[Taxacom] Progress in biogeography

John Grehan calabar.john at gmail.com
Sat Nov 5 10:58:12 CDT 2016

More food for thought below. John Grehan

Heads 2016 p. 64

Darwin to J.D. Hooker, April 23. 1861

“I quite agree with what you say on Lieut. Hutton’s review (who he is, I
know not): it struck me as very original: he is one of the very few who see
that the change of species cannot be directly proved & that the doctrine
must sink or swim according as it groups & explains phenomena. It is really
curious how few judge it in this way, which clearly the right way.”

Neither the chance dispersal model nor the vicariance/range expansion model
can be proved or disproved in a mathematical sense, but they can be
assessed by the efficiency with which they group and explain phenomena, and
provide solutions to concrete biogeographic problems. Does chance dispersal
provide coherent, testable explanations for common, repeated patterns? Or
does it just explain them away? Does the theory lead to any novel, bold
predictions that go beyond what is already accepted as true? Crisp et al
(2011) observed that “developing synergies between biogeography, ecology,
molecular dating and paleontology are providing novel data and
hypothesis-testing opportunities.” There is no doubt that the synergies are
providing impressive technical advances and a mountain of new data, but are
they leading to any new ideas?

Crisp et al. (2011) suggested one possibility: in the new studies
“understanding of how lineages become distributed s they are has changed
dramatically…it has been learned that the geographical evolution of biota
has been driven by a greater diversity or processes with a more complex
history than under a single vicariance (or dispersal) paradigm.” But this
is not new; all workers have accepted both vicariance and dispersal, at
lest in some sense. Dispersal theorists have accepted that vicariance has
occurred in some cases of local endemism (Mayr, 1965, as quoted earlier),
and no one has ever suggested that evolution works by pure vicariance, as
that would lead to every single area having a single, endemic clade.

Crisp et al. (2011) also wrote that new methods have led to “renewed
recognition that ecological factors (e.g. climatic tolerance and dispersal
limitation) underlie deep historical events (i.e. speciation, extinction
and distributional change)…” This is not a new concept; the same views are
presented in a founding document of the Modern Synthesis (Huxley, 1942, in
the sections on “ecological speciation”).

As another possible novelty, Crisp et al. (2011) proposed that “new
approaches are challenging the classic “Gondwana paradigm’…Surprisingly,
most trans-oceanic plant disjunctions…and many of those of animal taxa…have
been determined to be asynchronous or too young to be fully explained by
the break up of Gondwana.” Again, this repeats the Modern Synthesis view:
if the southern continents were ever joined, “they apparently separated to
long ago to leave any recognizable traces of the union in the distributions
of existing plants and animals…” (Darlington, 1964: 1085). The “new” views
are in fact the same as the Modern Synthesis position. This is not
surprising, as they are both based on the same conversion of
fossil-calibrated clade ages (minimum dates) into maximum dates, and on the
same assumptions about centers of origin and speciation by chance

To summarize, molecular work over the last decade or two has made stunning
technical advances and produced a fabulous wealth of new data, but in most
studies, the concepts used to interpret these data are inherited from the
Modern Synthesis. This may be changing though. Since the 1970’s, the
process of vicariance has been accepted by many authors, and a growing
number of contemporary authors are using vicariance in work on molecular

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