[Taxacom] more on iguanas

[John Grehan]

Daniel,

I think the nature of host changes is of great interest with regards to
these questions of the relationship between dispersal (as movement) and
differentiation. As I don't work with host-parasite evolution there is not
much I could offer in the way of informed comment. My impression is that
the ability of parasites to move between different hosts might be
comparable to a metapopulation where different hosts operate as 'islands'
or 'habitats' within the range of an ancestor. While this explains parasite
survival and persistence it may be another matter to consider
differentiation of parasite lineages with respect to their hosts. My only
direct observation is from an insect host plant example where a moth
species in New Zealand has several host plants which have different
biogeographic homologies - some being of Pacific origin, others Indian
Ocean (core Gondwana). This combination would suggest that the present host
rage is a composite that originated when Indian Ocean and Pacific plant
groups were brought into contact in what is now New Zealand, resulting in a
more extensive hostplant range for the moth (although I cannot confidently
predict which set of host plants represent later acquisitions as I am
currently not certain about its biogeographic homology).

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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> Nurturing nuance while assailing ambiguity for about 34 years, 1987-2021.
>