization or reticulation -
>it was enough to recognize that "form making" could simply flip any number
>of switches at any time.
>I will not go into John Grehans first statement - that "traditional
>evolutionary models" have neglected reticulate evolution. It just shows
>that he's not a botanist.
But yet botanical cladograms are constructed the same way as others, aren't
they? If so, and for as long as this has occurred and continues to occur,
that is a warning flag for the methodology as a whole, from my
perspective. And how about the degree to which cladistic (and other?)
botanical studies use chloroplast and ribosomal rather than nuclear genes
from which to generalize?
>Nationaal Herbarium Nederland - Leiden
>PO Box 9514
>2300 RA Leiden
>hovenkamp at nhn.leidenuniv.nl
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