[Taxacom] Dispersal clarifications
Robinwbruce at aol.com
Robinwbruce at aol.com
Mon Jun 6 09:36:44 CDT 2011
Hi John,
My take on dispersal in current usage is range expansion with the
possibility of localized speciation as a further product.
My take of dispersal in Croizat's sense was of broad fronts/sheets of
organisms expanding (or contracting) through time and space, and dependant on
the relative degree of mobilism vs. immobilism and the morphogenetic
potential of the organisms, generating either deep or shallow differences of form
which could then be recognised, classified and systematised. Organisms by
their nature generate differences of form by various modes of reproduction;
time and space then canalise these differences into what we recognise and
develop as taxonomic patterns; all ranking is derivative. The imperatives of
such a system are temporal and spatial coincidence, or at least proximity,
and the generative nature of organisms. Singular events, local effects and
special ranks are of little significance. Croizat's perspective was
always Pan-biogeographical.
The current either/or form of vicariance/dispersal threatens to drain much
meaning out of Croizat's efforts. Perhaps a more helpful and constructive
outlook would be a bi-polar or multi-polar one, reflecting the global
surface nature (assuming Tom Gold's ideas of deep organisms are just ideas) of
what we are trying to understand.
Robin
In a message dated 6/4/2011 2:21:27 P.M. GMT Daylight Time,
jgrehan at sciencebuff.org writes:
My take on dispersal and biogeography is that biogeography is, in some
ways, a test of the assumptions about the role of means of dispersal (or the
theorized ability to translate an ability of individuals to move into a
theory of how related taxa come to occupy different sectors of the earth).
In dispersalist biogeography the ability to disperse seems to be often
treated as an inherent quality of an organism, so there are 'good' dispersers
and 'bad' dispersers. The bad ones explain why they are absent from
particular sectors and visa versa, and if a bad disperser does turn up then it
obviously succeed by some amazing or mysterious (and that word or similar has
been used by biogeographers), or a good disperser does not turn up then its
just another vagary of chance.
Ecological work on the other hand seems to indicate that dispensability is
more an ecologically contingent quality, that one must measure the
ecological relationships between the organisms and the environment to understand
what dispersal is as a function of that relationship. Under one set of
ecological relationships the range of a species may be very stable, but under
another it may result in rapid and extensive changes in geographic range.
This must have occurred in many groups for there to be trans-continental or
global taxa. The correlation of such groups with Mesozoic tectonics suggests
that the ancestral range expansion did indeed occur in the Mesozoic. So its
not just a matter of whether or not to choose dispersal or vicariance, but
to have some way of estimating the spatial and temporal context for their
role in the origin of distribution in general.
The case of Juan Fernandez having closer affinities on the other side of
the Pacific for some taxa is an example of where biogeography gives an
insight that is not possible simply from the assuming stratigraphic age and
dispersal. JF is not the only such example and even for the Galapagos there are
comparable patterns that hit the Galapagos, but not the American mainland.
John Grehan
-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu
[mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Jason Mate
Sent: Friday, June 03, 2011 6:22 PM
To: Taxacom
Subject: Re: [Taxacom] Dispersal clarifications
Hi Michael, my thoughts on your comments below:
MH:About the terms you mention: 'dispersalist theory' and its advocates
accept chance dispersal as a mode of speciation.
JFM:So is dispersal of a species from a mainland source to an island
acceptable dispersal?
MH:An organism 'disperses across a barrier', often by mysterious means.
For example, an organism that moves a few centimeters in its lifetime is
theorized to have jumped - just once - 10 000 km across the Pacific. Vicariance
theory accepts that allopatric speciation is caused by vicariance and that
range expansion (physical movement) only explains overlap.
JFM:Are all purported cases of dispersal similarly extreme? Do you
consider the dispersal of flying organisms equally mysterious? Don´t you think
that the probability of dispersal correlates with the organisms ability to
disperse?
MH:Chance: this term can be used in the sense of sweeping something under
the carpet - 'oh, event x must have just been an accident'. Alternatively,
you can go out and collect data and actually work out the 'probabilities'.
These are also referred to as 'chance' - as in 'the chance of event x
happening', but 'chance' here is a completely different concept. For example, a
road accident may have happened on a corner where it turns out, following a
statistical analysis, 'accidents' regularly happen. It wasn't simply an
accident, there was a general, underlying explanation. This change in
approach was the psychological revolution of Fermat and Pascal that gave birth to
probability theory.
JFM:This is a qualitative remark. A 1 in a million event, although rare,
although ´chance´, still has a probability, and the sum of all the
probabilities describes and event. Thus, although accidents can cluster and these
are what people may find interesting, a well lit corner in a speed-controlled
zone with good visibility can also have the odd accident. The underlying
cause of this accident may be something completely random, with a low
repeatability (maybe old drivers with astigmatism at dusk fail to see
pedestrians). But this isolated case is still part of the totality of accidents for
the area under study. So I´d say that you are confusing probability with
repeatability. By studying the biota of an area you work out the frequency
(probability) of each mechanism. Maybe dispersal is not of interest to you
since you are only interested in vicariance, but to taxonomists or ecologists
dispersal can be equally interesting.
MH:Dispersal theory as seen in biogeography and systematics journals deals
with phylogeny and relies on 'chance' in the earlier sense. In contrast,
dispersal studies in ecological journals deal with observations (not
inferences) of simple movement and are full of probability analyses - they use
chance in the modern sense.
JFM:Chance in the earlier sense? You mean the magical sense? Each throw of
the dice is independent from the previous ones. So even if I could work
out the probability of dispersal under your system (I can´t because your
philosophical frame does not allow it) it would be based on studying the origin
of a certain number of clades and
MH:'Ecological dispersal' is just the simple movement of organisms
observed every day. A weed disperses into a garden, but it doesn't differentiate
into a new taxon. An organism can be where it is because it moved there or
because it evolved there.
JFM:The simple movement observed every day? So if you don´t observe it
every day it does not occur, i.e. migration? What about vagrant specimens?
Dispersal is not an everyday, week or even year event. It is rare, precisely
because it is a high failure, overwhelmingly lethal, no second prize
strategy.
MH:Neatly allopatric (dovetailing) geographic structure repeated in
different groups can't be explained by (mere) 'chance'. It could be the result of
a general process such as vicariance.
JFM:You are the only one here saying that vicariance is not an accepted
mechanism that accounts for organisms distributions. At the same time, you
would expect that sources of colonists would be limited to certain areas that
have some sort of advantage (i.e. currents, prevailing winds, shortest
distance).
MH:Vicariance might be falsified by finding that the distributions of taxa
in a region don't share similar phylogenetic/biogeographic breaks. But
this could be due to original allopatry followed by range expansion. Unlike
the text-book examples which everyone learns, most broad theories don't fall
over instantaneously because someone finds a single fact (see the detailed
critiques of 'naive falsificationism' by Lakatos, Feyerabend etc.). Usually
there is a slow accumulation of 'anomalous' data and eventually there is a
broad shift of opinion. If all the molecular phylogenies showed no
geographic structure and instead taxa tended to sort on colour or size or
hairiness, rather than locality, no-one would be looking at vicariance and it
would just fade away.
JFM:Once again, vicariance is an accepted mechanism.
MH:But the clades nearly all show impressive allopatry - this is why so
many authors are putting distribution maps of their clades in the graphical
abstracts at Mol. Phylogen. Evol. and lots of people are discussing
vicariance. If you Google scholar 'vicariance', here are the numbers of articles
mentioning the word for 5-year periods, starting with 1960-1965 when Croizat
wrote his big books on the topic: 27, 40, 66, 218, 770, 1040, 1380, 2280,
4140, 6600 (2005-2010).
JFM:So vicariance is the only mechanism allowable unless everything there
shows otherwise? Why would you assume that the entire biota of an area got
there by one mechanism only? What is the logical basis for this?
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