[Taxacom] more on iguanas

Wed Dec 1 19:55:30 CST 2021

One-off, unique events (like host switching) that are inferred to have 
happened based on e.g., host distributional data, but which have never 
been observed, are uncontroversial in parasitic lice, and, as I said, 
confound outdated ideas about strict co-speciation and strict host 
fidelity. They explain allohospitality, which is the parasite equivalent 
of allopatry. Moreover, they appear to be exceedingly common, and not 
the rare, negligible events that authors of the past believed.

If small parasites -- that cannot move when put on a non-feather 
surface, have no wings, and glue their eggs to the host, thus having no 
free-living stage at all -- can disperse through what you call "chance 
dispersal", why can this not happen with free-living organisms that do 
not have these kinds of "handicaps"?

(This is ignoring the fact that you just defined "chance dispersal" as 
"the dispersal that doesn't happen", and "ecological dispersal" as "any 
dispersal that happens", and assuming that this was not intended as it 
was written)

> Hi Daniel,
> 
> The 'chance' dispersal I was referring to is the conception of mostly 
> one off, unique events, that are imagined to have occurred but not 
> observed, to explain allopatry. This is different from normal means of 
> ecological dispersal by which organisms may move, to a greater or 
> lesser extent, which allows their continued survival through 
> generations and may result in range expansion or even contraction.
> 
> Discussion of such matters can continue indefinitely, as long as 
> taxonomists and systematists get involved with biogeography. No one is 
> forced to read or engage in such discussion.
> 
> Cheers, John Grehan
> 
> On Wed, Dec 1, 2021 at 7:29 PM Daniel Gustafsson via Taxacom 
> <taxacom at mailman.nhm.ku.edu> wrote:
> 
>> "the imaginary conception of 'chance' dispersal"
>> 
>> For minute, parasitic, wing-less insects that have no free-living 
>> stage,
>> never leave the host, cannot walk on a non-feather surface, are
>> constantly under threat of being killed by their host which is several
>> orders of magnitude larger than they are, have a life span after
>> hatching of about a month, and of course virtually always die when 
>> their
>> host dies -- for these, "chance dispersal" (or host switching) has 
>> been
>> shown to be a major issue confounding old ideas about strict
>> co-speciation in virtually every genus that has been studied to any
>> extent in the last 20 years, and is uncontroversial.
>> 
>> But for large, non-parasitic, animals that can walk, fly, swim etc. of
>> their own volition, may have life spans that cover several years, may
>> make regular large-scale movements across vast geographical areas, may
>> be omnivorous or at least not limited to a single kind of food, and 
>> have
>> life stages that do not consist of being attached to another organism 
>> --
>> for these chance dispersal is imaginary.
>> 
>> What a time to be alive.
>> 
>> Are these repetitious, and above all non-taxonomic, discussions on
>> panbiogeography ever going to end, or is this a preview of the hell 
>> that
>> all taxonomists go to when we die?
>> 
>>> To the biogeographic hobbyists: in my opinion, the primary problem of
>>> the
>>> molecular age underestimate papers is not even so much about their
>>> misrepresentation of fossil age calibration, but the way this 
>>> technique
>>> lets such authors escape from any responsibility to be familiar with
>>> biogeographic patterns in general. After all, if each individual 
>>> taxon
>>> has
>>> its own history of 'chance' dispersal, then there is no need to see 
>>> any
>>> connection with the biogeography of any other taxon or the 
>>> possibility
>>> of
>>> tectonic correlations. In effect, an intellectual curtain is drawn 
>>> over
>>> the
>>> biogeographic stage. What is not seen, does not exist. For the 
>>> iguanas,
>>> for
>>> example, there seems to be no comprehension of the basic 
>>> distributional
>>> facts as noted in Heads & Grehan (2021): "The Iguanidae and their
>>> sister,
>>> Agamidae (with Chamaeleonidae), are almost perfectly allopatric, and
>>> this
>>> is consistent with the origin of each clade more or less in situ, by
>>> vicariance in a global ancestor (Heads, 2014 p. 119). In this model,
>>> Iguanidae did not cross the Pacific in either direction. The origin 
>>> of
>>> the
>>> trans-Pacific affinity is explained by breaks in a global ancestor at
>>> sites
>>> that correspond with the western margin of the Pacific plate. The 
>>> only
>>> dispersal required in either Iguanidae or their sister group is in or
>>> around Madagascar, where the two clades overlap." I do hope the 
>>> critics
>>> on
>>> Taxacom make due note of the reference to the evidence for dispersal
>>> here!
>>> But of course, it is not the imaginary conception of 'chance'
>>> dispersal,
>>> but ordinary ecological dispersal (an observable phenomenon)
>>> responsible
>>> for range expansion. The real biogeographic issue has never been 
>>> about
>>> contesting vicariance against dispersal [which has generated the 
>>> trite
>>> conclusion that both are involved in different taxa], but coming to 
>>> an
>>> understanding about how the two processes are interrelated in the
>>> evolution
>>> of distributions. Croizat's work was, in my opinion, the first
>>> substantial
>>> effort to accomplish that - by making reference to the by far 
>>> greatest
>>> biodiversity resource available - the distributions of animal and 
>>> plant
>>> taxa that are made evident through the combined sciences of taxonomy,
>>> systematics, and geography.
>>> 
>>> As for my characterizations sometimes being seen to be 'over the 
>>> top',
>>> I
>>> suppose they might be. Perhaps from now on I will just refer to such
>>> papers
>>> as 'really, really, really terrible'. Hope that will be a widely
>>> acceptable
>>> expression of an opinion. And of course always, with reference to why
>>> that
>>> opinion is reached, since how one reaches an opinion in science is 
>>> more
>>> important than the opinion itself.
>>> 
>>> Interesting that those who are so outraged by language are evidently
>>> not
>>> able to come to the table with responses to questions about their
>>> assertions (as in recent questions by Heads). I think that says a 
>>> lot.
>>> 
>>> Cheers, John Grehan
>>> _______________________________________________
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>>> Nurturing nuance while assailing ambiguity for about 34 years,
>>> 1987-2021.
>> 
>> --
>> Dr. Daniel R. Gustafsson, Research Assistant Professor
>> Institute of Zoology Guangdong Academy of Sciences, Guangzhou, China.
>> 
>> Ask me about chewing lice!
>> _______________________________________________
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>> 
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>> http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
>> You can reach the person managing the list at: 
>> taxacom-owner at mailman.nhm.ku.edu
>> The Taxacom email archive back to 1992 can be searched at: 
>> http://taxacom.markmail.org
>> 
>> Nurturing nuance while assailing ambiguity for about 34 years, 
>> 1987-2021.

-- 
Dr. Daniel R. Gustafsson, Research Assistant Professor
Institute of Zoology Guangdong Academy of Sciences, Guangzhou, China.

Ask me about chewing lice!