[Taxacom] New paper on Coprosma biogeography

John Grehan calabar.john at gmail.com
Mon Feb 12 20:40:46 CST 2018

For those interested, the following paper was recently published:

Heads (2017) Metapopulation vicariance in the Pacific genus Coprosma
(Rubiaceae) and its Gondwanan relatives. Australian Systematic Botany,
2017, 30, 422–438.

A copy can be obtained from me or the author.

This paper is a must read for anyone interested in the various debates over
center of origin/dispersal, vicariance, molecular dating, priors, and
biogeographic-tectonic correlation. Heads examines all of these issue in
relation to Coprosma phylogeny and distribution. He also extensively
characterizes the concept of metapopulation vicariance where vicariance may
operate even without requiring formerly continuous land connections. He
also highlights the difference between normal ecological dispersal and
‘long-distance dispersal’ where the former is observed rather than
inferred, takes place all the time rather than once in millions or tens of
millions of years, involves whole populations rather than rare individuals,
and does not lead to speciation or differentiation. He points out that
normal dispersal is a key process enabling survival in a dynamic
environment and needs to be incorporated into biogeographic analysis.

The paper gives a detailed account of how vicariance may apply to the
geographic positioning of phylogenetic breaks between taxa with respect to
their distributions. Even though the group as a whole is widespread, the
subordinate taxa show precise patterns of distribution and tectonic
relationships that are also found in other taxa, something that should not
occur if dispersal is individual and unique as supposed in dispersalist
approaches that rule out vicariance.

The following conclusions are presented:

1. Fossil-calibrated methods for dating clades are misleading.
2. Standard methods for establishing ancestral areas are flawed.
3. Metapopulation vicariance accounts for distributions in oceanic groups.
4. Island biogeography is no different from continental biogeography.
5. Phylogenetic breaks and overlap around the Tasman basin can be explained
by tectonics
6. The sequence of phylogenetic-biogeographic breaks is consistent with
known aspects of tectonics.
7. The distribution of Coprosma is not the result of stochastic events and
is not unique.

*Abstract*. Coprosma is perhaps the most ubiquitous plant genus in New
Zealand. It belongs to the tribe Anthospermeae, which is distinctive in the
family Rubiaceae through its small, simple, wind-pollinated flowers and its
southern hemisphere distribution. The tribe comprises four main clades
found respectively in South Africa, Africa, Australia and the Pacific. The
high level of allopatry among the four subtribes is attributed here to
their origin by
vicariance. The Pacific clade, subtribe Coprosminae, is widespread around
the margins of the South Pacific and also occurs on most of the high
islands. Distributions of the main clades in the subtribe are mapped here
and are shown to be repeated in other groups. The distribution patterns
also coincide with features of regional geology. Large-scale volcanism has
persisted in the central Pacific region since at least the Jurassic. At
that time, the oldest of the Pacific large igneous provinces, the Shatsky
Rise, began to be erupted in the region now occupied by French Polynesia.
Large-scale volcanism in the central Pacific continued through the
Cretaceous and the Cenozoic. The sustained volcanism, along with details of
the clade distributions, both suggest that the Coprosminae have persisted
in the central Pacific by survival of metapopulations on individually
ephemeral islands. It is also likely that vicariance of metapopulations has
taken place, mediated by processes such as the subsidence of the Pacific
seafloor by thousands of metres, and rifting of active arcs by transform
faults. It is sometimes argued that a vicariance origin is unlikely for
groups on young, oceanic islands that have never been connected by
continuous land, but metapopulation vicariance does not require physical
contact between islands.


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