[Taxacom] How to generate biogeographic and ecological nonsense

John Grehan calabar.john at gmail.com
Wed Dec 28 17:46:39 CST 2016


As most list members no doubt appreciate, one cannot use the geological
ages of mountains or islands to uncritically make up maximum divergence
dates. A good example of the biogeographic mess that is otherwise created
(and remains a heavy burden on the scientific integrity of many New Zealand
studies) is illustrated by Heads for *Maoricicada* in New Zealand as
follows:


“Buckley et al. (2001) calibrated the timeline for phylogeny in*
Maoricicada* *campbelli* by assuming that *Maoricicada* was no older than
the current alpine habitat (~ 5 Ma), as the genus is mainly alpine. This
makes the assumption that taxa currently located in the alpine zone require
alpine habitat, and this is not necessarily correct. In any case, *M.
lindsayi* occurs at 0-500 m elevation, and *M. campbelli* itself is
recorded from 10 to 1580 m (Buckley and Simon, 2007). In fact, most of the
“alpine” species of *Maoricicada *are known from some localities below the
alpine zone; 10 out of 14 have populations at less than 1000 m (around the
level of the tree line) and several have populations in the lowlands
(Dugdale and Fleming, 1978). These records suggest that *Maoricicada* and
at least some of its diversity existed before the present alpine
environment developed, surviving in lowland areas of open vegetation. The
diverse ancestral complex would have been inherited by the mountains of the
Kaikoura orogeny, as they rose. If this was the history, it would undermine
a habitat calibration based on the age of the mountains.


Based on the “mountain age” *habitat *calibration, Buckley et al. (2001)
calculated that the earliest divergence within *M. campbelli* occurred at
2.3 Ma, and they concluded: “We cannot reconcile the earliest date of
divergence within M. campbelli with rifting along the Alpine Fault, a
process that began to accelerate 10 million years ago.” Yet the young age
they calculated for divergence in *M. campbelli* is based on the doubtful
assumption they made about the age of the genus.


Buckley et al. (2001) assumed that alpine taxa in Maoricicada could be no
older than their alpine habitats. In a subsequent study of the genus,
Buckley and Simon (2007) abandoned this method. Instead, though, they
calibrated a molecular clock for *Maoricicada *by assuming that related
cicadas, endemic on Norfolk Island and Kermadec Islands, could be no older
than their current *islands*. But islands are just another habitat type
within a region, as are mountains, leaf surfaces, or puddles. The existence
of *prior* islands in the region means that calibration using island age,
will, as will calibration using habitat age, give dates for clades that are
too young by an indeterminate amount. Buckley and Simon (2007) found that
*Maoricicada* diverged only in the mid-Miocene. This date is too young (by
how much is not known), but at least it confirms that *Maoricicada* existed
before the origin of high montane habitats.


Hill et al. (2009) carried out further studies on *Maoricicada campbelli*
and confirmed that the main break in *M. campbelli* is between a Central
Otago clade and the rest. They wrote “we can only speculated as to what
process initiated both the divergence of *M. campbelli *from the rest of
Maoricicada at <2.6 Ma, and the main *M. campbelli* split at 1.5-2.1 Ma”
(p. 13). As discussed, the “island age” calibration (from Buckley and
Simon, 2007) that these dates depend on is flawed.



The authors based their conclusions on these dates, but they did not refer
to major geological features at the boundaries, such as the Moonlight
tectonic zone, and instead suggested that increased sedimentation of river
gravel caused by rising mountains produced habitat “into which *M.
campbelli *evolved” (p. 14). The new sedimentation and new habitat might
explain local range expansion or contraction in the group during the
Kaikoura orogeny, but they do not account for the *location* of the mean
break in the species.”



 John Grehan


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