[Taxacom] Dispersal example

[John Grehan]

Peter,

I think it is less a case of considering long-distance dispersal as
unlikely and ask why it did not occur twice, but rather why did it occur
just once, time and time again for group after group, and in such a way as
to result in patterns that are correlated with tectonics that have nothing
to do with the origin of the group (according to the long-distance
dispersal premise). Even when statistics are attached to analyses that does
not meant that the analyses necessarily have any greater substance.
Long-distance dispersal biogeography has been around for a long time now
and with all the practitioners and economic resources there is still a lack
of statistical foundation to demonstrate the actuality of long-distance
dispersal as the mechanism of global biogeography. The molecular clock
divergence estimation method that was widely and prominently heralded as
the silver bullet proving dispersalist theory now proves to be a fiction.

John Grehan


On Mon, Apr 14, 2014 at 3:32 AM, Phovenkamp at casema Nl
<phovenkamp at casema.nl>wrote:

>  John and others,
>
> May I just note (again) that it is inconsistent to (a) consider
> long-distance dispersal as unlikely and (b) ask for an explanation why it
> did not occur twice. Clearly, when something is unlikely, the probability
> that it only occurred once is much higher than that it occured twice.
>
> And as a second remark, I would like notice any individual examples should
> be accompanied with some statistics. We're talking probabilities here, and
> to argue that "ad hoc hypotheses would also be
> needed for all the other groups" that share a same pattern is
> disingenuous. Some number of similar patterns will occur because of sheer
> coincidence. The question is, how large is that number? Only when the
> actual number well exceeds that expectation are we in need of an
> explanation.
>
> And then the expectation depends on a sizable number of factors (among
> which, to name a few, total number of taxa in either area, total number of
> taxa sharing a similar ecology, the probability of successful dispersal ?
> yes, even when ?unlikely? a probability can be given a value-, period over
> which the pattern may have developed, proportion of areas on both side with
> suitable ecological conditions...). I suggest that the number of factors,
> their actual values, and thus the result of the analysis, is close to
> intractable if not completely intractable. Actual probabilities cannot be
> assessed, confirmed or refuted. So all the talk about "argue against chance
> dispersal" is just rhetoric.
>
> Best,
>
> Peter Hovenkamp
>
> Op 10 april 2014 om 16:59 schreef John Grehan <calabar.john at gmail.com>:
>
>
>
> Following is a section of Heads' book regarding the problematic and
> contradictory nature of invoking chance long-distance trans-oceanic
> dispersal based on erroneous or unnecessary divergence predictions. Anyone
> wishing to make a critical response (that are welcome) might be advised to
> first examine Fig. 6.7.
>
> John Grehan
>
> In Craspedia clade 5 (Fig. 6.7), the New Zealand species are sister to a
> group in southeastern Australia and Tasmania. There are no dated fossils of
> the genus or its sister but 'very low' divergences between New Zealand and
> Australian/Tasmanian samples led Ford et al (2007) to suggest late Tertiary
> or Quarternary dispersal. The mode of dispersal is 'a matter of conjecture'
> as the pappus is deciduous and long-distance wind dispersal is unlikely.
> Ford et al (2007) suggested that carriage on muddy bird feet is
> 'plausible'. This has also been suggested for other trans-Tasman plant
> groups with no obvious means of long-distance dispersal, but it does not
> explain why successful dispersal in Craspedia only occurred once (the New
> Zealand species form a single clade). In addition, there are no obvious
> bird vectors. Ad hoc hypotheses could be proposed, such as extinction of
> suitable bird vectors, but then further ad hoc hypotheses would also be
> needed for all the other groups that share the trans-Tasman distribution of
> Craspedia while, as in that genus, having no obvious means of trans-oceanic
> dispersal. Finally, the high degree of endemism and vicariance in the tribe
> Gnaphalieae, the Australian members and the genus Craspedia (e.g. its
> absence from New Guinea) all argue against chance dispersal as the overall
> mode of spatial evolution. Its seems more likely that traditional ideas on
> the time course of evolution in Asteraceae (based on a literal reading of
> the fossil record) are incorrect.
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