[Taxacom] Dispersal example

[John Grehan]

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.