[Taxacom] What is possible or not in biogeography

John Grehan calabar.john at gmail.com
Wed Dec 21 17:39:32 CST 2016

Following the recent posting on biogeography an earnest colleague on
TAXACOM contacted me off list to ‘suggest’ that the genus *Meryta* a is a
good example of a distribution which cannot be explained other than by
dispersal of seeds by birds. Unfortunately he could not provide any
analytical evidence and could only assert that “the species are so disjunct
and scattered”. This brings forward the point that in analytical
biogeography there is no presumption about theorized dispersability or
distinction over the extent of disjunction or scattering of taxa. While my
colleague personally felt that bird dispersal was the only possibility,
this does not mean that is necessarily so.

Biogeographic analysis shows that allopatry, both within the genus and with
respect to potential relatives, is predominant and that the distribution of
*Meryta* is representative of many other taxa in the region. Bird mediated
dispersal of seeds may explain ecological survival, but not the geography
of differentiation (at least not without creating paradoxes). Below is the
excerpt from Heads (2012):

"Trees in the genus *Meryta *are distributed from Micronesia (Yap) to
south-eastern Polynesia (Lowry, 1988; Fig. 6-13). The two main clades in
the genus (not mapped here) are allopatric; on is only in Fiji and New
Zealand, the other widespread in the central Pacific (Micronesia, New
Caledonia, Vanuatu, and southern Polynesia). Tronchet et al. (2005)
accepted that the break between the two clades represent “ancient

Possible relatives of *Meryta* include a clade termed “Melanesian
*Schefflera*” (including *Plerandra, Dizygotheca*, Gabriellae group, etc.)
(Plunkett et al., 2005; Plunkett and Lowry, 2007; Fig. 6-13). The two
clades are largely vicariant, with a region of overlap: New Caledonia,
Vanuatu, and Fiji. Both clades have most of their diversity in New
Caledonia, which could be the result of the composite tectonic structure of
the island. *Pseudopanax* of New Zealand is another relative, and the three
groups may form a south-west Pacific clade (G. Plunkett, pers. comm.).
Another possibility is that “Melanesian *Schefflera*,” *Meryta*, and then
*Pseudopanax* are the three basal branches in a widespread Indo-Pacific
group, with the main, widespread clade being *Polyscias *s.lat (not shown)
(Plunkett and Lowry, 2010). In any case, *Meryta *is a distinctive genus
and a typical central Pacific group.”

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