Hybrid Speciation

John Grehan jrg13 at PSU.EDU
Mon Apr 20 09:43:38 CDT 1998


Some comments regarding notes by Les Kaufman on Croizat and Hennig and hybrids.
I have not read everything of Croizat's, and have not concentrated on his
considerations of species, but I am not aware that anything Les stated
about the complexity of landscape fragmentation and speciation would
conflict with Croizat's position. I do recall that Croizat was very
critical of cladistic splitting being representative of actual speciation
in many cases where he considered a polycotomous division to be more true
to the event even though the character relationships of the descendants
could be resolved to a dichotomous level.

The concept of "webbing at the node" would also appear to be in close
agreement with Croizat's concept of differentiated ancestor. Heads (Syst
Zoo 1985 pp 205-215) identified distinguished between the concepts of
differentiated and uniform ancestor. To the former he identified Croizat,
Simpson and Rosa, and the latter Darwin, Hennig, Brundin, Mayr, Nelson, and
Platnick.

On page 188 of Space, Time, Form, Croizat illustrates a model of
form-making ("speciation" in which the rearrangements of the landscape is
distinctly uneven. When considering the process of differentiation
(form-making) Croizat emphasizes character recombination of an acestor that
is already partially differentiated, both biologically and
geogeographically. The idea that vicariants may achieve the level of
semispecies, then anastomose back and forth would seem to be compatible
with Croizat. On page 184 Crioizat refers to hybridization of populations
within a species complex as part of the normal process of differentiation.

Heads (1990, New Zealand Journal of Zoology 16(4) p. 578), provides an
illustration of complex ancestral differentiation which he refers to as
hybrid swarms which may form and deform, with the possiblity that the
"crystallisation" of such swarms may result in descendant taxa with a very
high potential to hybrize. The concept of "hybrid swarm" and "webbing at
the node" would appear to have much in common, particularly in the
reference to node, since nodes are biogeographically and phylogenetically
complex points of differentiation in space and time.

John Grehan


>>A major challenge, and one that I believe we are having with the Lake
>>Victoria haplochromines, is that of adequately dealing with early stages
>>of lineage splitting.  In the real world, this does not happen exactly the
>>way that Croizat or Hennig seemed to have in mind.  Some landscapes
>>shatter, rather than divide evenly, producing scores of vicariants that
>>may alternately achieve the level of semispecies, then anastomose, back
>>and forth, until some decisive event puts an end to the confusion.   I've
>>been calling this "the webbing at the node."  Has anybody
>>else out there been thinking about this?
>>
>>Les Kaufman
>>Boston University Marine Program
>>lesk at bio.bu.edu
>
>What seems important to me, is the group you deal with. In marine algae,
>particularly corallines and other calcified groups, there are many species
>postulated to be living fossils. Many of these organisms have a sexual life
>history and have apparently coexisted with 'related' species for million of
>years. The fact that they are distinct today suggest that after all the way
>that Hennig (& Croizat ?) had in mind may be true (for these groups at
>least).




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